LM3670
LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
Literature Number: SNVS250D
LM3670
Miniature Step-Down DC-DC Converter for Ultra Low
Voltage Circuits
General Description
The LM3670 step-down DC-DC converter is optimized for
powering ultra-low voltage circuits from a single Li-Ion cell or
3 cell NiMH/NiCd batteries. It provides up to 350 mA load
current, over an input voltage range from 2.5V to 5.5V. There
are several different fixed voltage output options available as
well as an adjustable output voltage version (see ordering
information).
The device offers superior features and performance for
mobile phones and similar portable applications with com-
plex power management systems. Automatic intelligent
switching between PWM low-noise and PFM low-current
mode offers improved system control. During full-power op-
eration, a fixed-frequency 1 MHz (typ). PWM mode drives
loads from ∼70 mA to 350 mA max, with up to 95% efficiency.
Hysteretic PFM mode extends the battery life through reduc-
tion of the quiescent current to 15 µA (typ) during light
current loads and system standby. Internal synchronous rec-
tification provides high efficiency (90 to 95% typ. at loads
between 1 mA and 100 mA). In shutdown mode (Enable pin
pulled low) the device turns off and reduces battery con-
sumption to 0.1 µA (typ.).
The LM3670 is available in a SOT23-5 package. A high
switching frequency - 1 MHz (typ) - allows use of tiny
surface-mount components. Only three external surface-
mount components, an inductor and two ceramic capacitors,
are required.
Features
nV
OUT
= Adj (0.7V min), 1.2, 1.5, 1.6, 1.8, 1.875, 2.5,
3.3V
n2.5V ≤V
IN
≤5.5V
n15 µA typical quiescent current
n350 mA maximum load capability
n1 MHz PWM fixed switching frequency (typ.)
nAutomatic PFM/PWM mode switching
nAvailable in fixed output voltages as well as an
adjustable version
nSOT23-5 package
nLow drop out operation - 100% duty cycle mode
nInternal synchronous rectification for high efficiency
nInternal soft start
n0.1 µA typical shutdown current
nOperates from a single Li-Ion cell or 3 cell NiMH/NiCd
batteries
nOnly three tiny surface-mount external components
required (one inductor, two ceramic capacitors)
nCurrent overload protection
Applications
nMobile phones
nHandHeld
nPDAs
nPalm-top PCs
nPortable Instruments
nBattery Powered Devices
Typical Application
20075801
FIGURE 1. Fixed Output Voltage - Typical Application Circuit
January 2006
LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
© 2006 National Semiconductor Corporation DS200758 www.national.com
Typical Application (Continued)
Connection Diagram and Package Mark Information
Pin Descriptions
Pin # Name Description
1V
IN
Power supply input. Connect to the input filter capacitor (Figure 1).
2 GND Ground pin.
3 EN Enable input.
4 FB Feedback analog input. Connect to the output filter capacitor (Figure 1).
5 SW Switching node connection to the internal PFET switch and NFET synchronous
rectifier. Connect to an inductor with a saturation current rating that exceeds the
750 mA max. Switch Peak Current Limit specification.
20075830
FIGURE 2. Adjustable Output Voltage - Typical Application Circuit
SOT23-5 Package
NS Package Number MF05A
20075802
Note: The actual physical placement of the package marking will vary from part to part.
FIGURE 3. Top View
LM3670
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Ordering Information
Voltage Option
(V)
Order Number
(Level 95)
SPEC Package Marking Supplied As
(#/reel)
3.3 LM3670MF-3.3 NOPB SDEB 1000
LM3670MFX-3.3 NOPB 3000
LM3670MF-3.3 1000
LM3670MFX-3.3 3000
2.5 LM3670MF-2.5 NOPB SDDB 1000
LM3670MFX-2.5 NOPB 3000
LM3670MF-2.5 1000
LM3670MFX-2.5 3000
1.875 LM3670MF-1.875 NOPB SEFB 1000
LM3670MFX-1.875 NOPB 3000
LM3670MF-1.875 1000
LM3670MFX-1.875 3000
1.8 LM3670MF-1.8 NOPB SDCB 1000
LM3670MFX-1.8 NOPB 3000
LM3670MF-1.8 1000
LM3670MFX-1.8 3000
1.6 LM3670MF-1.6 NOPB SDBB 1000
LM3670MFX-1.6 NOPB 3000
LM3670MF-1.6 1000
LM3670MFX-1.6 3000
1.5 LM3670MF-1.5 NOPB S82B 1000
LM3670MFX-1.5 NOPB 3000
LM3670MF-1.5 1000
LM3670MFX-1.5 3000
1.2 LM3670MF-1.2 NOPB SCZB 1000
LM3670MFX-1.2 NOPB 3000
LM3670MF-1.2 1000
LM3670MFX-1.2 3000
Adjustable LM3670MF-ADJ NOPB SDFB 1000
LM3670MFX-ADJ NOPB 3000
LM3670MF-ADJ 1000
LM3670MFX-ADJ 3000
LM3670
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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.
V
IN
Pin: Voltage to GND −0.2V to 6.0V
EN Pin: Voltage to GND −0.2V to 6.0V
FB, SW Pin: (GND−0.2V) to
(V
IN
+ 0.2V)
Junction Temperature (T
J-MAX
) −45˚C to +125˚C
Storage Temperature Range −45˚C to +150˚C
Maximum Lead Temperature
(Soldering, 10 sec.)
260˚C
ESD Rating (Note 3)
Human Body Model:
VIN, SW, FB, EN, GND 2.0kV
Machine Model: 200V
Operating Ratings (Notes 1, 2)
Input Voltage Range 2.5V to 5.5V
Recommended Load Current 0A to 350 mA
Junction Temperature (T
J
) Range −40˚C to +125˚C
Ambient Temperature (T
A
) Range −40˚C to +85˚C
Thermal Properties
Junction-to-Ambient
Thermal Resistance (θ
JA
)
(SOT23-5)
250˚C/W
Electrical Characteristics Limits in standard typeface are for T
J
= 25˚C. Limits in boldface type apply over
the full operating junction temperature range (−40˚C ≤T
J
≤+125˚C). Unless otherwise noted V
IN
= 3.6V, V
OUT
= 1.8V, I
O
=
150mA, EN = V
IN
Symbol Parameter Condition Min Typ Max Units
V
IN
Input Voltage Range (Note 5) 2.5 5.5 V
V
OUT
Fixed Output Voltage: 1.2V 2.5V ≤V
IN
≤5.5V
I
O
=10mA
-2.0 +4.0 %
2.5V ≤V
IN
≤5.5V
0mA≤I
O
≤150 mA
-4.5 +4.0
Fixed Output Voltage: 1.5V 2.5V ≤V
IN
≤5.5V
I
O
=10mA
-2.5 +4.0 %
2.5V ≤V
IN
≤5.5V
0mA≤I
O
≤350 mA
-5.0 +4.0
Fixed Output Voltage: 1.6V,
1.875V
2.5V ≤V
IN
≤5.5V
I
O
=10mA
-2.5 +4.0 %
2.5V ≤V
IN
≤5.5V
0mA≤I
O
≤350 mA
-5.5 +4.0
Fixed Output Voltage: 1.8V 2.5V ≤V
IN
≤5.5V
I
O
=10mA
-1.5 +3.0 %
2.5V ≤V
IN
≤5.5V
0mA≤I
O
≤350 mA
−4.5 +3.0
Fixed Output Voltage: 2.5V,
3.3V
3.6V ≤V
IN
≤5.5V
I
O
=10mA
-2.0 +4.0 %
3.6V ≤V
IN
≤5.5V
0mA≤I
O
≤350 mA
-6.0 +4.0
Adjustable Output Voltage
(Note 4)
2.5V ≤V
IN
≤5.5V
I
O
=10mA
-2.5 +4.5 %
2.5V ≤V
IN
≤5.5V
0mA≤I
O
≤150 mA
-4.0 +4.5
Line_reg Line Regulation 2.5V ≤V
IN
≤5.5V
I
O
=10mA
0.26 %/V
Load_reg Load Regulation 150 mA ≤I
O
≤350 mA 0.0014 %/mA
V
REF
Internal Reference Voltage 0.5 V
I
Q_SHDN
Shutdown Supply Current T
A
=85oC 0.1 1 µA
LM3670
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Electrical Characteristics Limits in standard typeface are for T
J
= 25˚C. Limits in boldface type apply over
the full operating junction temperature range (−40˚C ≤T
J
≤+125˚C). Unless otherwise noted V
IN
= 3.6V, V
OUT
= 1.8V, I
O
=
150mA, EN = V
IN
(Continued)
Symbol Parameter Condition Min Typ Max Units
I
Q
DC Bias Current into V
IN
No load, device is not switching
(V
OUT
forced higher than
programmed output voltage)
15 30 µA
V
UVLO
Minimum V
IN
below which V
OUT
will be disabled 2.4 V
R
DSON (P)
Pin-Pin Resistance for PFET V
IN
=V
GS
=3.6V 360 690 mΩ
R
DSON (N)
Pin-Pin Resistance for NFET V
IN
=V
GS
=3.6V 250 660 mΩ
I
LKG (P)
P Channel Leakage Current V
DS
=5.5V 0.1 1 µA
I
LKG (N)
N Channel Leakage Current V
DS
=5.5V 0.1 1.5 µA
I
LIM
Switch Peak Current Limit 400 620 750 mA
ηEfficiency
(V
IN
= 3.6V, V
OUT
= 1.8V)
I
LOAD
=1mA 91
%
I
LOAD
=10mA 94
I
LOAD
= 100 mA 94
I
LOAD
= 200 mA 94
I
LOAD
= 300 mA 92
I
LOAD
= 350 mA 90
V
IH
Logic High Input 1.3 V
V
IL
Logic Low Input 0.4 V
I
EN
Enable (EN) Input Current 0.01 1µA
F
OSC
Internal Oscillator Frequency PWM Mode 550 1000 1300 kHz
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩresistor into each pin. The machine model is a 200 pF capacitor discharged
directly into each pin. MIL-STD-883 3015.7
Note 4: Output voltage specification for the adjustable version includes tolerance of the external resistor divider.
Note 5: The input voltage range recommended for the specified output voltages are given below:
VIN = 2.5V to 5.5V for 0.7V ≤VOUT <1.875V
VIN =(V
OUT +V
DROP OUT) to 5.5V for 1.875 ≤VOUT≤3.3V
Where VDROP OUT =I
LOAD *(R
DSON (P) +R
INDUCTOR)
LM3670
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20075832
FIGURE 4. Simplified Functional Diagram
LM3670
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Typical Performance Characteristics (unless otherwise stated: V
IN
= 3.6V, V
OUT
= 1.8V)
I
Q
(Non-switching) vs. V
IN
I
Q
vs. Temp
20075804
20075805
V
OUT
vs. V
IN
V
OUT
vs. I
OUT
20075806 20075807
Efficiency vs. I
OUT
Efficiency vs. V
IN
20075808 20075809
LM3670
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Typical Performance Characteristics (unless otherwise stated: V
IN
= 3.6V, V
OUT
= 1.8V) (Continued)
Frequency vs. Temperature
R
DSON
vs. V
IN
P & N Channel
20075810 20075811
Line Transient
(V
IN
= 2.6V to 3.6V, I
LOAD
= 100 mA)
Line Transient
(V
IN
= 3.6V to 4.6V , I
LOAD
= 100 mA)
20075812
20075813
Load Transient
I
LOAD
= 3mA to 280mA
Load Transient
I
LOAD
= 0mA to 70mA
20075816 20075817
LM3670
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Typical Performance Characteristics (unless otherwise stated: V
IN
= 3.6V, V
OUT
= 1.8V) (Continued)
Load Transient
I
LOAD
= 0mA to 280mA
Load Transient
I
LOAD
= 0mA to 350mA
20075818 20075819
Load Transient
I
LOAD
= 50mA to 350mA
Load Transient
I
LOAD
= 100mA to 300mA
20075820 20075821
PFM Mode
V
SW
,V
OUT
,I
INDUCTOR
vs. Time
PWM Mode
V
SW
,V
OUT
,I
INDUCTOR
vs. Time
20075822 20075823
LM3670
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Typical Performance Characteristics (unless otherwise stated: V
IN
= 3.6V, V
OUT
= 1.8V) (Continued)
Soft Start
V
IN
,V
OUT
,I
INDUCTOR
vs. Time
(I
LOAD
= 350mA)
20075824
Operation Description
DEVICE INFORMATION
The LM3670, a high efficiency step down DC-DC switching
buck converter, delivers a constant voltage from either a
single Li-Ion or three cell NiMH/NiCd battery to portable
devices such as cell phones and PDAs. Using a voltage
mode architecture with synchronous rectification, the
LM3670 has the ability to deliver up to 350 mA depending on
the input voltage and output voltage (voltage head room),
and the inductor chosen (maximum current capability).
There are three modes of operation depending on the cur-
rent required - PWM (Pulse Width Modulation), PFM (Pulse
Frequency Modulation), and shutdown. PWM mode handles
current loads of approximately 70 mA or higher. Lighter
output current loads cause the device to automatically switch
into PFM for reduced current consumption (I
Q
= 15 µA typ)
and a longer battery life. Shutdown mode turns off the de-
vice, offering the lowest current consumption
(I
SHUTDOWN
= 0.1 µA typ).
The LM3670 can operate up to a 100% duty cycle (PMOS
switch always on) for low drop out control of the output
voltage. In this way the output voltage will be controlled
down to the lowest possible input voltage.
Additional features include soft-start, under voltage lock out,
current overload protection, and thermal overload protection.
As shown in Figure 1, only three external power components
are required for implementation.
CIRCUIT OPERATION
The LM3670 operates as follows. During the first portion of
each switching cycle, the control block in the LM3670 turns
on the internal PFET switch. This allows current to flow from
the input through the inductor to the output filter capacitor
and load. The inductor limits the current to a ramp with a
slope of
by storing energy in a magnetic field. During the second
portion of each cycle, the controller turns the PFET switch
off, blocking current flow from the input, and then turns the
NFET synchronous rectifier on. The inductor draws current
from ground through the NFET to the output filter capacitor
and load, which ramps the inductor current down with a
slope of
The output filter stores charge when the inductor current is
high, and releases it when low, smoothing the voltage across
the load.
PWM OPERATION
During PWM operation the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve excellent load and line regulation.
The DC gain of the power stage is proportional to the input
voltage. To eliminate this dependence, feed forward in-
versely proportional to the input voltage is introduced.
Internal Synchronous Rectification
While in PWM mode, the LM3670 uses an internal NFET as
a synchronous rectifier to reduce rectifier forward voltage
drop and associated power loss. Synchronous rectification
provides a significant improvement in efficiency whenever
the output voltage is relatively low compared to the voltage
drop across an ordinary rectifier diode.
Current Limiting
A current limit feature allows the LM3670 to protect itself and
external components during overload conditions PWM mode
implements cycle-by-cycle current limiting using an internal
comparator that trips at 620 mA (typ).
PFM OPERATION
At very light load, the converter enters PFM mode and
operates with reduced switching frequency and supply cur-
rent to maintain high efficiency.
LM3670
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Operation Description (Continued)
The part automatically transition into PFM mode when either
of two conditions occurs for a duration of 32 or more clock
cycles:
A. The inductor current becomes discontinuous
B. The peak PMOS switch current drops below the I
MODE
level:
During PFM operation, the converter positions the output
voltage slightly higher than the nominal output voltage in
PWM operation, allowing additional headroom for voltage
drop during a load transient from light to heavy load. The
PFM comparator senses the output voltage via the feedback
pin and control the switching of the output FETs such that the
output voltage ramps between 0.8% and 1.6% (typ) above
the nominal PWM output voltage. If the output voltage is
below the ‘high’ PFM comparator threshold, the PMOS
power switch is turned on. It remains on until the output
voltage exceeds the ‘high’ PFM threshold or the peak current
exceeds the I
PFM
level set for PFM mode. The peak current
in PFM mode is:
Once the PMOS power switch is turned off, the NMOS
power switch is turned on until the inductor current ramps to
zero. When the NMOS zero-current condition is detected,
the NMOS power switch is turned off. If the output voltage is
below the ‘high’ PFM comparator threshold (see Figure 5),
the PMOS switch is again turned on and the cycle is re-
peated until the output reaches the desired level. Once the
output reaches the ‘high’ PFM threshold, the NMOS switch is
turned on briefly to ramp the inductor current to zero and
then both output switches are turned off and the part enters
an extremely low power mode. Quiescent supply current
during this ‘sleep’ mode is less than 30 µA, which allows the
part to achieve high efficiencies under extremely light load
conditions. When the output drops below the ‘low’ PFM
threshold, the cycle repeats to restore the output voltage to
∼1.6% above the nominal PWM output voltage.
If the load current should increase during PFM mode (see
Figure 5) causing the output voltage to fall below the ‘low2’
PFM threshold, the part automatically transitions into fixed-
frequency PWM mode.
Soft-Start
The LM3670 has a soft-start circuit that limits in-rush current
during start-up. Typical start-up times with a 10µF output
capacitor and 350mA load is 400µs:
Inrush Current (mA) Duration (µSec)
032
70 224
140 256
Inrush Current (mA) Duration (µSec)
280 256
620 until soft start ends
Note 6: The first 32µS are to allow the bias currents to stabilize
20075803
FIGURE 5. Operation in PFM Mode and Transition to PWM Mode
LM3670
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Operation Description (Continued)
LDO - Low Drop Out Operation
The LM3670 can operate at 100% duty cycle (no switching,
PMOS switch is completely on) for low drop out support of
the output voltage. In this way the output voltage is con-
trolled down to the lowest possible input voltage.
The minimum input voltage needed to support the output
voltage is
•I
LOAD
Load current
•R
DSON,
PFET
Drain to source resistance of PFET switch
in the triode region
•R
INDUCTOR
Inductor resistance
Application Information
OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE
LM3670
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from V
OUT
to V
FB
then to GND. V
OUT
is adjusted to make V
FB
equal to
0.5V. The resistor from V
FB
to GND (R2) should be at least
100KΩto keep the current sunk through this network well
below the 15µA quiescent current level (PFM mode with no
switching) but large enough that it is not susceptible to noise.
If R
2
is 200KΩ, and V
FB
is 0.5V, then the current through the
resistor feedback network is 2.5µA ( I
FB
=0.5V/R
2
). The
output voltage formula is:
•V
OUT
Output Voltage (V)
•V
FB
Feedback Voltage (0.5V typ)
•R
1
Resistor from V
OUT
to V
FB
(Ω)
•R
2
Resistor from V
OUT
to GND (Ω)
For any output voltage greater than or equal to 0.7V a
frequency zero must be added at 10kHz for stability. The
formula is:
For any output voltages below 0.7 and above or equal to
2.5V, a pole must also be placed at 10kHz as well. The
lowest output voltage possible is 0.7V. At low output voltages
the duty cycle is very small and, as the input voltage in-
creases, the duty cycle decreases even further. Since the
duty cycle is so low any change due to noise is an appre-
ciable percentage. In other words, it is susceptible to noise.
Capacitors C
1
and C
2
act as noise filters rather than fre-
quency poles and zeros. If the pole and zero are at the same
frequency the formula is:
A pole can also be used at higher output voltages. For
example, in the table Table 3, there is an entry for 1.24V with
both a pole and zero at approximately 10kHz for noise
rejection.
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor current should not saturate, and the inductor
current ripple is small enough to achieve the desired output
voltage ripple.
There are two methods to choose the inductor current rating.
Method 1:
The total current is the sum of the load and the inductor
ripple current. This can be written as
•I
LOAD
load current
•V
IN
input voltage
•L inductor
•f switching frequency
•I
RIPPLE
peak-to-peak
Method 2:
A more conservative approach is to choose an inductor that
can handle the current limit of 700 mA.
Given a peak-to-peak current ripple (I
PP
) the inductor needs
to be at least
A 10 µH inductor with a saturation current rating of at least
800 mA is recommended for most applications. The induc-
tor’s resistance should be less than around 0.3Ωfor good
efficiency. Table 1 lists suggested inductors and suppliers.
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is
to lay out the board with overlapping footprints of both types
for design flexibility. This allows substitution of a low-noise
toroidal inductor, in the event that noise from low-cost bobbin
models is unacceptable.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 µF is sufficient for most
applications. A larger value may be used for improved input
voltage filtering. The input filter capacitor supplies current to
the PFET switch of the LM3670 in the first half of each cycle
and reduces voltage ripple imposed on the input power
source. A ceramic capcitor’s low ESR provides the best
LM3670
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Application Information (Continued)
noise filtering of the input voltage spikes due to this rapidly
changing current. Select an input filter capacitor with a surge
current rating sufficient for the power-up surge from the input
power source. The power-up surge current is approximately
the capacitor’s value (µF) times the voltage rise rate (V/µs).
The input current ripple can be calculated as:
TABLE 1. Suggested Inductors and Their Suppliers
Model Vendor Phone FAX
IDC2512NB100M Vishay 408-727-2500 408-330-4098
DO1608C-103 Coilcraft 847-639-6400 847-639-1469
ELL6RH100M Panasonic 714-373-7366 714-373-7323
CDRH5D18-100 Sumida 847-956-0666 847-956-0702
OUTPUT CAPACITOR SELECTION
The output filter capacitor smoothes out current flow from the
inductor to the load, maintaining a steady output voltage
during transient load changes and reduces output voltage
ripple. These capacitors must be selected with sufficient
capacitance and sufficiently low ESR to perform these func-
tions.
The output ripple current can be calculated as:
Voltage peak-to-peak ripple due to capacitance =
Voltage peak-to-peak ripple due to ESR =
Voltage peak-to-peak ripple, root mean squared =
Note that the output ripple is dependent on the current ripple
and the equivalent series resistance of the output capacitor
(R
ESR
).
Because these two components are out of phase the rms
value is used. The R
ESR
is frequency dependent (as well as
temperature dependent); make sure the frequency of the
R
ESR
given is the same order of magnitude as the switching
frequency.
TABLE 2. Suggested Capacitors and Their Suppliers
Model Type Vendor Phone FAX
10 µF for C
OUT
VJ1812V106MXJAT Ceramic Vishay 408-727-2500 408-330-4098
LMK432BJ106MM Ceramic Taiyo-Yuden 847-925-0888 847-925-0899
JMK325BJ106MM Ceramic Taiyo-Yuden 847-925-0888 847-925-0899
4.7 µF for C
IN
VJ1812V475MXJAT Ceramic Vishay 408-727-2500 408-330-4098
EMK325BJ475MN Ceramic Taiyo-Yuden 847-925-0888 847-925-0899
C3216X5R0J475M Ceramic TDK 847-803-6100 847-803-6296
TABLE 3. Adjustable LM3670 Configurations for Various V
OUT
VOUT (V) R1 (KΩ)R2(KΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF)
0.7 80.6 200 200 150 4.7 4.7 10
0.8 120 200 130 none 4.7 4.7 10
0.9 160 200 100 none 4.7 4.7 10
1.0 200 200 82 none 4.7 4.7 10
LM3670
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Application Information (Continued)
TABLE 3. Adjustable LM3670 Configurations for Various V
OUT
(Continued)
VOUT (V) R1 (KΩ)R2(KΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF)
1.1 240 200 68 none 4.7 4.7 10
1.2 280 200 56 none 4.7 4.7 10
1.24 300 200 56 none 4.7 4.7 10
1.24 221 150 75 120 4.7 4.7 10
1.5 402 200 39 none 10 4.7 10
1.6 442 200 39 none 10 4.7 10
1.7 487 200 33 none 10 4.7 10
1.875 549 200 30 none 10 4.7 14.7
Note: (10 ||
4.7)
2.5 806 200 22 82 10 4.7 22
LM3670
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Application Information (Continued)
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter
design. Poor board layout can disrupt the performance of a
DC-DC converter and surrounding circuitry by contributing to
EMI, ground bounce, and resistive voltage loss in the traces.
These can send erroneous signals to the DC-DC converter
IC, resulting in poor regulation or instability.
Good layout for the LM3670 can be implemented by follow-
ing a few simple design rules, as illustrated in .
1. Place the LM3670, inductor and filter capacitors close
together and make the traces short. The traces between
these components carry relatively high switching cur-
rents and act as antennas. Following this rule reduces
radiated noise. Place the capacitors and inductor within
0.2 in. (5 mm) of the LM3670.
2. Arrange the components so that the switching current
loops curl in the same direction. During the first half of
each cycle, current flows from the input filter capacitor,
through the LM3670 and inductor to the output filter
capacitor and back through ground, forming a current
loop. In the second half of each cycle, current is pulled
up from ground, through the LM3670 by the inductor, to
the output filter capacitor and then back through ground,
forming a second current loop. Routing these loops so
the current curls in the same direction prevents mag-
netic field reversal between the two half-cycles and re-
duces radiated noise.
3. Connect the ground pins of the LM3670, and filter ca-
pacitors together using generous component-side cop-
per fill as a pseudo-ground plane. Then, connect this to
the ground-plane (if one is used) with several vias. This
reduces ground-plane noise by preventing the switching
currents from circulating through the ground plane. It
also reduces ground bounce at the LM3670 by giving it
a low-impedance ground connection.
4. Use wide traces between the power components and for
power connections to the DC-DC converter circuit. This
reduces voltage errors caused by resistive losses across
the traces.
5. Route noise sensitive traces, such as the voltage feed-
back path, away from noisy traces between the power
components. The voltage feedback trace must remain
close to the LM3670 circuit and should be direct but
should be routed opposite to noisy components. This
reduces EMI radiated onto the DC-DC converter’s own
voltage feedback trace.
6. Place noise sensitive circuitry, such as radio IF blocks,
away from the DC-DC converter, CMOS digital blocks
and other noisy circuitry. Interference with noise-
sensitive circuitry in the system can be reduced through
distance.
In mobile phones, for example, a common practice is to
place the DC-DC converter on one corner of the board,
20075831
FIGURE 6. Board Layout Design Rules for the LM3670
LM3670
www.national.com15
Application Information (Continued)
arrange the CMOS digital circuitry around it (since this also
generates noise), and then place sensitive preamplifiers and
IF stages on the diagonally opposing corner. Often, the
sensitive circuitry is shielded with a metal pan and power to
it is post-regulated to reduce conducted noise, using low-
dropout linear regulators.
LM3670
www.national.com 16
Physical Dimensions inches (millimeters) unless otherwise noted
5-Lead SOT23-5 Package
NS Package Number MF05A
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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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
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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.
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www.national.com
LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
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