LM2578A,LM3578A
LM2578A/LM3578A Switching Regulator
Literature Number: SNVS767D
LM2578A/LM3578A
Switching Regulator
General Description
The LM2578A is a switching regulator which can easily be
set up for such DC-to-DC voltage conversion circuits as the
buck, boost, and inverting configurations. The LM2578A fea-
tures a unique comparator input stage which not only has
separate pins for both the inverting and non-inverting inputs,
but also provides an internal 1.0V reference to each input,
thereby simplifying circuit design and p.c. board layout. The
output can switch up to 750 mA and has output pins for its
collector and emitter to promote design flexibility. An external
current limit terminal may be referenced to either the ground
or the V
in
terminal, depending upon the application. In addi-
tion, the LM2578A has an on board oscillator, which sets the
switching frequency with a single external capacitor from <1
Hz to 100 kHz (typical).
The LM2578A is an improved version of the LM2578, offer-
ing higher maximum ratings for the total supply voltage and
output transistor emitter and collector voltages.
Features
nInverting and non-inverting feedback inputs
n1.0V reference at inputs
nOperates from supply voltages of 2V to 40V
nOutput current up to 750 mA, saturation less than 0.9V
nCurrent limit and thermal shut down
nDuty cycle up to 90%
Applications
nSwitching regulators in buck, boost, inverting, and
single-ended transformer configurations
nMotor speed control
nLamp flasher
Connection Diagram and Ordering Information
Dual-In-Line Package
00871129
Order Number LM3578AM, LM2578AN or LM3578AN
See NS Package Number M08A or N08E
February 2005
LM2578A/LM3578A Switching Regulator
© 2005 National Semiconductor Corporation DS008711 www.national.com
Functional Diagram
00871101
LM2578A/LM3578A
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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.
Total Supply Voltage 50V
Collector Output to Ground −0.3V to +50V
Emitter Output to Ground (Note 2) −1V to +50V
Power Dissipation (Note 3) Internally limited
Output Current 750 mA
Storage Temperature −65˚C to +150˚C
Lead Temperature
(soldering, 10 seconds) 260˚C
Maximum Junction Temperature 150˚C
ESD Tolerance (Note 4) 2 kV
Operating Ratings
Ambient Temperature Range
LM2578A −40˚C ≤T
A
≤+85˚C
LM3578A 0˚C ≤T
A
≤+70˚C
Junction Temperature Range
LM2578A −40˚C ≤T
J
≤+125˚C
LM3578A 0˚C ≤T
J
≤+125˚C
Electrical Characteristics
These specifications apply for 2V ≤V
IN
≤40V (2.2V ≤V
IN
≤40V for T
J
≤−25˚C), timing capacitor C
T
= 3900 pF, and 25% ≤
duty cycle ≤75%, unless otherwise specified. Values in standard typeface are for T
J
= 25˚C; values in boldface type apply for
operation over the specified operating junction temperature range.
LM2578A/
Symbol Parameter Conditions Typical LM3578A Units
(Note 5) Limit (Note 6)
OSCILLATOR
f
OSC
Frequency 20 kHz
24 kHz (max)
16 kHz (min)
∆f
OSC
/∆T Frequency Drift with Temperature −0.13 %/˚C
Amplitude 550 mV
p-p
REFERENCE/COMPARATOR (Note 7)
V
R
Input Reference I
1
=I
2
= 0 mA and 1.0 V
Voltage I
1
=I
2
=1mA±1% (Note 8) 1.050/1.070 V (max)
0.950/0.930 V (min)
∆V
R
/∆V
IN
Input Reference Voltage Line
Regulation
I
1
=I
2
= 0 mA and 0.003 %/V
I
1
=I
2
=1mA±1% (Note 8) 0.01/0.02 %/V (max)
I
INV
Inverting Input Current I
1
=I
2
= 0 mA, duty cycle = 25% 0.5 µA
Level Shift Accuracy Level Shift Current=1mA 1.0 %
10/13 % (max)
∆V
R
/∆t Input Reference Voltage Long Term
Stability
100 ppm/1000h
OUTPUT
V
C
(sat) Collector Saturation Voltage I
C
= 750 mA pulsed, Emitter
grounded
0.7 V
0.90/1.2 V (max)
V
E
(sat) Emitter Saturation Voltage I
O
= 80 mA pulsed, 1.4 V
V
IN
=V
C
= 40V 1.7/2.0 V (max)
I
CES
Collector Leakage Current V
IN
=V
CE
= 40V, Emitter grounded,
Output OFF
0.1 µA
200/250 µA (max)
BV
CEO(SUS)
Collector-Emitter Sustaining Voltage I
SUST
= 0.2A (pulsed), V
IN
= 0 60 V
50 V (min)
CURRENT LIMIT
V
CL
Sense Voltage Shutdown Level Referred to V
IN
or Ground 110 mV
(Note 9) 80 mV (min)
160 mV (max)
LM2578A/LM3578A
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Electrical Characteristics (Continued)
These specifications apply for 2V ≤V
IN
≤40V (2.2V ≤V
IN
≤40V for T
J
≤−25˚C), timing capacitor C
T
= 3900 pF, and 25% ≤
duty cycle ≤75%, unless otherwise specified. Values in standard typeface are for T
J
= 25˚C; values in boldface type apply for
operation over the specified operating junction temperature range.
LM2578A/
Symbol Parameter Conditions Typical LM3578A Units
(Note 5) Limit (Note 6)
CURRENT LIMIT
∆V
CL
/∆T Sense Voltage Temperature Drift 0.3 %/˚C
I
CL
Sense Bias Current Referred to V
IN
4.0 µA
Referred to ground 0.4 µA
DEVICE POWER CONSUMPTION
I
S
Supply Current Output OFF, V
E
= 0V 2.0 mA
3.5/4.0 mA (max)
Output ON, I
C
= 750 mA pulsed, 14 mA
V
E
=0V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its rated operating conditions.
Note 2: For TJ≥100˚C, the Emitter pin voltage should not be driven more than 0.6V below ground (see Application Information).
Note 3: At elevated temperatures, devices must be derated based on package thermal resistance. The device in the 8-pin DIP must be derated at 95˚C/W, junction
to ambient. The device in the surface-mount package must be derated at 150˚C/W, junction-to-ambient.
Note 4: Human body model, 1.5 kΩin series with 100 pF.
Note 5: Typical values are for TJ= 25˚C and represent the most likely parametric norm.
Note 6: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). Room temperature limits are 100% production
tested. Limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
AOQL.
Note 7: Input terminals are protected from accidental shorts to ground but if external voltages higher than the reference voltage are applied, excessive current will
flow and should be limited to less than 5 mA.
Note 8: I1and I2are the external sink currents at the inputs (refer to Test Circuit).
Note 9: Connection of a 10 kΩresistor from pin 1 to pin 4 will drive the duty cycle to its maximum, typically 90%. Applying the minimum Current Limit Sense Voltage
to pin 7 will not reduce the duty cycle to less than 50%. Applying the maximum Current Limit Sense Voltage to pin 7 is certain to reduce the duty cycle below 50%.
Increasing this voltage by 15 mV may be required to reduce the duty cycle to 0%, when the Collector output swing is 40V or greater (see Ground-Referred Current
Limit Sense Voltage typical curve).
Typical Performance Characteristics
Oscillator Frequency Change
with Temperature Oscillator Voltage Swing
00871132 00871133
LM2578A/LM3578A
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Typical Performance Characteristics (Continued)
Input Reference Voltage
Drift with Temperature
Collector Saturation Voltage
(Sinking Current,
Emitter Grounded)
00871134 00871135
Emitter Saturation Voltage
(Sourcing Current,
Collector at V
in
)
Ground Referred
Current Limit Sense Voltage
00871136 00871137
Current Limit Sense Voltage
Drift with Temperature
Current Limit Response Time
for Various Over Drives
00871138 00871139
LM2578A/LM3578A
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Typical Performance Characteristics (Continued)
Current Limit Sense Voltage
vs Supply Voltage Supply Current
00871140 00871141
Supply Current
Collector Current with
Emitter Output Below Ground
00871142 00871143
Test Circuit*
Parameter tests can be made using the test circuit shown.
Select the desired V
in
, collector voltage and duty cycle with
adjustable power supplies. A digital volt meter with an input
resistance greater than 100 MΩshould be used to measure
the following:
Input Reference Voltage to Ground; S1 in either position.
Level Shift Accuracy (%) = (T
P3
(V)/1V) x 100%; S1 at I
1
=I
2
=1mA
Input Current (mA) = (1V − T
p3
(V))/1 MΩ:S1atI
1
=I
2
=
0 mA.
Oscillator parameters can be measured at T
p4
using a fre-
quency counter or an oscilloscope.
The Current Limit Sense Voltage is measured by connecting
an adjustable 0-to-1V floating power supply in series with the
current limit terminal and referring it to either the ground or
the V
in
terminal. Set the duty cycle to 90% and monitor test
point T
P5
while adjusting the floating power supply voltage
until the LM2578A’s duty cycle just reaches 0%. This voltage
is the Current Limit Sense Voltage.
The Supply Current should be measured with the duty cycle
at 0% and S1 in the I
1
=I
2
= 0 mA position.
*LM2578A specifications are measured using automated
test equipment. This circuit is provided for the customer’s
convenience when checking parameters. Due to possible
variations in testing conditions, the measured values from
these testing procedures may not match those of the factory.
LM2578A/LM3578A
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Test Circuit* (Continued)
00871103
Op amp supplies are ±15V
DVM input resistance >100 MΩ
*LM2578 max duty cycle is 90%
Definition of Terms
Input Reference Voltage: The voltage (referred to ground)
that must be applied to either the inverting or non-inverting
input to cause the regulator switch to change state (ON or
OFF).
Input Reference Current: The current that must be drawn
from either the inverting or non-inverting input to cause the
regulator switch to change state (ON or OFF).
Input Level Shift Accuracy: This specification determines
the output voltage tolerance of a regulator whose output
control depends on drawing equal currents from the inverting
and non-inverting inputs (see the Inverting Regulator of Fig-
ure 21, and the RS-232 Line Driver Power Supply of Figure
23).
Level Shift Accuracy is tested by using two equal-value
resistors to draw current from the inverting and non-inverting
input terminals, then measuring the percentage difference in
the voltages across the resistors that produces a controlled
duty cycle at the switch output.
Collector Saturation Voltage: With the inverting input ter-
minal grounded thru a 10 kΩresistor and the output transis-
tor’s emitter connected to ground, the Collector Saturation-
Voltage is the collector-to-emitter voltage for a given
collector current.
Emitter Saturation Voltage: With the inverting input termi-
nal grounded thru a 10 kΩresistor and the output transistor’s
collector connected to V
in
, the Emitter Saturation Voltage is
the collector-to-emitter voltage for a given emitter current.
Collector Emitter Sustaining Voltage: The collector-
emitter breakdown voltage of the output transistor, mea-
sured at a specified current.
Current Limit Sense Voltage: The voltage at the Current
Limit pin, referred to either the supply or the ground terminal,
which (via logic circuitry) will cause the output transistor to
turn OFF and resets cycle-by-cycle at the oscillator fre-
quency.
Current Limit Sense Current: The bias current for the
Current Limit terminal with the applied voltage equal to the
Current Limit Sense Voltage.
Supply Current: The IC power supply current, excluding the
current drawn through the output transistor, with the oscilla-
tor operating.
Functional Description
The LM2578A is a pulse-width modulator designed for use
as a switching regulator controller. It may also be used in
other applications which require controlled pulse-width volt-
age drive.
A control signal, usually representing output voltage, fed into
the LM2578A’s comparator is compared with an internally-
generated reference. The resulting error signal and the os-
cillator’s output are fed to a logic network which determines
when the output transistor will be turned ON or OFF. The
following is a brief description of the subsections of the
LM2578A.
COMPARATOR INPUT STAGE
The LM2578A’s comparator input stage is unique in that both
the inverting and non-inverting inputs are available to the
user, and both contain a 1.0V reference. This is accom-
plished as follows: A 1.0V reference is fed into a modified
voltage follower circuit (see FUNCTIONAL DIAGRAM).
When both input pins are open, no current flows through R1
LM2578A/LM3578A
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Functional Description (Continued)
and R2. Thus, both inputs to the comparator will have the
potential of the 1.0V reference, V
A
. When one input, for
example the non-inverting input, is pulled ∆V away from V
A
,
a current of ∆V/R1 will flow through R1. This same current
flows through R2, and the comparator sees a total voltage of
2∆V between its inputs. The high gain of the system, through
feedback, will correct for this imbalance and return both
inputs to the 1.0V level.
This unusual comparator input stage increases circuit flex-
ibility, while minimizing the total number of external compo-
nents required for a voltage regulator system. The inverting
switching regulator configuration, for example, can be set up
without having to use an external op amp for feedback
polarity reversal (see TYPICAL APPLICATIONS).
OSCILLATOR
The LM2578A provides an on-board oscillator which can be
adjusted up to 100 kHz. Its frequency is set by a single
external capacitor, C
1
, as shown in Figure 1, and follows the
equation
f
OSC
= 8x10
−5
/C
1
The oscillator provides a blanking pulse to limit maximum
duty cycle to 90%, and a reset pulse to the internal circuitry.
OUTPUT TRANSISTOR
The output transistor is capable of delivering up to 750 mA
with a saturation voltage of less than 0.9V. (see Collector
Saturation Voltage and Emitter Saturation Voltage curves).
The emitter must not be pulled more than 1V below ground
(this limit is 0.6V for T
J
≥100˚C). Because of this limit, an
external transistor must be used to develop negative output
voltages (see the Inverting Regulator Typical Application).
Other configurations may need protection against violation
of this limit (see the Emitter Output section of the Applica-
tions Information).
CURRENT LIMIT
The LM2578A’s current limit may be referenced to either the
ground or the V
in
pins, and operates on a cycle-by-cycle
basis.
The current limit section consists of two comparators: one
with its non-inverting input referenced to a voltage 110 mV
below V
in
, the other with its inverting input referenced
110 mV above ground (see FUNCTIONAL DIAGRAM). The
current limit is activated whenever the current limit terminal
is pulled 110 mV away from either V
in
or ground.
Applications Information
CURRENT LIMIT
As mentioned in the functional description, the current limit
terminal may be referenced to either the V
in
or the ground
terminal. Resistor R3 converts the current to be sensed into
a voltage for current limit detection.
CURRENT LIMIT TRANSIENT SUPPRESSION
When noise spikes and switching transients interfere with
proper current limit operation, R1 and C1 act together as a
low pass filter to control the current limit circuitry’s response
time.
Because the sense current of the current limit terminal varies
according to where it is referenced, R1 should be less
than 2 kΩwhen referenced to ground, and less than 100Ω
when referenced to V
in
.
00871104
FIGURE 1. Value of Timing Capacitor vs
Oscillator Frequency
00871115
FIGURE 2. Current Limit, Ground Referred
00871116
FIGURE 3. Current Limit, V
in
Referred
LM2578A/LM3578A
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Applications Information (Continued)
C.L. SENSE VOLTAGE MULTIPLICATION
When a larger sense resistor value is desired, the voltage
divider network, consisting of R1 and R2, may be used. This
effectively multiplies the sense voltage by (1 + R1/R2). Also,
R1 can be replaced by a diode to increase current limit
sense voltage to about 800 mV (diode V
f
+ 110 mV).
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout is accomplished with few external
components. When V
in
becomes lower than the zener
breakdown voltage, the output transistor is turned off. This
occurs because diode D1 will then become forward biased,
allowing resistor R3 to sink a greater current from the non-
inverting input than is sunk by the parallel combination of R1
and R2 at the inverting terminal. R3 should be one-fifth of the
value of R1 and R2 in parallel.
MAXIMUM DUTY CYCLE LIMITING
The maximum duty cycle can be externally limited by adjust-
ing the charge to discharge ratio of the oscillator capacitor
with a single external resistor. Typical values are 50 µA for
the charge current, 450 µA for the discharge current, and a
voltage swing from 200 mV to 750 mV. Therefore, R1 is
selected for the desired charging and discharging slopes
and C1 is readjusted to set the oscillator frequency.
00871117
FIGURE 4. Current Limit Transient Suppressor,
Ground Referred
00871118
FIGURE 5. Current Limit Transient Suppressor,
V
in
Referred
00871119
FIGURE 6. Current Limit Sense Voltage Multiplication,
Ground Referred
00871120
FIGURE 7. Current Limit Sense Voltage Multiplication,
V
in
Referred
00871122
FIGURE 8. Under-Voltage Lockout
LM2578A/LM3578A
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Applications Information (Continued)
DUTY CYCLE ADJUSTMENT
When manual or mechanical selection of the output transis-
tor’s duty cycle is needed, the cirucit shown below may be
used. The output will turn on with the beginning of each
oscillator cycle and turn off when the current sunk by R2 and
R3 from the non-inverting terminal becomes greater than the
current sunk from the inverting terminal.
With the resistor values as shown, R3 can be used to adjust
the duty cycle from 0% to 90%.
When the sum of R2 and R3 is twice the value of R1, the
duty cycle will be about 50%. C1 may be a large electrolytic
capacitor to lower the oscillator frequency below 1 Hz.
REMOTE SHUTDOWN
The LM2578A may be remotely shutdown by sinking a
greater current from the non-inverting input than from the
inverting input. This may be accomplished by selecting re-
sistor R3 to be approximately one-half the value of R1 and
R2 in parallel.
EMITTER OUTPUT
When the LM2578A output transistor is in the OFF state, if
the Emitter output swings below the ground pin voltage, the
output transistor will turn ON because its base is clamped
near ground. The Collector Current with Emitter Output Be-
low Ground curve shows the amount of Collector current
drawn in this mode, vs temperature and Emitter voltage.
When the Collector-Emitter voltage is high, this current will
cause high power dissipation in the output transistor and
should be avoided.
This situation can occur in the high-current high-voltage
buck application if the Emitter output is used and the catch
diode’s forward voltage drop is greater than 0.6V. A fast-
recovery diode can be added in series with the Emitter
output to counter the forward voltage drop of the catch diode
(see Figure 2). For better efficiency of a high output current
buck regulator, an external PNP transistor should be used as
shown in Figure 16.
SYNCHRONIZING DEVICES
When several devices are to be operated at once, their
oscillators may be synchronized by the application of an
external signal. This drive signal should be a pulse waveform
with a minimum pulse width of 2 µs. and an amplitude from
00871121
FIGURE 9. Maximum Duty Cycle Limiting
00871123
FIGURE 10. Duty Cycle Adjustment
00871124
FIGURE 11. Shutdown Occurs when V
L
is High
00871130
FIGURE 12. D1 Prevents Output Transistor from
Improperly Turning ON due to D2’s Forward Voltage
LM2578A/LM3578A
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Applications Information (Continued)
1.5V to 2.0V. The signal source must be capable of 1.)
driving capacitive loads and 2.) delivering up to 500 µA for
each LM2578A.
Capacitors C1 thru CN are to be selected for a 20% slower
frequency than the synchronization frequency.
Typical Applications
The LM2578A may be operated in either the continuous or
the discontinuous conduction mode. The following applica-
tions (except for the Buck-Boost Regulator) are designed for
continuous conduction operation. That is, the inductor cur-
rent is not allowed to fall to zero. This mode of operation has
higher efficiency and lower EMI characteristics than the dis-
continuous mode.
BUCK REGULATOR
The buck configuration is used to step an input voltage down
to a lower level. Transistor Q1 in Figure 14 chops the input
DC voltage into a squarewave. This squarewave is then
converted back into a DC voltage of lower magnitude by the
low pass filter consisting of L1 and C1. The duty cycle, D, of
the squarewave relates the output voltage to the input volt-
age by the following equation:
V
out
=DxV
in
=V
in
x(t
on
)/(t
on
+t
off
).
Figure 15 is a 15V to 5V buck regulator with an output
current, I
o
, of 350 mA. The circuit becomes discontinuous at
20% of I
o(max)
, has 10 mV of output voltage ripple, an effi-
ciency of 75%, a load regulation of 30 mV (70 mA to 350 mA)
and a line regulation of 10 mV (12 ≤V
in
≤18V).
Component values are selected as follows:
R1=(V
o
− 1) x R2 where R2 = 10 kΩ
R3 = V/I
sw(max)
R3 = 0.15Ω
where:
V is the current limit sense voltage, 0.11V
I
sw(max)
is the maximum allowable current thru the output
transistor.
L1 is the inductor and may be found from the inductance
calculation chart (Figure 16) as follows:
Given V
in
= 15V
V
o
=5V
I
o(max)
= 350 mA
f
OSC
=50kHz
Discontinuous at 20% of I
o(max)
.
Note that since the circuit will become discontinuous at 20%
of I
o(max)
, the load current must not be allowed to fall below
70 mA.
00871125
FIGURE 13. Synchronizing Devices
00871105
FIGURE 14. Basic Buck Regulator
LM2578A/LM3578A
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Typical Applications (Continued)
00871106
V
in
= 15V R3 = 0.15Ω
V
o
= 5V C1 = 1820 pF
V
ripple
=10mV C2=220µF
I
o
= 350 mA C3 = 20 pF
f
osc
= 50 kHz L1 = 470 µH
R1=40kΩD1 = 1N5818
R2=10kΩ
FIGURE 15. Buck or Step-Down Regulator
LM2578A/LM3578A
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Typical Applications (Continued)
00871131
FIGURE 16. DC/DC Inductance Calculator
LM2578A/LM3578A
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Typical Applications (Continued)
Step 1: Calculate the maximum DC current through the
inductor, I
L(max)
. The necessary equations are indicated at
the top of the chart and show that I
L(max)
=I
o(max)
for the
buck configuration. Thus, I
L(max)
= 350 mA.
Step 2: Calculate the inductor Volts-sec product, E-T
op
,
according to the equations given from the chart. For the
Buck:
E-T
op
=(V
in
−V
o
)(V
o
/V
in
) (1000/f
osc
)
=(15 − 5) (5/15) (1000/50)
= 66V-µs.
with the oscillator frequency, f
osc
, expressed in kHz.
Step 3: Using the graph with axis labeled “Discontinuous At
%I
OUT
” and “I
L(max, DC)
” find the point where the desired
maximum inductor current, I
L(max, DC)
intercepts the desired
discontinuity percentage.
In this example, the point of interest is where the 0.35A line
intersects with the 20% line. This is nearly the midpoint of the
horizontal axis.
Step 4: This last step is merely the translation of the point
found in Step 3 to the graph directly below it. This is accom-
plished by moving straight down the page to the point which
intercepts the desired E-T
op
. For this example, E-T
op
is
66V-µs and the desired inductor value is 470 µH. Since this
example was for 20% discontinuity, the bottom chart could
have been used directly, as noted in step 3 of the chart
instructions.
For a full line of standard inductor values, contact Pulse
Engineering (San Diego, Calif.) regarding their PE526XX
series, or A. I. E. Magnetics (Nashville, Tenn.).
A more precise inductance value may be calculated for the
Buck, Boost and Inverting Regulators as follows:
BUCK
L=V
o
(V
in
−V
o
)/(∆I
L
V
in
f
osc
)
BOOST
L=V
in
(V
o
−V
in
)/(∆I
L
f
osc
V
o
)
INVERT
L=V
in
|V
o
|/[∆I
L
(V
in
+|V
o
|)f
osc
]
where ∆I
L
is the current ripple through the inductor. ∆I
L
is
usually chosen based on the minimum load current expected
of the circuit. For the buck regulator, since the inductor
current I
L
equals the load current I
O
,
∆I
L
=2•I
O(min)
∆I
L
= 140 mA for this circuit. ∆I
L
can also be interpreted as
∆I
L
=2•(Discontinuity Factor) •I
L
where the Discontinuity Factor is the ratio of the minimum
load current to the maximum load current. For this example,
the Discontinuity Factor is 0.2.
The remainder of the components of Figure 15 are chosen
as follows:
C1 is the timing capacitor found in Figure 1.
C2 ≥V
o
(V
in
−V
o
)/(8f
osc 2
V
in
V
ripple
L1)
where V
ripple
is the peak-to-peak output voltage ripple.
C3 is necessary for continuous operation and is generally in
the 10 pF to 30 pF range.
D1 should be a Schottky type diode, such as the 1N5818 or
1N5819.
BUCK WITH BOOSTED OUTPUT CURRENT
For applications requiring a large output current, an external
transistor may be used as shown in Figure 17. This circuit
steps a 15V supply down to 5V with 1.5A of output current.
The output ripple is 50 mV, with an efficiency of 80%, a load
regulation of 40 mV (150 mA to 1.5A), and a line regulation
of 20 mV (12V ≤V
in
≤18V).
Component values are selected as outlined for the buck
regulator with a discontinuity factor of 10%, with the addition
of R4 and R5:
R4 = 10V
BE1
B
f
/I
p
R5=(V
in
−V−V
BE1
−V
sat
)B
f
/(I
L(max, DC)
+I
R4
)
where:
V
BE1
is the V
BE
of transistor Q1.
V
sat
is the saturation voltage of the LM2578A output transis-
tor.
V is the current limit sense voltage.
B
f
is the forced current gain of transistor Q1 (B
f
= 30 for
Figure 17 ).
I
R4
=V
BE1
/R4
I
p
=I
L(max, DC)
+ 0.5∆I
L
LM2578A/LM3578A
www.national.com 14
Typical Applications (Continued)
BOOST REGULATOR
The boost regulator converts a low input voltage into a
higher output voltage. The basic configuration is shown in
Figure 18. Energy is stored in the inductor while the transis-
tor is on and then transferred with the input voltage to the
output capacitor for filtering when the transistor is off. Thus,
V
o
=V
in
+V
in
(t
on
/t
off
).
The circuit of Figure 19 converts a 5V supply into a 15V
supply with 150 mA of output current, a load regulation of
14 mV (30 mA to 140 mA), and a line regulation of 35 mV
(4.5V ≤V
in
≤8.5V).
R1=(V
o
− 1) R2 where R2 = 10 kΩ.
R3 = V/(I
L(max, DC)
+ 0.5 ∆I
L
)
where:
∆I
L
= 2(I
LOAD(min)
)(V
o
/V
in
)
∆I
L
is 200 mA in this example.
R4, C3 and C4 are necessary for continuous operation and
are typically 220 kΩ, 20 pF, and 0.0022 µF respectively.
C1 is the timing capacitor found in Figure 1.
C2 ≥I
o
(V
o
−V
in
)/(f
osc
V
o
V
ripple
).
00871108
V
in
= 15V R4 = 200Ω
V
o
=5V R5=330Ω
V
ripple
= 50 mV C1 = 1820 pF
I
o
= 1.5A C2 = 330 µF
f
osc
=50kHz C3=20pF
R1=40kΩL1 = 220 µH
R2=10kΩD1 = 1N5819
R3 = 0.05ΩQ1 = D45
FIGURE 17. Buck Converter with Boosted Output Current
00871109
FIGURE 18. Basic Boost Regulator
00871111
V
in
=5V R4=200kΩ
V
o
= 15V C1 = 1820 pF
V
ripple
=10mV C2=470µF
I
o
= 140 mA C3 = 20 pF
f
osc
= 50 kHz C4 = 0.0022 µF
R1 = 140 kΩL1 = 330 µH
R2=10kΩD1 = 1N5818
R3 = 0.15Ω
FIGURE 19. Boost or Step-Up Regulator
LM2578A/LM3578A
www.national.com15
Typical Applications (Continued)
D1 is a Schottky type diode such as a 1N5818 or 1N5819.
L1 is found as described in the buck converter section, using
the inductance chart for Figure 16 for the boost configuration
and 20% discontinuity.
INVERTING REGULATOR
Figure 20 shows the basic configuration for an inverting
regulator. The input voltage is of a positive polarity, but the
output is negative. The output may be less than, equal to, or
greater in magnitude than the input. The relationship be-
tween the magnitude of the input voltage and the output
voltage is V
o
=V
in
x(t
on
/t
off
).
Figure 21 shows an LM2578A configured as a 5V to −15V
polarity inverter with an output current of 300 mA, a load
regulation of 44 mV (60 mA to 300 mA) and a line regulation
of 50 mV (4.5V ≤V
in
≤8.5V).
R1 = (|V
o
| +1) R2 where R2 = 10 kΩ.
R3 = V/(I
L(max, DC)
+ 0.5 ∆I
L
).
R4 = 10V
BE1
B
f
/(I
L (max, DC)
+ 0.5 ∆I
L
)
where:
V, V
BE1
,V
sat
, and B
f
are defined in the “Buck Converter with
Boosted Output Current” section.
∆I
L
= 2(I
LOAD(min)
)(V
in
+|V
o
|)/V
IN
R5 is defined in the “Buck with Boosted Output Current”
section.
R6 serves the same purpose as R4 in the Boost Regulator
circuit and is typically 220 kΩ.
C1, C3 and C4 are defined in the “Boost Regulator” section.
C2 ≥I
o
|V
o
|/[f
osc
(|V
o
|+V
in
)V
ripple
]
L1 is found as outlined in the section on buck converters,
using the inductance chart of Figure 16 for the invert con-
figuration and 20% discontinuity.
BUCK-BOOST REGULATOR
The Buck-Boost Regulator, shown in Figure 22, may step a
voltage up or down, depending upon whether or not the
desired output voltage is greater or less than the input
voltage. In this case, the output voltage is 12V with an input
voltage from 9V to 15V. The circuit exhibits an efficiency of
75%, with a load regulation of 60 mV (10 mA to 100 mA) and
a line regulation of 52 mV.
R1=(V
o
− 1) R2 where R2 = 10 kΩ
R3 = V/0. 75A
R4, C1, C3 and C4 are defined in the “Boost Regulator”
section.
D1 and D2 are Schottky type diodes such as the 1N5818 or
1N5819.
where:
V
d
is the forward voltage drop of the diodes.
V
sat
is the saturation voltage of the LM2578A output transis-
tor.
V
sat1
is the saturation voltage of transistor Q1.
L1 ≥(V
in
−V
sat
−V
sat1
)(t
on
/I
p
)
00871110
FIGURE 20. Basic Inverting Regulator
00871112
V
in
=5V R4=190Ω
V
o
= −15V R5 = 82Ω
V
ripple
=5mV R6=220kΩ
I
o
= 300 mA C1 = 1820 pF
I
min
= 60 mA C2 = 1000 µF
f
osc
=50kHz C3=20pF
R1 = 160 kΩC4 = 0.0022 µF
R2=10kΩL1 = 150 µH
R3 = 0.01ΩD1 = 1N5818
FIGURE 21. Inverting Regulator
LM2578A/LM3578A
www.national.com 16
Typical Applications (Continued)
where:
RS-232 LINE DRIVER POWER SUPPLY
The power supply, shown in Figure 23, operates from an
input voltage as low as 4.2V (5V nominal), and delivers an
output of ±12V at ±40 mA with better than 70% efficiency.
The circuit provides a load regulation of ±150 mV (from 10%
to 100% of full load) and a line regulation of ±10 mV. Other
notable features include a cycle-by-cycle current limit and an
output voltage ripple of less than 40 mVp-p.
A unique feature of this circuit is its use of feedback from
both outputs. This dual feedback configuration results in a
sharing of the output voltage regulation by each output so
that neither side becomes unbalanced as in single feedback
systems. In addition, since both sides are regulated, it is not
necessary to use a linear regulator for output regulation.
The feedback resistors, R2 and R3, may be selected as
follows by assuming a value of 10 kΩfor R1;
R2=(V
o
− 1V)/45.8 µA = 240 kΩ
R3 = (|V
o
| +1V)/54.2 µA = 240 kΩ
Actually, the currents used to program the values for the
feedback resistors may vary from 40 µA to 60 µA, as long as
their sum is equal to the 100 µA necessary to establish the
1V threshold across R1. Ideally, these currents should be
equal (50 µA each) for optimal control. However, as was
done here, they may be mismatched in order to use standard
resistor values. This results in a slight mismatch of regulation
between the two outputs.
The current limit resistor, R4, is selected by dividing the
current limit threshold voltage by the maximum peak current
level in the output switch. For our purposes R4 = 110 mV/
750 mA = 0.15Ω. A value of 0.1Ωwas used.
Capacitor C1 sets the oscillator frequency and is selected
from Figure 1.
Capacitor C2 serves as a compensation capacitor for syn-
chronous operation and a value of 10 to 50 pF should be
sufficient for most applications.
00871113
9V ≤V
in
≤15V R5 = 270
V
o
= 12V C1 = 1820 pF
I
o
= 100 mA C2 = 220 µF
V
ripple
=50mV C3=20pF
f
osc
= 50 kHz C4 = 0.0022 µF
R1 = 110k L1 = 220 µH
R2 = 10k D1, D2 = 1N5819
R3 = 0.15 Q1 = D44
R4 = 220k
FIGURE 22. Buck-Boost Regulator
00871114
V
in
= 5V R4 = 0.15Ω
V
o
±12V C1 = 820 pF
I
o
=±40 mA C2 = 10 pF
f
osc
= 80 kHz C3 = 220 µF
R1=10kΩD1, D2, D3 = 1N5819
R2 = 240 kΩT1 = PE-64287
R3 = 240 kΩ
FIGURE 23. RS-232 Line Driver Power Supply
LM2578A/LM3578A
www.national.com17
Typical Applications (Continued)
A minimum value for an ideal output capacitor C3, could be
calculated asC=I
o
xt/∆V where I
o
is the load current, t is
the transistor on time (typically 0.4/f
osc
), and ∆V is the peak-
to-peak output voltage ripple. A larger output capacitor than
this theoretical value should be used since electrolytics have
poor high frequency performance. Experience has shown
that a value from 5 to 10 times the calculated value should
be used.
For good efficiency, the diodes must have a low forward
voltage drop and be fast switching. 1N5819 Schottky diodes
work well.
Transformer selection should be picked for an output tran-
sistor “on” time of 0.4/f
osc
, and a primary inductance high
enough to prevent the output transistor switch from ramping
higher than the transistor’s rating of 750 mA. Pulse Engi-
neering (San Diego, Calif.) and Renco Electronics, Inc.
(Deer Park, N.Y.) can provide further assistance in selecting
the proper transformer for a specific application need. The
transformer used in Figure 23 was a Pulse Engineering
PE-64287.
LM2578A/LM3578A
www.national.com 18
Physical Dimensions inches (millimeters)
unless otherwise noted
Plastic Surface-Mount Package (M)
Order Number LM3578AM
NS Package Number M08A
LM2578A/LM3578A
www.national.com19
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Molded Dual-In-Line Package (N)
Order Number LM2578AN or LM3578AN
NS Package Number N08E
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.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products
Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain
no ‘‘Banned Substances’’ as defined in CSP-9-111S2.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
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Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
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Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM2578A/LM3578A Switching Regulator
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