LM2599
LM2599 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage
Regulator, with Features
Literature Number: SNVS123B
LM2599
SIMPLE SWITCHER®Power Converter 150 kHz 3A
Step-Down Voltage Regulator, with Features
General Description
The LM2599 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving a 3A load with
excellent line and load regulation. These devices are avail-
able in fixed output voltages of 3.3V, 5V, 12V, and an adjust-
able output version.
This series of switching regulators is similar to the LM2596
series, with additional supervisory and performance features
added.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation†, improved line and load specifications, fixed-
frequency oscillator, Shutdown/Soft-start, error flag delay
and error flag output.
The LM2599 series operates at a switching frequency of 150
kHz thus allowing smaller sized filter components than what
would be needed with lower frequency switching regulators.
Available in a standard 7-lead TO-220 package with several
different lead bend options, and a 7-lead TO-263 Surface
mount package.
A standard series of inductors (both through hole and sur-
face mount types) are available from several different manu-
facturers optimized for use with the LM2599 series. This
feature greatly simplifies the design of switch-mode power
supplies.
Other features include a guaranteed ±4% tolerance on out-
put voltage under all conditions of input voltage and output
load conditions, and ±15% on the oscillator frequency. Ex-
ternal shutdown is included, featuring typically 80 µA
standby current. Self protection features include a two stage
current limit for the output switch and an over temperature
shutdown for complete protection under fault conditions.
Features
n3.3V, 5V, 12V, and adjustable output versions
nAdjustable version output voltage range, 1.2V to 37V
±4% max over line and load conditions
nGuaranteed 3A output current
nAvailable in 7-pin TO-220 and TO-263 (surface mount)
Package
nInput voltage range up to 40V
n150 kHz fixed frequency internal oscillator
nShutdown/Soft-start
nOut of regulation error flag
nError output delay
nLow power standby mode, I
Q
typically 80 µA
nHigh Efficiency
nUses readily available standard inductors
nThermal shutdown and current limit protection
Applications
nSimple high-efficiency step-down (buck) regulator
nEfficient pre-regulator for linear regulators
nOn-card switching regulators
nPositive to Negative converter
Note: †Patent Number 5,382,918.
Typical Application (Fixed Output Voltage
Versions)
01258201
SIMPLE SWITCHER®and Switchers Made Simple®are registered trademarks of National Semiconductor Corporation.
December 2000
LM2599 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator, with
Features
© 2004 National Semiconductor Corporation DS012582 www.national.com
Connection Diagrams and Order Information
Bent and Staggered Leads, Through Hole Package
7-Lead TO-220 (T)
Surface Mount Package
7-Lead TO-263 (S)
01258250
Order Number LM2599T-3.3, LM2599T-5.0,
LM2599T-12 or LM2599T-ADJ
See NS Package Number TA07B
01258223
Order Number LM2599S-3.3, LM2599S-5.0,
LM2599S-12 or LM2599S-ADJ
See NS Package Number TS7B
LM2599
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Voltage (V
IN
) 45V
SD /SS Pin Input Voltage (Note 2) 6V
Delay Pin Voltage (Note 2) 1.5V
Flag Pin Voltage −0.3 ≤V≤45V
Feedback Pin Voltage −0.3 ≤V≤+25V
Output Voltage to Ground
(Steady State) −1V
Power Dissipation Internally limited
Storage Temperature Range −65˚C to +150˚C
ESD Susceptibility
Human Body Model (Note 3) 2 kV
Lead Temperature
S Package
Vapor Phase (60 sec.) +215˚C
Infrared (10 sec.) +245˚C
T Package (Soldering, 10 sec.) +260˚C
Maximum Junction Temperature +150˚C
Operating Conditions
Temperature Range −40˚C ≤T
J
≤+125˚C
Supply Voltage 4.5V to 40V
LM2599-3.3
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-3.3 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
OUT
Output Voltage 4.75V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 3.3 V
3.168/3.135 V(min)
3.432/3.465 V(max)
ηEfficiency V
IN
= 12V, I
LOAD
=3A 73 %
LM2599-5.0
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-5.0 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
OUT
Output Voltage 7V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 5 V
4.800/4.750 V(min)
5.200/5.250 V(max)
ηEfficiency V
IN
= 12V, I
LOAD
=3A 80 %
LM2599
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LM2599-12
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-12 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
OUT
Output Voltage 15V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 12 V
11.52/11.40 V(min)
12.48/12.60 V(max)
ηEfficiency V
IN
= 25V, I
LOAD
=3A 90 %
LM2599-ADJ
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-ADJ Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
FB
Feedback Voltage 4.5V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 1.230 V
V
OUT
programmed for 3V. Circuit of Figure 1. 1.193/1.180 V(min)
1.267/1.280 V(max)
ηEfficiency V
IN
= 12V, V
OUT
= 3V, I
LOAD
=3A 73 %
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version and V
IN
= 24V for the 12V version.
I
LOAD
= 500 mA
Symbol Parameter Conditions LM2599-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
DEVICE PARAMETERS
I
b
Feedback Bias Current Adjustable Version Only, V
FB
= 1.3V 10 nA
50/100 nA (max)
f
O
Oscillator Frequency (Note 7) 150 kHz
127/110 kHz(min)
173/173 kHz(max)
V
SAT
Saturation Voltage I
OUT
= 3A (Note 8) (Note 9) 1.16 V
1.4/1.5 V(max)
DC Max Duty Cycle (ON) (Note 9) 100 %
Min Duty Cycle (OFF) (Note 10) 0
I
CL
Current Limit Peak Current, (Note 8) (Note 9) 4.5 A
3.6/3.4 A(min)
6.9/7.5 A(max)
I
L
Output Leakage Current (Note 8) (Note 10) (Note 11) Output = 0V 50 µA(max)
Output = −1V 2 mA
30 mA(max)
I
Q
Operating Quiescent SD /SS Pin Open (Note 10) 5mA
Current 10 mA(max)
LM2599
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All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version and V
IN
= 24V for the 12V version.
I
LOAD
= 500 mA
Symbol Parameter Conditions LM2599-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
DEVICE PARAMETERS
I
STBY
Standby Quiescent SD /SS pin = 0V (Note 11) 80 µA
Current 200/250 µA(max)
θ
JC
Thermal Resistance TO220 or TO263 Package, Junction to Case 2 ˚C/W
θ
JA
TO220 Package, Juncton to Ambient (Note 12) 50 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 13) 50 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 14) 30 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 15) 20 ˚C/W
SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 1
V
SD
Shutdown Threshold 1.3 V
Voltage Low, (Shutdown Mode) 0.6 V(max)
High, (Soft-start Mode) 2V(min)
V
SS
Soft-start Voltage V
OUT
= 20% of Nominal Output Voltage 2 V
V
OUT
= 100% of Nominal Output Voltage 3
I
SD
Shutdown Current V
SHUTDOWN
= 0.5V 5µA
10 µA(max)
I
SS
Soft-start Current V
Soft-start
= 2.5V 1.6 µA
5 µA(max)
FLAG/DELAY CONTROL Test Circuit of Figure 1
Regulator Dropout Detector Low (Flag ON) 96 %
Threshold Voltage 92 %(min)
98 %(max)
VF
SAT
Flag Output Saturation I
SINK
= 3 mA 0.3 V
Voltage V
DELAY
= 0.5V 0.7/1.0 V(max)
IF
L
Flag Output Leakage Current V
FLAG
= 40V 0.3 µA
Delay Pin Threshold 1.25 V
Voltage Low (Flag ON) 1.21 V(min)
High (Flag OFF) and V
OUT
Regulated 1.29 V(max)
Delay Pin Source Current V
DELAY
= 0.5V 3 µA
6 µA(max)
Delay Pin Saturation Low (Flag ON) 55 mV
350/400 mV(max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 4: Typical numbers are at 25˚C and represent the most likely norm.
Note 5: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used
to calculate Average Outgoing Quality Level (AOQL).
Note 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2599
is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 7: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current
overload.
Note 8: No diode, inductor or capacitor connected to output pin.
Note 9: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 10: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version, and 15V for the 12V version, to force the output transistor
switch OFF.
LM2599
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All Output Voltage Versions
Electrical Characteristics (Continued)
Note 11: VIN = 40V.
Note 12: Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the leads soldered to
a printed circuit board with (1 oz.) copper area of approximately 1 in2.
Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2of (1 oz.) copper area.
Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2of (1 oz.) copper area.
Note 15: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2of (1 oz.) copper area on
the LM2599S side of the board, and approximately 16 in2of copper on the other side of the p-c board. See application hints in this data sheet and the thermal model
in Switchers Made Simple version 4.2.1 (or later) software.
Typical Performance Characteristics
(Circuit of Figure 1)
Normalized
Output Voltage Line Regulation
01258202 01258203
Efficiency
Switch Saturation
Voltage
01258204 01258205
LM2599
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Typical Performance Characteristics (Circuit of Figure 1) (Continued)
Shutdown/Soft-start
Threshold Voltage
Continuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
=2A
L=32µH,C
OUT
= 220 µF, C
OUT
ESR=50mΩ
01258253
01258220
A: Output Pin Voltage, 10V/div.
B: Inductor Current 1A/div.
C: Output Ripple Voltage, 50 mV/div.
Horizontal Time Base: 2 µs/div.
Discontinuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA
L=10µH,C
OUT
= 330 µF, C
OUT
ESR=45mΩ
Load Transient Response for Continuous Mode
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA to 2A
L=32µH,C
OUT
= 220 µF, C
OUT
ESR=50mΩ
01258219
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 100 mV/div.
Horizontal Time Base: 2 µs/div.
01258221
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Horizontal Time Base: 50 µs/div.
Load Transient Response for Discontinuous Mode
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA to 2A
L=10µH,C
OUT
= 330 µF, C
OUT
ESR=45mΩ
01258222
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Horizontal Time Base: 200 µs/div.
LM2599
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Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
01258224
Component Values shown are for VIN = 15V,
VOUT = 5V, ILOAD = 3A.
CIN — 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 220 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 5A, 40V Schottky Rectifier, 1N5825
L1 — 68 µH, L38
Typical Values
CSS — 0.1 µF
CDELAY — 0.1 µF
RPull Up — 4.7k
LM2599
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Test Circuit and Layout Guidelines (Continued)
As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance can
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the wires indicated by
heavy lines should be wide printed circuit traces and
should be kept as short as possible. For best results,
external components should be located as close to the
switcher lC as possible using ground plane construction or
single point grounding.
If open core inductors are used, special care must be
taken as to the location and positioning of this type of induc-
tor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and C
OUT
wiring can cause problems.
When using the adjustable version, special care must be
taken as to the location of the feedback resistors and the
associated wiring. Physically locate both resistors near the
IC, and route the wiring away from the inductor, especially an
open core type of inductor. (See application section for more
information.)
Adjustable Output Voltage Versions
01258225
where VREF = 1.23V
Select R1to be approximately 1 kΩ, use a 1% resistor for best stability.
Component Values shown are for VIN = 20V,
VOUT = 10V, ILOAD = 3A.
CIN: — 470 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”
COUT: — 220 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 5A, 30V Schottky Rectifier, 1N5824
L1 — 68 µH, L38
R1—1kΩ,1%
R2— 7.15k, 1%
CFF — 3.3 nF, See Application Information Section
RFF —3kΩ, See Application Information Section
Typical Values
CSS — 0.1 µF
CDELAY — 0.1 µF
RPULL UP — 4.7k
FIGURE 1. Standard Test Circuits and Layout Guides
LM2599
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LM2599 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given:
V
OUT
= Regulated Output Voltage (3.3V, 5V or 12V)
V
IN
(max) = Maximum DC Input Voltage
I
LOAD
(max) = Maximum Load Current
Given:
V
OUT
=5V
V
IN
(max) = 12V
I
LOAD
(max) = 3A
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from
Figure 4,Figure 5,or6. (Output voltages of 3.3V, 5V, or
12V respectively.) For all other voltages, see the design
procedure for the adjustable version.
B. From the inductor value selection guide, identify the
inductance region intersected by the Maximum Input
Voltage line and the Maximum Load Current line. Each
region is identified by an inductance value and an
inductor code (LXX).
C. Select an appropriate inductor from the four
manufacturer’s part numbers listed in Figure 8.
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version
shown in Figure 5.
B. From the inductor value selection guide shown in
Figure 5, the inductance region intersected by the 12V
horizontal line and the 3A vertical line is 33 µH, and the
inductor code is L40. C. The inductance value required is
33 µH. From the table in Figure 8, go to the L40 line and
choose an inductor part number from any of the four
manufacturers shown. (In most instance, both through
hole and surface mount inductors are available.)
2. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR (Equivalent
Series Resistance) electrolytic capacitors between 82 µF
and 820 µF and low ESR solid tantalum capacitors
between 10 µF and 470 µF provide the best results. This
capacitor should be located close to the IC using short
capacitor leads and short copper traces. Do not use
capacitors larger than 820 µF.
For additional information, see section on output
capacitors in application information section.
B. To simplify the capacitor selection procedure, refer to
the quick design component selection table shown in
Figure 2. This table contains different input voltages,
output voltages, and load currents, and lists various
inductors and output capacitors that will provide the best
design solutions.
C. The capacitor voltage rating for electrolytic capacitors
should be at least 1.5 times greater than the output
voltage, and often much higher voltage ratings are
needed to satisfy the low ESR requirements for low
output ripple voltage.
D. For computer aided design software, see Switchers
Made Simple (version 4.2.1 or later).
2. Output Capacitor Selection (C
OUT
)
A. See section on output capacitors in application
information section.
B. From the quick design component selection table
shown in Figure 2, locate the 5V output voltage section.
In the load current column, choose the load current line
that is closest to the current needed in your application,
for this example, use the 3A line. In the maximum input
voltage column, select the line that covers the input
voltage needed in your application, in this example, use
the 15V line. Continuing on this line are recommended
inductors and capacitors that will provide the best
overall performance.
The capacitor list contains both through hole electrolytic
and surface mount tantalum capacitors from four
different capacitor manufacturers. It is recommended
that both the manufacturers and the manufacturer’s
series that are listed in the table be used.
In this example aluminum electrolytic capacitors from
several different manufacturers are available with the
range of ESR numbers needed.
330 µF 35V Panasonic HFQ Series
330 µF 35V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least 7.5V
or more is needed. But even a low ESR, switching grade,
220 µF 10V aluminum electrolytic capacitor would exhibit
approximately 225 mΩof ESR (see the curve in Figure 17
for the ESR vs voltage rating). This amount of ESR would
result in relatively high output ripple voltage. To reduce
the ripple to 1% of the output voltage, or less, a capacitor
with a higher value or with a higher voltage rating (lower
ESR) should be selected. A 16V or 25V capacitor will
reduce the ripple voltage by approximately half.
LM2599
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LM2599 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3
times greater than the maximum load current. Also, if the
power supply design must withstand a continuous
output short, the diode should have a current rating
equal to the maximum current limit of the LM2599. The
most stressful condition for this diode is an overload or
shorted output condition.
B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time)
and must be located close to the LM2599 using short
leads and short printed circuit traces. Because of their
fast switching speed and low forward voltage drop,
Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in
low output voltage applications. Ultra-fast recovery, or
High-Efficiency rectifiers also provide good results.
Ultra-fast recovery diodes typically have reverse
recovery times of 50 ns or less. Rectifiers such as the
IN5400 series are much too slow and should not be used.
3. Catch Diode Selection (D1)
A. Refer to the table shown in Figure 11. In this example,
a 5A, 20V, 1N5823 Schottky diode will provide the best
performance, and will not be overstressed even for a
shorted output.
4. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground to prevent
large voltage transients from appearing at the input. In
addition, the RMS current rating of the input capacitor
should be selected to be at least
1
⁄
2
the DC load current.
The capacitor manufacturers data sheet must be checked
to assure that this current rating is not exceeded. The
curve shown in Figure 16 shows typical RMS current
ratings for several different aluminum electrolytic
capacitor values.
This capacitor should be located close to the IC using
short leads and the voltage rating should be
approximately 1.5 times the maximum input voltage.
If solid tantalum input capacitors are used, it is
recomended that they be surge current tested by the
manufacturer.
Use caution when using ceramic capacitors for input
bypassing, because it may cause severe ringing at the
V
IN
pin.
For additional information, see section on input
capacitors in Application Information section.
4. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the
input voltage rating and the RMS current rating. With a
nominal input voltage of 12V, an aluminum electrolytic
capacitor with a voltage rating greater than 18V (1.5 x V
IN
)
would be needed. The next higher capacitor voltage
rating is 25V.
The RMS current rating requirement for the input
capacitor in a buck regulator is approximately
1
⁄
2
the DC
load current. In this example, with a 3A load, a capacitor
with a RMS current rating of at least 1.5A is needed. The
curves shown in Figure 16 can be used to select an
appropriate input capacitor. From the curves, locate the
35V line and note which capacitor values have RMS
current ratings greater than 1.5A. A 680 µF, 35V capacitor
could be used.
For a through hole design, a 680 µF/35V electrolytic
capacitor (Panasonic HFQ series or Nichicon PL series
or equivalent) would be adequate. other types or other
manufacturers capacitors can be used provided the RMS
ripple current ratings are adequate.
For surface mount designs, solid tantalum capacitors are
recommended. The TPS series available from AVX, and
the 593D series from Sprague are both surge current
tested.
LM2599
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LM2599 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
Given:
V
OUT
= Regulated Output Voltage
V
IN
(max) = Maximum Input Voltage
I
LOAD
(max) = Maximum Load Current
F = Switching Frequency (Fixed at a nominal 150 kHz).
Given:
V
OUT
= 20V
V
IN
(max) = 28V
I
LOAD
(max) = 3A
F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R
1
and R
2
,as
shown in Figure 1)
Use the following formula to select the appropriate resistor
values.
Select a value for R
1
between 240Ωand 1.5 kΩ. The lower
resistor values minimize noise pickup in the sensitive feed-
back pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)
1. Programming Output Voltage (Selecting R
1
and R
2
,as
shown in Figure 1)
Select R
1
to be 1 kΩ, 1%. Solve for R
2
.
R
2
= 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
R
2
= 15.4 kΩ.
Conditions Inductor Output Capacitor
Through Hole Electrolytic Surface Mount Tantalum
Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague
Voltage Current Voltage (µH) (#) HFQ Series PL Series Series 595D Series
(V) (A) (V) (µF/V) (µF/V) (µF/V) (µF/V)
3.3
3
5 22 L41 470/25 560/16 330/6.3 390/6.3
7 22 L41 560/35 560/35 330/6.3 390/6.3
10 22 L41 680/35 680/35 330/6.3 390/6.3
40 33 L40 560/35 470/35 330/6.3 390/6.3
6 22 L33 470/25 470/35 330/6.3 390/6.3
2 10 33 L32 330/35 330/35 330/6.3 390/6.3
40 47 L39 330/35 270/50 220/10 330/10
5
3
8 22 L41 470/25 560/16 220/10 330/10
10 22 L41 560/25 560/25 220/10 330/10
15 33 L40 330/35 330/35 220/10 330/10
40 47 L39 330/35 270/35 220/10 330/10
9 22 L33 470/25 560/16 220/10 330/10
2 20 68 L38 180/35 180/35 100/10 270/10
40 68 L38 180/35 180/35 100/10 270/10
12
3
15 22 L41 470/25 470/25 100/16 180/16
18 33 L40 330/25 330/25 100/16 180/16
30 68 L44 180/25 180/25 100/16 120/20
40 68 L44 180/35 180/35 100/16 120/20
15 33 L32 330/25 330/25 100/16 180/16
2 20 68 L38 180/25 180/25 100/16 120/20
40 150 L42 82/25 82/25 68/20 68/25
FIGURE 2. LM2599 Fixed Voltage Quick Design Component Selection Table
LM2599
www.national.com 14
LM2599 Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
2. Inductor Selection (L1)
A. Calculate the inductor Volt •microsecond constant
E•T(V•µs), from the following formula:
where V
SAT
= internal switch saturation voltage = 1.16V and
V
D
= diode forward voltage drop = 0.5V
B. Use the E •T value from the previous formula and match it
with the E •T number on the vertical axis of the Inductor Value
Selection Guide shown in Figure 7.
C. on the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E •T value
and the Maximum Load Current value. Each region is identi-
fied by an inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufacturer’s
part numbers listed in Figure 8.
2. Inductor Selection (L1)
A. Calculate the inductor Volt •microsecond constant
(E •T),
B. E•T = 34.2 (V •µs)
C. I
LOAD
(max) = 3A
D. From the inductor value selection guide shown in Figure 7,
the inductance region intersected by the 34 (V •µs) horizontal
line and the 3A vertical line is 47 µH, and the inductor code is
L39.
E. From the table in Figure 8, locate line L39, and select an
inductor part number from the list of manufacturers part num-
bers.
3. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 820 µF provide the
best results. This capacitor should be located close to the IC
using short capacitor leads and short copper traces. Do not
use capacitors larger than 820 µF. For additional
information, see section on output capacitors in
application information section.
B. To simplify the capacitor selection procedure, refer to the
quick design table shown in Figure 3. This table contains
different output voltages, and lists various output capacitors
that will provide the best design solutions.
C. The capacitor voltage rating should be at least 1.5 times
greater than the output voltage, and often much higher
voltage ratings are needed to satisfy the low ESR
requirements needed for low output ripple voltage.
3. Output Capacitor SeIection (C
OUT
)
A. See section on C
OUT
in Application Information
section.
B. From the quick design table shown in Figure 3,
locate the output voltage column. From that column,
locate the output voltage closest to the output voltage
in your application. In this example, select the 24V line.
Under the output capacitor section, select a capacitor
from the list of through hole electrolytic or surface
mount tantalum types from four different capacitor
manufacturers. It is recommended that both the
manufacturers and the manufacturers series that are
listed in the table be used.
In this example, through hole aluminum electrolytic
capacitors from several different manufacturers are
available.
220/35 Panasonic HFQ Series
150/35 Nichicon PL Series
C. For a 20V output, a capacitor rating of at least 30V or
more is needed. In this example, either a 35V or 50V
capacitor would work. A 50V rating was chosen because
it has a lower ESR which provides a lower output ripple
voltage.
Other manufacturers or other types of capacitors may
also be used, provided the capacitor specifications
(especially the 100 kHz ESR) closely match the types
listed in the table. Refer to the capacitor manufacturers
data sheet for this information.
LM2599
www.national.com15
LM2599 Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
4. Feedforward Capacitor (C
FF
)(See Figure 1)
For output voltages greater than approximately 10V, an
additional capacitor is required. The compensation capacitor
is typically between 100 pF and 33 nF, and is wired in parallel
with the output voltage setting resistor, R
2
. It provides
additional stability for high output voltages, low input-output
voltages, and/or very low ESR output capacitors, such as
solid tantalum capacitors.
This capacitor type can be ceramic, plastic, silver mica, etc.
(Because of the unstable characteristics of ceramic capacitors
made with Z5U material, they are not recommended.)
4. Feedforward Capacitor (C
FF
)
The table shown in Figure 3 contains feed forward
capacitor values for various output voltages. In this
example, a 560 pF capacitor is needed.
5. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3
times greater than the maximum load current. Also, if the
power supply design must withstand a continuous
output short, the diode should have a current rating
equal to the maximum current limit of the LM2599. The
most stressful condition for this diode is an overload or
shorted output condition.
B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time)
and must be located close to the LM2599 using short
leads and short printed circuit traces. Because of their
fast switching speed and low forward voltage drop,
Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in
low output voltage applications. Ultra-fast recovery, or
High-Efficiency rectifiers are also a good choice, but
some types with an abrupt turn-off characteristic may
cause instability or EMl problems. Ultra-fast recovery
diodes typically have reverse recovery times of 50 ns or
less. Rectifiers such as the 1N4001 series are much too
slow and should not be used.
5. Catch Diode Selection (D1)
A. Refer to the table shown in Figure 11. Schottky
diodes provide the best performance, and in this
example a 3A, 40V, 1N5825 Schottky diode would be a
good choice. The 3A diode rating is more than adequate
and will not be overstressed even for a shorted output.
LM2599
www.national.com 16
LM2599 Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
6. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground to prevent
large voltage transients from appearing at the input. In
addition, the RMS current rating of the input capacitor
should be selected to be at least
1
⁄
2
the DC load current.
The capacitor manufacturers data sheet must be
checked to assure that this current rating is not
exceeded. The curve shown in Figure 16 shows typical
RMS current ratings for several different aluminum
electrolytic capacitor values.
This capacitor should be located close to the IC using
short leads and the voltage rating should be
approximately 1.5 times the maximum input voltage.
If solid tantalum input capacitors are used, it is
recomended that they be surge current tested by the
manufacturer.
Use caution when using a high dielectric constant
ceramic capacitor for input bypassing, because it may
cause severe ringing at the V
IN
pin.
For additional information, see section on input
capacitor in application information section.
6. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the
input voltage rating and the RMS current rating. With a
nominal input voltage of 28V, an aluminum electrolytic
aluminum electrolytic capacitor with a voltage rating
greater than 42V (1.5 x V
IN
) would be needed. Since the
the next higher capacitor voltage rating is 50V, a 50V
capacitor should be used. The capacitor voltage rating
of (1.5 x V
IN
) is a conservative guideline, and can be
modified somewhat if desired.
The RMS current rating requirement for the input
capacitor of a buck regulator is approximately
1
⁄
2
the DC
load current. In this example, with a 3A load, a capacitor
with a RMS current rating of at least 1.5A is needed.
The curves shown in Figure 16 can be used to select an
appropriate input capacitor. From the curves, locate the
50V line and note which capacitor values have RMS
current ratings greater than 1.5A. Either a 470 µF or
680 µF, 50V capacitor could be used.
For a through hole design, a 680 µF/50V electrolytic
capacitor (Panasonic HFQ series or Nichicon PL series
or equivalent) would be adequate. Other types or other
manufacturers capacitors can be used provided the
RMS ripple current ratings are adequate.
For surface mount designs, solid tantalum capacitors
can be used, but caution must be exercised with regard
to the capacitor sure current rating (see Application
Information or input capacitors in this data sheet). The
TPS series available from AVX, and the 593D series
from Sprague are both surge current tested.
To further simplify the buck regulator design procedure,
National Semiconductor is making available computer
design software to be used with the Simple Switcher
line ot switching regulators. Switchers Made Simple
(version 4.2.1 or later) is available on a 3
1
⁄
2
" diskette for
IBM compatible computers.
Output
Voltage
(V)
Through Hole Output Capacitor Surface Mount Output Capacitor
Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward
HFQ Series Series Capacitor Series 595D Series Capacitor
(µF/V) (µF/V) (µF/V) (µF/V)
2820/35 820/35 33 nF 330/6.3 470/4 33 nF
4560/35 470/35 10 nF 330/6.3 390/6.3 10 nF
6470/25 470/25 3.3 nF 220/10 330/10 3.3 nF
9330/25 330/25 1.5 nF 100/16 180/16 1.5 nF
12 330/25 330/25 1 nF 100/16 180/16 1 nF
15 220/35 220/35 680 pF 68/20 120/20 680 pF
24 220/35 150/35 560 pF 33/25 33/25 220 pF
28 100/50 100/50 390 pF 10/35 15/50 220 pF
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
LM2599
www.national.com17
LM2599 Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES
(For Continuous Mode Operation)
01258226
FIGURE 4. LM2599-3.3
01258227
FIGURE 5. LM2599-5.0
01258228
FIGURE 6. LM2599-12
01258229
FIGURE 7. LM2599-ADJ
LM2599
www.national.com 18
LM2599 Series Buck Regulator Design Procedure (Continued)
Inductance
(µH)
Current
(A)
Schott Renco Pulse Engineering Coilcraft
Through Surface Through Surface Through Surface Surface
Hole Mount Hole Mount Hole Mount Mount
L15 22 0.99 67148350 67148460 RL-1284-22-43 RL1500-22 PE-53815 PE-53815-S DO3308-223
L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DO3316-683
L22 47 1.17 67144080 67144460 RL-5471-6 — PE-53822 PE-53822-S DO3316-473
L23 33 1.40 67144090 67144470 RL-5471-7 — PE-53823 PE-53823-S DO3316-333
L24 22 1.70 67148370 67148480 RL-1283-22-43 — PE-53824 PE-53825-S DO3316-223
L25 15 2.1 67148380 67148490 RL-1283-15-43 — PE-53825 PE-53824-S DO3316-153
L26 330 0.80 67144100 67144480 RL-5471-1 — PE-53826 PE-53826-S DOS022P-334
L27 220 1.00 67144110 67144490 RL-5471-2 — PE-53827 PE-53827-S DOS022P-224
L28 150 1.20 67144120 67144500 RL-5471-3 — PE-53828 PE-53828-S DOS022P-154
L29 100 1.47 67144130 67144510 RL-5471-4 — PE-53829 PE-53829-S DOS022P-104
L30 68 1.78 67144140 67144520 RL-5471-5 — PE-53830 PE-53830-S DOS022P-683
L31 47 2.2 67144150 67144530 RL-5471-6 — PE-53831 PE-53831-S DOS022P-473
L32 33 2.5 67144160 67144540 RL-5471-7 — PE-53932 PE-53932-S DOS022P-333
L33 22 3.1 67148390 67148500 RL-1283-22-43 — PE-53933 PE-53933-S DOS022P-223
L34 15 3.4 67148400 67148790 RL-1283-15-43 — PE-53934 PE-53934-S DOS022P-153
L35 220 1.70 67144170 — RL-5473-1 — PE-53935 PE-53935-S —
L36 150 2.1 67144180 — RL-5473-4 — PE-54036 PE-54036-S —
L37 100 2.5 67144190 — RL-5472-1 — PE-54037 PE-54037-S —
L38 68 3.1 67144200 — RL-5472-2 — PE-54038 PE-54038-S —
L39 47 3.5 67144210 — RL-5472-3 — PE-54039 PE-54039-S —
L40 33 3.5 67144220 67148290 RL-5472-4 — PE-54040 PE-54040-S —
L41 22 3.5 67144230 67148300 RL-5472-5 — PE-54041 PE-54041-S —
L42 150 2.7 67148410 — RL-5473-4 — PE-54042 PE-54042-S —
L43 100 3.4 67144240 — RL-5473-2 — PE-54043 — —
L44 68 3.4 67144250 — RL-5473-3 — PE-54044 — —
FIGURE 8. Inductor Manufacturers Part Numbers
Coilcraft Inc. Phone (800) 322-2645
FAX (708) 639-1469
Coilcraft Inc., Europe Phone +11 1236 730 595
FAX +44 1236 730 627
Pulse Engineering Inc. Phone (619) 674-8100
FAX (619) 674-8262
Pulse Engineering Inc., Phone +353 93 24 107
Europe FAX +353 93 24 459
Renco Electronics Inc. Phone (800) 645-5828
FAX (516) 586-5562
Schott Corp. Phone (612) 475-1173
FAX (612) 475-1786
FIGURE 9. Inductor Manufacturers Phone Numbers
LM2599
www.national.com19
LM2599 Series Buck Regulator Design Procedure (Continued)
Nichicon Corp. Phone (708) 843-7500
FAX (708) 843-2798
Panasonic Phone (714) 373-7857
FAX (714) 373-7102
AVX Corp. Phone (803) 448-9411
FAX (803) 448-1943
Sprague/Vishay Phone (207) 324-4140
FAX (207) 324-7223
FIGURE 10. Capacitor Manufacturers Phone Numbers
VR
3 Amp Diodes 4 to 6 Amp Diodes
Surface Mount Through Hole Surface Mount Through Hole
Schottky Ultra Fast Schottky Ultra Fast Schottky Ultra Fast Schottky Ultra Fast
Recovery Recovery Recovery Recovery
20V
All of 1N5820 All of All of SR502 All of
SK32 these SR302 these these 1N5823 these
diodes MBR320 diodes diodes SB520 diodes
30V
30WQ03 are rated 1N5821 are rated are rated are rated
SK33 to at MBR330 to at 50WQ03 to at SR503 to at
least 31DQ03 least least 1N5824 least
40V
50V. 1N5822 50V. 50V. SB530 50V.
SK34 SR304 50WQ04 SR504
MBRS340 MBR340 1N5825
30WQ04 MURS320 31DQ04 MUR320 MURS620 SB540 MUR620
50V
or
more
SK35 30WF10 SR305 50WF10 HER601
MBRS360 MBR350 50WQ05 SB550
30WQ05 31DQ05 50SQ080
FIGURE 11. Diode Selection Table
LM2599
www.national.com 20
Application Information
PIN FUNCTIONS
+V
IN
(Pin 1) — This is the positive input supply for the IC
switching regulator. A suitable input bypass capacitor must
be present at this pin to minimize voltage transients and to
supply the switching currents needed by the regulator.
Ground (Pin 4) — Circuit ground.
Output (Pin 2) — Internal switch. The voltage at this pin
switches between approximately (+V
IN
−V
SAT
) and approxi-
mately −0.5V, with a duty cycle of V
OUT
/V
IN
. To minimize
coupling to sensitive circuitry, the PC board copper area
connected to this pin should be kept to a minimum.
Feedback (Pin 6) — Senses the regulated output voltage to
complete the feedback loop.
Shutdown /Soft-start (Pin 7) — This dual function pin pro-
vides the following features: (a) Allows the switching regula-
tor circuit to be shut down using logic level signals thus
dropping the total input supply current to approximately
80 µA. (b) Adding a capacitor to this pin provides a soft-start
feature which minimizes startup current and provides a con-
trolled ramp up of the output voltage.
Error Flag (Pin 3) — Open collector output that provides a
low signal (flag transistor ON) when the regulated output
voltage drops more than 5% from the nominal output volt-
age. On start up, Error Flag is low until V
OUT
reaches 95% of
the nominal output voltage and a delay time determined by
the Delay pin capacitor. This signal can be used as a reset to
a microprocessor on power-up.
Delay (Pin 5) — At power-up, this pin can be used to provide
a time delay between the time the regulated output voltage
reaches 95% of the nominal output voltage, and the time the
error flag output goes high.
Special Note If any of the above three features (Shutdown
/Soft-start, Error Flag, or Delay) are not used, the respective
pins should be left open.
EXTERNAL COMPONENTS
SOFT-START CAPACITOR
C
SS
— A capacitor on this pin provides the regulator with a
Soft-start feature (slow start-up). When the DC input voltage
is first applied to the regulator, or when the Shutdown /Soft-
start pin is allowed to go high, a constant current (approxi-
mately 5 µA begins charging this capacitor). As the capacitor
voltage rises, the regulator goes through four operating re-
gions (See the bottom curve in Figure 13).
1. Regulator in Shutdown. When the SD /SS pin voltage is
between 0V and 1.3V, the regulator is in shutdown, the
output voltage is zero, and the IC quiescent current is ap-
proximately 85 µA.
2. Regulator ON, but the output voltage is zero. With the
SD /SS pin voltage between approximately 1.3V and 1.8V,
the internal regulator circuitry is operating, the quiescent
current rises to approximately 5 mA, but the output voltage is
still zero. Also, as the 1.3V threshold is exceeded, the Soft-
start capacitor charging current decreases from 5 µA down
to approximately 1.6 µA. This decreases the slope of capaci-
tor voltage ramp.
Block Diagram
01258230
FIGURE 12.
LM2599
www.national.com21
Application Information (Continued)
3. Soft-start Region. When the SD /SS pin voltage is be-
tween 1.8V and 2.8V (@25˚C), the regulator is in a Soft-start
condition. The switch (Pin 2) duty cycle initially starts out
very low, with narrow pulses and gradually get wider as the
capacitor SD /SS pin ramps up towards 2.8V. As the duty
cycle increases, the output voltage also increases at a con-
trolled ramp up. See the center curve in Figure 13. The input
supply current requirement also starts out at a low level for
the narrow pulses and ramp up in a controlled manner. This
is a very useful feature in some switcher topologies that
require large startup currents (such as the inverting configu-
ration) which can load down the input power supply.
Note: The lower curve shown in Figure 13 shows the Soft-start region from
0% to 100%. This is not the duty cycle percentage, but the output
voltage percentage. Also, the Soft-start voltage range has a negative
temperature coefficient associated with it. See the Soft-start curve in
the electrical characteristics section.
4. Normal operation. Above 2.8V, the circuit operates as a
standard Pulse Width Modulated switching regulator. The
capacitor will continue to charge up until it reaches the
internal clamp voltage of approximately 7V. If this pin is
driven from a voltage source, the current must be limited to
about 1 mA.
If the part is operated with an input voltage at or below the
internal soft-start clamp voltage of approximately 7V, the
voltage on the SD/SS pin tracks the input voltage and can be
disturbed by a step in the voltage. To maintain proper func-
tion under these conditions, it is strongly recommended that
the SD/SS pin be clamped externally between the 3V maxi-
mum soft-start threshold and the 4.5V minimum input volt-
age. Figure 15 is an example of an external 3.7V (approx.)
clamp that prevents a line-step related glitch but does not
interfere with the soft-start behavior of the device.
01258231
FIGURE 13. Soft-start, Delay, Error, Output
LM2599
www.national.com 22
Application Information (Continued)
DELAY CAPACITOR
C
DELAY
— Provides delay for the error flag output. See the
upper curve in Figure 13, and also refer to timing diagrams in
Figure 14. A capacitor on this pin provides a time delay
between the time the regulated output voltage (when it is
increasing in value) reaches 95% of the nominal output
voltage, and the time the error flag output goes high. A 3 µA
constant current from the delay pin charges the delay ca-
pacitor resulting in a voltage ramp. When this voltage
reaches a threshold of approximately 1.3V, the open collec-
tor error flag output (or power OK) goes high. This signal can
be used to indicate that the regulated output has reached the
correct voltage and has stabilized.
If, for any reason, the regulated output voltage drops by 5%
or more, the error output flag (Pin 3) immediately goes low
(internal transistor turns on). The delay capacitor provides
very little delay if the regulated output is dropping out of
regulation. The delay time for an output that is decreasing is
approximately a 1000 times less than the delay for the rising
output. For a 0.1 µF delay capacitor, the delay time would be
approximately 50 ms when the output is rising and passes
through the 95% threshold, but the delay for the output
dropping would only be approximately 50 µs.
R
Pull Up
— The error flag output, (or power OK) is the col-
lector of a NPN transistor, with the emitter internally
grounded. To use the error flag, a pullup resistor to a positive
voltage is needed. The error flag transistor is rated up to a
maximum of 45V and can sink approximately 3 mA. If the
error flag is not used, it can be left open.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
C
FF
- A Feedforward Capacitor C
FF
, shown across R2 in
Figure 1 is used when the output voltage is greater than 10V
01258232
FIGURE 14. Timing Diagram for 5V Output
01258265
FIGURE 15. External 3.7V Soft-Start Clamp
LM2599
www.national.com23
Application Information (Continued)
or when C
OUT
has a very low ESR. This capacitor adds lead
compensation to the feedback loop and increases the phase
margin for better loop stability. For C
FF
selection, see the
design procedure section.
If the output ripple is large (>5% of the nominal output
voltage), this ripple can be coupled to the feedback pin
through the feedforward capacitor and cause the error com-
parator to trigger the error flag. In this situation, adding a
resistor, R
FF
, in series with the feedforward capacitor, ap-
proximately 3 times R1, will attenuate the ripple voltage at
the feedback pin.
INPUT CAPACITOR
C
IN
— A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground pin. It must be
located near the regulator using short leads. This capacitor
prevents large voltage transients from appearing at the in-
put, and provides the instantaneous current needed each
time the switch turns on.
The important parameters for the Input capacitor are the
voltage rating and the RMS current rating. Because of the
relatively high RMS currents flowing in a buck regulator’s
input capacitor, this capacitor should be chosen for its RMS
current rating rather than its capacitance or voltage ratings,
although the capacitance value and voltage rating are di-
rectly related to the RMS current rating.
The RMS current rating of a capacitor could be viewed as a
capacitor’s power rating. The RMS current flowing through
the capacitors internal ESR produces power which causes
the internal temperature of the capacitor to rise. The RMS
current rating of a capacitor is determined by the amount of
current required to raise the internal temperature approxi-
mately 10˚C above an ambient temperature of 105˚C. The
ability of the capacitor to dissipate this heat to the surround-
ing air will determine the amount of current the capacitor can
safely sustain. Capacitors that are physically large and have
a large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage elec-
trolytic capacitor will be physically larger than a lower voltage
capacitor, and thus be able to dissipate more heat to the
surrounding air, and therefore will have a higher RMS cur-
rent rating.
The consequences of operating an electrolytic capacitor
above the RMS current rating is a shortened operating life.
The higher temperature speeds up the evaporation of the
capacitor’s electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manu-
facturers data sheet for maximum allowable RMS ripple
current. For a maximum ambient temperature of 40˚C, a
general guideline would be to select a capacitor with a ripple
current rating of approximately 50% of the DC load current.
For ambient temperatures up to 70˚C, a current rating of
75% of the DC load current would be a good choice for a
conservative design. The capacitor voltage rating must be at
least 1.25 times greater than the maximum input voltage,
and often a much higher voltage capacitor is needed to
satisfy the RMS current requirements.
A graph shown in Figure 16 shows the relationship between
an electrolytic capacitor value, its voltage rating, and the
RMS current it is rated for. These curves were obtained from
the Nichicon “PL” series of low ESR, high reliability electro-
lytic capacitors designed for switching regulator applications.
Other capacitor manufacturers offer similar types of capaci-
tors, but always check the capacitor data sheet.
“Standard” electrolytic capacitors typically have much higher
ESR numbers, lower RMS current ratings and typically have
a shorter operating lifetime.
Because of their small size and excellent performance, sur-
face mount solid tantalum capacitors are often used for input
bypassing, but several precautions must be observed. A
small percentage of solid tantalum capacitors can short if the
inrush current rating is exceeded. This can happen at turn on
when the input voltage is suddenly applied, and of course,
higher input voltages produce higher inrush currents. Sev-
eral capacitor manufacturers do a 100% surge current test-
ing on their products to minimize this potential problem. If
high turn on currents are expected, it may be necessary to
limit this current by adding either some resistance or induc-
tance before the tantalum capacitor, or select a higher volt-
age capacitor. As with aluminum electrolytic capacitors, the
RMS ripple current rating must be sized to the load current.
OUTPUT CAPACITOR
C
OUT
— An output capacitor is required to filter the output
and provide regulator loop stability. Low impedance or low
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When select-
ing an output capacitor, the important capacitor parameters
01258233
FIGURE 16. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
01258234
FIGURE 17. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
LM2599
www.national.com 24
Application Information (Continued)
are; the 100 kHz Equivalent Series Resistance (ESR), the
RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
important parameter.
The output capacitor requires an ESR value that has an
upper and lower limit. For low output ripple voltage, a low
ESR value is needed. This value is determined by the maxi-
mum allowable output ripple voltage, typically 1% to 2% of
the output voltage. But if the selected capacitor’s ESR is
extremely low, there is a possibility of an unstable feedback
loop, resulting in an oscillation at the output. Using the
capacitors listed in the tables, or similar types, will provide
design solutions under all conditions.
If very low output ripple voltage (less than 15 mV) is re-
quired, refer to the section on Output Voltage Ripple and
Transients for a post ripple filter.
An aluminum electrolytic capacitor’s ESR value is related to
the capacitance value and its voltage rating. In most cases,
higher voltage electrolytic capacitors have lower ESR values
(see Figure 17). Often, capacitors with much higher voltage
ratings may be needed to provide the low ESR values re-
quired for low output ripple voltage.
The output capacitor for many different switcher designs
often can be satisfied with only three or four different capaci-
tor values and several different voltage ratings. See the
quick design component selection tables in Figure 2 and 3
for typical capacitor values, voltage ratings, and manufactur-
ers capacitor types.
Electrolytic capacitors are not recommended for tempera-
tures below −25˚C. The ESR rises dramatically at cold tem-
peratures and typically rises 3X @−25˚C and as much as
10X at −40˚C. See curve shown in Figure 18.
Solid tantalum capacitors have a much better ESR spec for
cold temperatures and are recommended for temperatures
below −25˚C.
CATCH DIODE
Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
a fast diode and must be located close to the LM2599 using
short leads and short printed circuit traces.
Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turnoff charac-
teristic may cause instability or EMI problems. Ultra-fast
recovery diodes typically have reverse recovery times of 50
ns or less. Rectifiers such as the IN5400 series are much too
slow and should not be used.
INDUCTOR SELECTION
All switching regulators have two basic modes of operation;
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulators
performance and requirements. Most switcher designs will
operate in the discontinuous mode when the load current is
low.
The LM2599 (or any of the Simple Switcher family) can be
used for both continuous or discontinuous modes of opera-
tion.
In many cases the preferred mode of operation is the con-
tinuous mode. It offers greater output power, lower peak
switch, inductor and diode currents, and can have lower
output ripple voltage. But it does require larger inductor
values to keep the inductor current flowing continuously,
especially at low output load currents and/or high input volt-
ages.
To simplify the inductor selection process, an inductor selec-
tion guide (nomograph) was designed (see Figure 4 through
7). This guide assumes that the regulator is operating in the
continuous mode, and selects an inductor that will allow a
peak-to-peak inductor ripple current to be a certain percent-
age of the maximum design load current. This peak-to-peak
inductor ripple current percentage is not fixed, but is allowed
to change as different design load currents are selected.
(See Figure 19).
01258235
FIGURE 18. Capacitor ESR Change vs Temperature
LM2599
www.national.com25
Application Information (Continued)
By allowing the percentage of inductor ripple current to
increase for low load currents, the inductor value and size
can be kept relatively low.
When operating in the continuous mode, the inductor current
waveform ranges from a triangular to a sawtooth type of
waveform (depending on the input voltage), with the average
value of this current waveform equal to the DC output load
current.
Inductors are available in different styles such as pot core,
toroid, E-core, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least
expensive, the bobbin, rod or stick core, consists of wire
wound on a ferrite bobbin. This type of construction makes
for an inexpensive inductor, but since the magnetic flux is not
completely contained within the core, it generates more
Electro-Magnetic Interference (EMl). This magnetic flux can
induce voltages into nearby printed circuit traces, thus caus-
ing problems with both the switching regulator operation and
nearby sensitive circuitry, and can give incorrect scope read-
ings because of induced voltages in the scope probe. Also
see section on Open Core Inductors.
When multiple switching regulators are located on the same
PC board, open core magnetics can cause interference
between two or more of the regulator circuits, especially at
high currents. A torroid or E-core inductor (closed magnetic
structure) should be used in these situations.
The inductors listed in the selection chart include ferrite
E-core construction for Schott, ferrite bobbin core for Renco
and Coilcraft, and powdered iron toroid for Pulse Engineer-
ing.
Exceeding an inductor’s maximum current rating may cause
the inductor to overheat because of the copper wire losses,
or the core may saturate. If the inductor begins to saturate,
the inductance decreases rapidly and the inductor begins to
look mainly resistive (the DC resistance of the winding). This
can cause the switch current to rise very rapidly and force
the switch into a cycle-by-cycle current limit, thus reducing
the DC output load current. This can also result in overheat-
ing of the inductor and/or the LM2599. Different inductor
types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.
The inductor manufacturer’s data sheets include current and
energy limits to avoid inductor saturation.
DISCONTINUOUS MODE OPERATION
The selection guide chooses inductor values suitable for
continuous mode operation, but for low current applications
and/or high input voltages, a discontinuous mode design
may be a better choice. It would use an inductor that would
be physically smaller, and would need only one half to one
third the inductance value needed for a continuous mode
design. The peak switch and inductor currents will be higher
in a discontinuous design, but at these low load currents (1A
and below), the maximum switch current will still be less than
the switch current limit.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design. The
output pin (switch) waveform can have some damped sinu-
soidal ringing present. (See Typical Performance Character-
istics photo titled Discontinuous Mode Switching Wave-
forms) This ringing is normal for discontinuous operation,
and is not caused by feedback loop instabilities. In discon-
tinuous operation, there is a period of time where neither the
switch or the diode are conducting, and the inductor current
has dropped to zero. During this time, a small amount of
energy can circulate between the inductor and the switch/
diode parasitic capacitance causing this characteristic ring-
ing. Normally this ringing is not a problem, unless the ampli-
tude becomes great enough to exceed the input voltage, and
even then, there is very little energy present to cause dam-
age.
Different inductor types and/or core materials produce differ-
ent amounts of this characteristic ringing. Ferrite core induc-
tors have very little core loss and therefore produce the most
ringing. The higher core loss of powdered iron inductors
produce less ringing. If desired, a series RC could be placed
in parallel with the inductor to dampen the ringing. The
computer aided design software Switchers Made Simple
(version 4.3) will provide all component values for continu-
ous and discontinuous modes of operation.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply operating in
the continuous mode will contain a sawtooth ripple voltage at
the switcher frequency, and may also contain short voltage
spikes at the peaks of the sawtooth waveform.
The output ripple voltage is a function of the inductor saw-
tooth ripple current and the ESR of the output capacitor. A
typical output ripple voltage can range from approximately
01258236
FIGURE 19. (∆I
IND
) Peak-to-Peak Inductor Ripple
Current (as a Percentage of the
Load Current) vs Load Current
01258237
FIGURE 20. Post Ripple Filter Waveform
LM2599
www.national.com 26
Application Information (Continued)
0.5% to 3% of the output voltage. To obtain low ripple
voltage, the ESR of the output capacitor must be low, how-
ever, caution must be exercised when using extremely low
ESR capacitors because they can affect the loop stability,
resulting in oscillation problems. If very low output ripple
voltage is needed (less than 20 mV), a post ripple filter is
recommended (See Figure 1). The inductance required is
typically between 1 µH and 5 µH, with low DC resistance, to
maintain good load regulation. A low ESR output filter ca-
pacitor is also required to assure good dynamic load re-
sponse and ripple reduction. The ESR of this capacitor may
be as low as desired, because it is out of the regulator
feedback loop. The photo shown in Figure 20 shows a
typical output ripple voltage, with and without a post ripple
filter.
When observing output ripple with a scope, it is essential
that a short, low inductance scope probe ground connection
be used. Most scope probe manufacturers provide a special
probe terminator which is soldered onto the regulator board,
preferable at the output capacitor. This provides a very short
scope ground thus eliminating the problems associated with
the 3 inch ground lead normally provided with the probe, and
provides a much cleaner and more accurate picture of the
ripple voltage waveform.
The voltage spikes are caused by the fast switching action of
the output switch, the diode, and the parasitic inductance of
the output filter capacitor, and its associated wiring. To mini-
mize these voltage spikes, the output capacitor should be
designed for switching regulator applications, and the lead
lengths must be kept very short. Wiring inductance, stray
capacitance, as well as the scope probe used to evaluate
these transients, all contribute to the amplitude of these
spikes.
When a switching regulator is operating in the continuous
mode, the inductor current waveform ranges from a triangu-
lar to a sawtooth type of waveform (depending on the input
voltage). For a given input and output voltage, the peak-to-
peak amplitude of this inductor current waveform remains
constant. As the load current increases or decreases, the
entire sawtooth current waveform also rises and falls. The
average value (or the center) of this current waveform is
equal to the DC load current.
If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will smoothly change from a continuous to a discon-
tinuous mode of operation. Most switcher designs (irregard-
less how large the inductor value is) will be forced to run
discontinuous if the output is lightly loaded. This is a per-
fectly acceptable mode of operation.
In a switching regulator design, knowing the value of the
peak-to-peak inductor ripple current (∆I
IND
) can be useful for
determining a number of other circuit parameters. Param-
eters such as, peak inductor or peak switch current, mini-
mum load current before the circuit becomes discontinuous,
output ripple voltage and output capacitor ESR can all be
calculated from the peak-to-peak ∆I
IND
. When the inductor
nomographs shown in Figure 4 through 7are used to select
an inductor value, the peak-to-peak inductor ripple current
can immediately be determined. The curve shown in Figure
21 shows the range of (∆I
IND
) that can be expected for
different load currents. The curve also shows how the peak-
to-peak inductor ripple current (∆I
IND
) changes as you go
from the lower border to the upper border (for a given load
current) within an inductance region. The upper border rep-
resents a higher input voltage, while the lower border repre-
sents a lower input voltage (see Inductor Selection Guides).
These curves are only correct for continuous mode opera-
tion, and only if the inductor selection guides are used to
select the inductor value
Consider the following example:
V
OUT
= 5V, maximum load current of 2.5A
V
IN
= 12V, nominal, varying between 10V and 16V.
The selection guide in Figure 5 shows that the vertical line
for a 2.5A load current, and the horizontal line for the 12V
input voltage intersect approximately midway between the
upper and lower borders of the 33 µH inductance region. A
33 µH inductor will allow a peak-to-peak inductor current
(∆I
IND
) to flow that will be a percentage of the maximum load
current. Referring to Figure 21, follow the 2.5A line approxi-
mately midway into the inductance region, and read the
peak-to-peak inductor ripple current (∆I
IND
) on the left hand
axis (approximately 620 mA p-p).
As the input voltage increases to 16V, it approaches the
upper border of the inductance region, and the inductor
ripple current increases. Referring to the curve in Figure 21,
it can be seen that for a load current of 2.5A, the peak-to-
peak inductor ripple current (∆I
IND
) is 620 mA with 12V in,
and can range from 740 mA at the upper border (16V in) to
500 mA at the lower border (10V in).
Once the ∆I
IND
value is known, the following formulas can be
used to calculate additional information about the switching
regulator circuit.
1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage = (∆I
IND
)x(ESR of C
OUT
)
= 0.62Ax0.1Ω=62 mV p-p
01258249
FIGURE 21. Peak-to-Peak Inductor
Ripple Current vs Load Current
LM2599
www.national.com27
Application Information (Continued)
4.
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or
unstable operation is from an open core inductor. Ferrite
bobbin or stick inductors have magnetic lines of flux flowing
through the air from one end of the bobbin to the other end.
These magnetic lines of flux will induce a voltage into any
wire or PC board copper trace that comes within the induc-
tor’s magnetic field. The strength of the magnetic field, the
orientation and location of the PC copper trace to the mag-
netic field, and the distance between the copper trace and
the inductor, determine the amount of voltage generated in
the copper trace. Another way of looking at this inductive
coupling is to consider the PC board copper trace as one
turn of a transformer (secondary) with the inductor winding
as the primary. Many millivolts can be generated in a copper
trace located near an open core inductor which can cause
stability problems or high output ripple voltage problems.
If unstable operation is seen, and an open core inductor is
used, it’s possible that the location of the inductor with
respect to other PC traces may be the problem. To deter-
mine if this is the problem, temporarily raise the inductor
away from the board by several inches and then check
circuit operation. If the circuit now operates correctly, then
the magnetic flux from the open core inductor is causing the
problem. Substituting a closed core inductor such as a tor-
roid or E-core will correct the problem, or re-arranging the
PC layout may be necessary. Magnetic flux cutting the IC
device ground trace, feedback trace, or the positive or nega-
tive traces of the output capacitor should be minimized.
Sometimes, locating a trace directly beneath a bobbin in-
ductor will provide good results, provided it is exactly in the
center of the inductor (because the induced voltages cancel
themselves out), but if it is off center one direction or the
other, then problems could arise. If flux problems are
present, even the direction of the inductor winding can make
a difference in some circuits.
This discussion on open core inductors is not to frighten the
user, but to alert the user on what kind of problems to watch
out for when using them. Open core bobbin or “stick” induc-
tors are an inexpensive, simple way of making a compact
efficient inductor, and they are used by the millions in many
different applications.
THERMAL CONSIDERATIONS
The LM2599 is available in two packages, a 7-pin TO-220
(T) and a 7-pin surface mount TO-263 (S).
The TO-220 package needs a heat sink under most condi-
tions. The size of the heat sink depends on the input voltage,
the output voltage, the load current and the ambient tem-
perature. The curves in Figure 22 show the LM2599T junc-
tion temperature rises above ambient temperature for a 3A
load and different input and output voltages. The data for
these curves was taken with the LM2599T (TO-220 pack-
age) operating as a buck switching regulator in an ambient
temperature of 25˚C (still air). These temperature rise num-
bers are all approximate and there are many factors that can
affect these temperatures. Higher ambient temperatures re-
quire more heat sinking.
The TO-263 surface mount package tab is designed to be
soldered to the copper on a printed circuit board. The copper
and the board are the heat sink for this package and the
other heat producing components, such as the catch diode
and inductor. The pc board copper area that the package is
soldered to should be at least 0.4 in
2
, and ideally should
have 2 or more square inches of 2 oz. (0.0028 in) copper.
Additional copper area improves the thermal characteristics,
but with copper areas greater than approximately 6 in
2
, only
small improvements in heat dissipation are realized. If fur-
ther thermal improvements are needed, double sided, mul-
tilayer pc-board with large copper areas and/or airflow are
recommended.
The curves shown in Figure 23 show the LM2599S (TO-263
package) junction temperature rise above ambient tempera-
ture with a 2A load for various input and output voltages. This
data was taken with the circuit operating as a buck switching
regulator with all components mounted on a pc board to
simulate the junction temperature under actual operating
conditions. This curve can be used for a quick check for the
approximate junction temperature for various conditions, but
be aware that there are many factors that can affect the
junction temperature. When load currents higher than 2A are
used, double sided or multilayer pc-boards with large copper
areas and/or airflow might be needed, especially for high
ambient temperatures and high output voltages.
01258238
Circuit Data for Temperature Rise Curve TO-220
Package (T)
Capacitors Through hole electrolytic
Inductor Through hole Renco
Diode Through hole, 5A 40V, Schottky
PC board 3 square inches single sided 2 oz. copper
(0.0028")
FIGURE 22. Junction Temperature Rise, TO-220
LM2599
www.national.com 28
Application Information (Continued)
01258239
For the best thermal performance, wide copper traces and
generous amounts of printed circuit board copper should be
used in the board layout. (One exception to this is the output
(switch) pin, which should not have large areas of copper.)
Large areas of copper provide the best transfer of heat
(lower thermal resistance) to the surrounding air, and moving
air lowers the thermal resistance even further.
Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors
that will affect these numbers. Some of these factors include
board size, shape, thickness, position, location, and even
board temperature. Other factors are, trace width, total
printed circuit copper area, copper thickness, single- or
double-sided, multilayer board and the amount of solder on
the board. The effectiveness of the pc board to dissipate
heat also depends on the size, quantity and spacing of other
components on the board, as well as whether the surround-
ing air is still or moving. Furthermore, some of these com-
ponents such as the catch diode will add heat to the pc
board and the heat can vary as the input voltage changes.
For the inductor, depending on the physical size, type of core
material and the DC resistance, it could either act as a heat
sink taking heat away from the board, or it could add heat to
the board.
SHUTDOWN /SOFT-START
The circuit shown in Figure 26 is a standard buck regulator
with 20V in, 12V out, 1A load, and using a 0.068 µF Soft-start
capacitor. The photo in Figure 24 Figure 25 show the effects
of Soft-start on the output voltage, the input current, with,
and without a Soft-start capacitor. The reduced input current
required at startup is very evident when comparing the two
photos. The Soft-start feature reduces the startup current
from 2.6A down to 650 mA, and delays and slows down the
output voltage rise time.
This reduction in start up current is useful in situations where
the input power source is limited in the amount of current it
can deliver. In some applications Soft-start can be used to
replace undervoltage lockout or delayed startup functions.
If a very slow output voltage ramp is desired, the Soft-start
capacitor can be made much larger. Many seconds or even
minutes are possible.
If only the shutdown feature is needed, the Soft-start capaci-
tor can be eliminated.
Circuit Data for Temperature Rise Curve TO-263
Package (S)
Capacitors Surface mount tantalum, molded “D” size
Inductor Surface mount, Pulse engineering, 68 µH
Diode Surface mount, 5A 40V, Schottky
PC board 9 square inches single sided 2 oz. copper
(0.0028")
FIGURE 23. Junction Temperature Rise, TO-263
01258240
FIGURE 24. Output Voltage, Input Current,
at Start-Up, WITH Soft-start
01258241
FIGURE 25. Output Voltage, Input Current,
at Start-Up, WITHOUT Soft-start
LM2599
www.national.com29
Application Information (Continued)
lNVERTING REGULATOR
The circuit in Figure 27 converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulator’s ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
This example uses the LM2599-5 to generate a −5V output,
but other output voltages are possible by selecting other
output voltage versions, including the adjustable version.
Since this regulator topology can produce an output voltage
that is either greater than or less than the input voltage, the
maximum output current greatly depends on both the input
and output voltage. The curve shown in Figure 28 provides a
guide as to the amount of output load current possible for the
different input and output voltage conditions.
The maximum voltage appearing across the regulator is the
absolute sum of the input and output voltage, and this must
be limited to a maximum of 40V. In this example, when
converting +20V to −5V, the regulator would see 25V be-
tween the input pin and ground pin. The LM2599 has a
maximum input voltage rating of 40V.
01258242
FIGURE 26. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
01258243
FIGURE 27. Inverting −5V Regulator With Shutdown and Soft-start
LM2599
www.national.com 30
Application Information (Continued)
An additional diode is required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the C
IN
capacitor to the output, under light
or no load conditions. Also, this diode isolation changes the
topology to closely resemble a buck configuration thus pro-
viding good closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a IN5400 diode could be
used.
Because of differences in the operation of the inverting
regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 33 µH,
3.5A inductor is the best choice. Capacitor selection can also
be narrowed down to just a few values. Using the values
shown in Figure 27 will provide good results in the majority of
inverting designs.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2599 current limit
(approximately 4.5A) are needed for 2 ms or more, until the
output reaches its nominal output voltage. The actual time
depends on the output voltage and the size of the output
capacitor. Input power sources that are current limited or
sources that can not deliver these currents without getting
loaded down, may not work correctly. Because of the rela-
tively high startup currents required by the inverting topology,
the Soft-start feature shown in Figure 27 is recommended.
Also shown in Figure 27 are several shutdown methods for
the inverting configuration. With the inverting configuration,
some level shifting is required, because the ground pin of the
regulator is no longer at ground, but is now at the negative
output voltage. The shutdown methods shown accept
ground referenced shutdown signals.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. Figure
29 contains a undervoltage lockout circuit for a buck configu-
ration, while Figure 30 and 30 are for the inverting types
(only the circuitry pertaining to the undervoltage lockout is
shown). Figure 29 uses a zener diode to establish the
threshold voltage when the switcher begins operating. When
the input voltage is less than the zener voltage, resistors R1
and R2 hold the Shutdown /Soft-start pin low, keeping the
regulator in the shutdown mode. As the input voltage ex-
ceeds the zener voltage, the zener conducts, pulling the
Shutdown /Soft-start pin high, allowing the regulator to begin
switching. The threshold voltage for the undervoltage lockout
feature is approximately 1.5V greater than the zener voltage.
Figure 30 and 30 apply the same feature to an inverting
circuit. Figure 30 features a constant threshold voltage for
turn on and turn off (zener voltage plus approximately one
volt). If hysteresis is needed, the circuit in Figure 31 has a
turn ON voltage which is different than the turn OFF voltage.
The amount of hysteresis is approximately equal to the value
of the output voltage. Since the SD /SS pin has an internal
7V zener clamp, R2 is needed to limit the current into this pin
to approximately 1 mA when Q1 is on.
Inverting Regulator
01258244
FIGURE 28. Maximum Load Current for
Inverting Regulator Circuit
01258245
FIGURE 29. Undervoltage Lockout for a Buck
Regulator
01258247
FIGURE 30. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
01258246
FIGURE 31. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
LM2599
www.national.com31
Application Information (Continued)
NEGATIVE VOLTAGE CHARGE PUMP
Occasionally a low current negative voltage is needed for
biasing parts of a circuit. A simple method of generating a
negative voltage using a charge pump technique and the
switching waveform present at the OUT pin, is shown in
Figure 32. This unregulated negative voltage is approxi-
mately equal to the positive input voltage (minus a few volts),
and can supply up to a 600 mA of output current. There is a
requirement however, that there be a minimum load of 1.2A
on the regulated positive output for the charge pump to work
correctly. Also, resistor R1 is required to limit the charging
current of C1 to some value less than the LM2599 current
limit (typically 4.5A).
This method of generating a negative output voltage without
an additional inductor can be used with other members of
the Simple Switcher Family, using either the buck or boost
topology.
01258248
FIGURE 32. Charge Pump for Generating a
Low Current, Negative Output Voltage
LM2599
www.national.com 32
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED
01258251
CIN: — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”
COUT: — 330 µF, 35V, Aluminum Electrolytic Panasonic, “HFQ Series”
D1: — 5A, 40V Schottky Rectifier, 1N5825
L1: — 47 µH, L39, Renco, Through Hole
RPULL UP: — 10k
CDELAY: — 0.1 µF
CSD/SS: — 0.1 µF
Thermalloy Heat Sink #7020
LM2599
www.national.com33
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED
01258252
CIN: — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”
COUT: — 220 µF, 35V Aluminum Electrolytic Panasonic, “HFQ Series”
D1: — 5A, 40V Schottky Rectifier, 1N5825
L1: — 47 µH, L39, Renco, Through Hole
R1:—1kΩ,1%
R2: — Use formula in Design Procedure
CFF: — See Figure 4.
RFF: — See Application Information Section (CFF Section)
RPULL UP: — 10k
CDELAY: — 0.1 µF
CSD/SS: — 0.1 µF
Thermalloy Heat Sink #7020
FIGURE 33. PC Board Layout
LM2599
www.national.com 34
Physical Dimensions inches (millimeters)
unless otherwise noted
7-Lead TO-220 Bent and Staggered Package
Order Number LM2599T-3.3, LM2599T-5.0,
LM2599T-12 or LM2599T-ADJ
NS Package Number TA07B
7-Lead TO-263 Bent and Formed Package
Order Number LM2599S-3.3, LM2599S-5.0, LM2599S-12 or LM2599S-ADJ
NS Package Number TS7B
LM2599
www.national.com35
Notes
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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LM2599 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator, with
Features
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