October 28, 2008
LM3671
2MHz, 600mA Step-Down DC-DC Converter
General Description
The LM3671 step-down DC-DC converter is optimized for
powering low voltage circuits from a single Li-Ion cell battery
and input voltage rails from 2.7V to 5.5V. It provides up to
600mA load current, over the entire input voltage range.
There are several different fixed voltage output options avail-
able as well as an adjustable output voltage version range
from 1.1V to 3.3V.
The device offers superior features and performance for mo-
bile phones and similar portable systems. Automatic intelli-
gent switching between PWM low-noise and PFM low-current
mode offers improved system control. During PWM mode, the
device operates at a fixed-frequency of 2 MHz (typ.). Hys-
teretic PFM mode extends the battery life by reducing the
quiescent current to 16 µA (typ.) during light load and standby
operation. Internal synchronous rectification provides high ef-
ficiency during PWM mode operation. In shutdown mode, the
device turns off and reduces battery consumption to 0.01 µA
(typ.).
The LM3671 is available in SOT23-5, tiny 5-bump micro SMD
and a 6-pin LLP packages in leaded (PB) and lead-free (NO
PB) versions. A high switching frequency of 2 MHz (typ.) al-
lows use of tiny surface-mount components. Only three ex-
ternal surface-mount components, an inductor and two
ceramic capacitors, are required.
Features
16 µA typical quiescent current
600 mA maximum load capability
2 MHz PWM fixed switching frequency (typ.)
Automatic PFM/PWM mode switching
Internal synchronous rectification for high efficiency
Internal soft start
0.01 µA typical shutdown current
Operates from a single Li-Ion cell battery
Only three tiny surface-mount external components
required (one inductor, two ceramic capacitors)
Current overload and Thermal shutdown protection
Available in fixed output voltages and adjustable version
SOT23-5, 5-bump micro SMD and 6-pin LLP packages
Applications
Mobile phones
PDAs
MP3 players
W-LAN
Portable instruments
Digital still cameras
Portable Hard disk drives
Typical Application Circuits
20108401
FIGURE 1. Typical Application Circuit
© 2008 National Semiconductor Corporation 201084 www.national.com
LM3671 2MHz, 600mA Step-Down DC-DC Converter
20108431
FIGURE 2. Typical Application Circuit for ADJ version
Connection Diagram and Package Mark Information
20108402
FIGURE 3. Top View
SOT23-5 Package NS Package Number MF05A (2.92mm x 2.84mm x 1.2mm)
20108444
FIGURE 4. 5-Bump Micro SMD Package NS Package Number TLA05CBA (1.05mm x 1.38mm x 0.6mm)
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LM3671
20108462
FIGURE 5. 6-Pin LLP Package Number LCA06B (2mm x 2mm x 0.6mm)
Pin Descriptions (SOT23-5)
Pin # Name Description
1 VIN Power supply input. Connect to the input filter capacitor (Figure 1).
2 GND Ground pin.
3 EN Enable pin. The device is in shutdown mode when voltage to this pin is <0.4V and enabled
when >1.0V. Do not leave this pin floating.
4 FB Feedback analog input. Connect directly to the output filter capacitor for fixed voltage
versions. For adjustable version external resistor dividers are required (Figure 2). The
internal resistor dividers are disabled for the adjustable version.
5 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier.
Pin Descriptions (5-Bump Micro SMD)
Pin # Name Description
A1 VIN Power supply input. Connect to the input filter capacitor (Figure 1).
A3 GND Ground pin.
C1 EN Enable pin. The device is in shutdown mode when voltage to this pin is <0.4V and enabled
when >1.0V. Do not leave this pin floating.
C3 FB Feedback analog input. Connect directly to the output filter capacitor for fixed voltage
versions. For adjustable version external resistor dividers are required (Figure 2). The
internal resistor dividers are disabled for the adjustable version.
B2 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier.
Pin Descriptions (6-Pin LLP)
Pin # Name Description
1 EN Enable pin. The device is in shutdown mode when voltage to this pin is <0.4V and enabled
when >1.0V. Do not leave this pin floating.
2 Pgnd Ground pin.
3 VIN Power supply input. Connect to the input filter capacitor ().
4 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier.
5 Sgnd Singnal ground (feedback ground).
6 FB Feedback analog input. Connect directly to the output filter capacitor for fixed voltage
versions. For adjustable version external resistor dividers are required (Figure 2). The
internal resistor dividers are disabled for the adjustable version.
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LM3671
Ordering Information (SOT23-5)
Voltage Option Order Number Spec Package Marking Supplied As
ADJ LM3671MF-ADJ NOPB
SBTB
1000 units, Tape-and-Reel
LM3671MFX-ADJ NOPB 3000 units, Tape-and-Reel
LM3671MF-ADJ 1000 units, Tape-and-Reel
LM3671MFX-ADJ 3000 units, Tape-and-Reel
1.2 LM3671MF-1.2 NOPB
SBPB
1000 units, Tape-and-Reel
LM3671MFX-1.2 NOPB 3000 units, Tape-and-Reel
LM3671MF-1.2 1000 units, Tape-and-Reel
LM3671MFX-1.2 3000 units, Tape-and-Reel
1.25 LM3671MF-1.25 NOPB
SDRB
1000 units, Tape-and-Reel
LM3671MFX-1.25 NOPB 3000 units, Tape-and-Reel
LM3671MF-1.25 1000 units, Tape-and-Reel
LM3671MFX-1.25 3000 units, Tape-and-Reel
1.375 LM3671MF-1.375 NOPB
SEDB
1000 units, Tape-and-Reel
LM3671MFX-1.375 NOPB 3000 units, Tape-and-Reel
LM3671MF-1.375 1000 units, Tape-and-Reel
LM3671MFX-1.375 3000 units, Tape-and-Reel
1.5 LM3671MF-1.5 NOPB
SBRB
1000 units, Tape-and-Reel
LM3671MFX-1.5 NOPB 3000 units, Tape-and-Reel
LM3671MF-1.5 1000 units, Tape-and-Reel
LM3671MFX-1.5 3000 units, Tape-and-Reel
1.6 LM3671MF-1.6 NOPB
SDUB
1000 units, Tape-and-Reel
LM3671MFX-1.6 NOPB 3000 units, Tape-and-Reel
LM3671MF-1.6 1000 units, Tape-and-Reel
LM3671MFX-1.6 3000 units, Tape-and-Reel
1.8 LM3671MF-1.8 NOPB
SBSB
1000 units, Tape-and-Reel
LM3671MFX-1.8 NOPB 3000 units, Tape-and-Reel
LM3671MF-1.8 1000 units, Tape-and-Reel
LM3671MFX-1.8 3000 units, Tape-and-Reel
1.875 LM3671MF-1.875 NOPB
SDVB
1000 units, Tape-and-Reel
LM3671MFX-1.875 NOPB 3000 units, Tape-and-Reel
LM3671MF-1.875 1000 units, Tape-and-Reel
LM3671MFX-1.875 3000 units, Tape-and-Reel
2.5 LM3671MF-2.5 NOPB
SJRB
1000 units, Tape-and-Reel
LM3671MFX-2.5 NOPB 3000 units, Tape-and-Reel
LM3671MF-2.5 1000 units, Tape-and-Reel
LM3671MFX-2.5 3000 units, Tape-and-Reel
2.8 LM3671MF-2.8 NOPB
SJSB
1000 units, Tape-and-Reel
LM3671MFX-2.8 NOPB 3000 units, Tape-and-Reel
LM3671MF-2.8 1000 units, Tape-and-Reel
LM3671MFX-2.8 3000 units, Tape-and-Reel
3.3 LM3671MF-3.3 NOPB
SJEB
1000 units, Tape-and-Reel
LM3671MFX-3.3 NOPB 3000 units, Tape-and-Reel
LM3671MF-3.3 1000 units, Tape-and-Reel
LM3671MFX-3.3 3000 units, Tape-and-Reel
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LM3671
Ordering Information (5-bump Micro SMD)
Voltage Option Order Number Spec Package Marking Supplied As
ADJ LM3671TL-ADJ NOPB
E
250 units, Tape-and-Reel
LM3671TLX-ADJ NOPB 3000 units, Tape-and-Reel
LM3671TL-ADJ 250 units, Tape-and-Reel
LM3671TLX-ADJ 3000 units, Tape-and-Reel
1.2 LM3671TL-1.2 NOPB
C
250 units, Tape-and-Reel
LM3671TLX-1.2 NOPB 3000 units, Tape-and-Reel
LM3671TL-1.2 250 units, Tape-and-Reel
LM3671TLX-1.2 3000 units, Tape-and-Reel
1.5 LM3671TL-1.5 NOPB
D
250 units, Tape-and-Reel
LM3671TLX-1.5 NOPB 3000 units, Tape-and-Reel
LM3671TL-1.5 250 units, Tape-and-Reel
LM3671TLX-1.5 3000 units, Tape-and-Reel
1.8 LM3671TL-1.8 NOPB
B
250 units, Tape-and-Reel
LM3671TLX-1.8 NOPB 3000 units, Tape-and-Reel
LM3671TL-1.8 250 units, Tape-and-Reel
LM3671TLX-1.8 3000 units, Tape-and-Reel
1.875 LM3671TL-1.875 NOPB
S
250 units, Tape-and-Reel
LM3671TLX-1.875 NOPB 3000 units, Tape-and-Reel
LM3671TL-1.875 250 units, Tape-and-Reel
LM3671TLX-1.875 3000 units, Tape-and-Reel
2.5 LM3671TL-2.5 NOPB
L
250 units, Tape-and-Reel
LM3671TLX-2.5 NOPB 3000 units, Tape-and-Reel
LM3671TL-2.5 250 units, Tape-and-Reel
LM3671TLX-2.5 3000 units, Tape-and-Reel
2.8 LM3671TL-2.8 NOPB
K
250 units, Tape-and-Reel
LM3671TLX-2.8 NOPB 3000 units, Tape-and-Reel
LM3671TL-2.8 250 units, Tape-and-Reel
LM3671TLX-2.8 3000 units, Tape-and-Reel
3.3 LM3671TL-3.3 NOPB
J
250 units, Tape-and-Reel
LM3671TLX-3.3 NOPB 3000 units, Tape-and-Reel
LM3671TL-3.3 250 units, Tape-and-Reel
LM3671TLX-3.3 3000 units, Tape-and-Reel
Ordering Information (6-Pin LLP)
Voltage Option Order Number Spec Package Marking Supplied As
1.2 LM3671LC-1.2 NOPB S39 1000 units, Tape-and-Reel
LM3671LCX-1.2 NOPB 4500 units, Tape-and-Reel
1.3 LM3671LC-1.3 NOPB S40 1000 units, Tape-and-Reel
LM3671LCX-1.3 NOPB 4500 units, Tape-and-Reel
1.6 LM3671LC-1.6 NOPB S41 1000 units, Tape-and-Reel
LM3671LCX-1.6 NOPB 4500 units, Tape-and-Reel
1.8 LM3671LC-1.8 NOPB S42 1000 units, Tape-and-Reel
LM3671LCX-1.8 NOPB 4500 units, Tape-and-Reel
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LM3671
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN Pin: Voltage to GND −0.2V to 6.0V
FB, SW, EN Pin: (GND−0.2V) to
(VIN + 0.2V)
Continuous Power Dissipation
(Note 3)
Internally Limited
Junction Temperature (TJ-MAX) +125°C
Storage Temperature Range −65°C to +150°C
Maximum Lead Temperature
(Soldering, 10 sec.)
260°C
ESD Rating (Note 4)
Human Body Model 2 kV
Machine Model 200V
Operating Ratings (Notes 1, 2)
Input Voltage Range (Note 10) 2.7V to 5.5V
Recommended Load Current 0mA to 600 mA
Junction Temperature (TJ) Range −30°C to +125°C
Ambient Temperature (TA) Range (Note
5)
−30°C to +85°C
Thermal Properties
Junction-to-Ambient Thermal
Resistance (θJA) (SOT23-5) for 4
layer board (Note 6)
130°C/W
Junction-to-Ambient Thermal
Resistance (θJA) (Micro SMD) for 4
layer board (Note 6)
85°C/W
Junction-to-Ambient Thermal
Resistance (θJA) (COL) for 4 layer
board (Note 6)
165°C/W
Electrical Characteristics (Notes 2, 8, 9) Limits in standard typeface are for TJ = 25°C. Limits in boldface type
apply over the full operating ambient temperature range (−30°C TA +85°C). Unless otherwise noted, specifications apply to
the LM3671MF/TL/LC with VIN = EN = 3.6V
Symbol Parameter Condition Min Typ Max Units
VIN Input Voltage (Note 10) 2.7 5.5 V
VFB Feedback Voltage (Fixed) MF PWM mode (Note 12) -4 +4 %
Feedback Voltage (Fixed) TL -2.5 +2.5
Feedback Voltage (Fixed) LC -4 +4
Feedback Voltage (ADJ) MF
(Note 11)
PWM mode (Note 12) -4 +4 %
Feedback Voltage (ADJ) TL -2.5 +2.5
Line Regulation 2.7V VIN 5.5V
IO = 10 mA
0.031 %/V
Load Regulation 100 mA IO 600 mA
VIN= 3.6V
0.0013 %/mA
VREF Internal Reference Voltage 0.5 V
ISHDN Shutdown Supply Current EN = 0V 0.01 1µA
IQDC Bias Current into VIN No load, device is not switching (FB
forced higher than programmed
output voltage)
16 35 µA
RDSON (P) Pin-Pin Resistance for PFET VIN= VGS= 3.6V 380 500 m
RDSON (N) Pin-Pin Resistance for NFET VIN= VGS= 3.6V 250 400 m
ILIM Switch Peak Current Limit Open Loop (Note 7) 830 1020 1150 mA
VIH Logic High Input 1.0 V
VIL Logic Low Input 0.4 V
IEN Enable (EN) Input Current 0.01 1µA
FOSC Internal Oscillator Frequency PWM Mode (Note 12) 1.6 22.6 MHz
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device 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.
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LM3671
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ= 150°C (typ.) and disengages at
TJ= 130°C (typ.).
Note 4: The Human body model is a 100 pF capacitor discharged through a 1.5 k resistor into each pin. The machine model is a 200 pF capacitor discharged
directly into each pin. MIL-STD-883 3015.7
Note 5: In Applications where high power dissipation and/or poor package resistance is present, the maximum ambient temperature may have to be derated.
Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX), the maximum power dissipation of the device in
the application (PD-MAX) and the junction to ambient thermal resistance of the package (θJA) in the application, as given by the following equation:TA-MAX= TJ-MAX
− (θJAx PD-MAX). Refer to Dissipation rating table for PD-MAX values at different ambient temperatures.
Note 6: Junction to ambient thermal resistance is highly application and board layout dependent. In applications where high power dissipation exists, special care
must be given to thermal dissipation issues in board design. Specified value of 130 °C/W for SOT23-5 is based on a 4 layer, 4" x 3", 2/1/1/2 oz. Cu board as per
JEDEC standards is used.
Note 7: Refer to datasheet curves for closed loop data and its variation with regards to supply voltage and temperature. Electrical Characteristic table reflects
open loop data (FB=0V and current drawn from SW pin ramped up until cycle by cycle current limit is activated). Closed loop current limit is the peak inductor
current measured in the application circuit by increasing output current until output voltage drops by 10%.
Note 8: Min and Max limits are guaranteed by design, test or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.
Note 9: The parameters in the electrical characteristic table are tested at VIN= 3.6V unless otherwise specified. For performance over the input voltage range
refer to datasheet curves.
Note 10: The input voltage range recommended for ideal applications performance for the specified output voltages are given below:
VIN = 2.7V to 4.5V for 1.1V VOUT < 1.5V
VIN = 2.7V to 5.5V for 1.5V VOUT < 1.8V
VIN = (VOUT+ VDROPOUT) to 5.5V for 1.8V VOUT 3.3V
where VDROPOUT = ILOAD *( RDSON, PFET + RINDUCTOR)
Note 11: ADJ version is configured to 1.5V output. For ADJ output version:
VIN = 2.7V to 4.5V for 0.90V VOUT < 1.1V
VIN = 2.7V to 5.5V for 1.1V VOUT < 3.3V
Note 12: Test condition: for VOUT less than 2.5V, VIN = 3.6V; for VOUT greater than or equal to 2.5V, VIN = VOUT+ 1V.
Dissipation Rating Table
θJA TA 25°C
Power Rating
TA= 60°C
Power Rating
TA= 85°C
Power Rating
130°C/W (4 layer board)
SOT23-5
770mW 500mW 310mW
85°C/W (4 layer board) 5-bump
Micro SMD
1179mW 765mW 470mW
165°C/W (4 layer board) 6-pin
LLP
606mW 394mW 242mW
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LM3671
Block Diagram
20108418
FIGURE 6. Simplified Functional Diagram
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LM3671
Typical Performance Characteristics
LM3671MF/TL/LC, Circuit of Figure 1, VIN= 3.6V, VOUT= 1.5V, TA= 25°C, unless otherwise noted.
Quiescent Supply Current vs. Supply Voltage
20108404
Shutdown Current vs. Temp
20108405
Feedback Bias Current vs. Temp
20108440
Switching Frequency vs. Temperature
20108447
RDS(ON) vs. Temperature
20108433
Open/Closed Loop Current Limit vs. Temperature
20108448
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LM3671
Output Voltage vs. Supply Voltage
(VOUT = 1.5V)
20108429
Output Voltage vs. Supply Voltage
(VOUT = 2.5V)
20108458
Output Voltage vs. Temperature
(VOUT = 1.5V)
20108406
Output Voltage vs. Temperature
(VOUT = 2.5V)
20108459
Output Voltage vs. Output Current
(VOUT = 1.5V)
20108407
Output Voltage vs. Output Current
(VOUT = 2.5V)
20108460
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LM3671
Efficiency vs. Output Current
(VOUT = 1.5V, L= 2.2 µH)
20108408
Efficiency vs. Output Current
(VOUT = 1.8V, L= 2.2 µH)
20108409
Efficiency vs. Output Current
(VOUT = 2.5V, L= 2.2 µH)
20108441
Efficiency vs. Output Current
(VOUT = 3.3V, L= 2.2 µH)
20108442
Line Transient Response
VOUT = 1.5V (PWM Mode)
20108412
Line Transient Response
VOUT = 2.5V (PWM Mode)
20108450
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LM3671
Load Transient Response
VOUT = 1.5V (PWM Mode)
20108413
Load Transient Response
VOUT = 2.5V (PWM Mode)
20108451
Load Transient Response (VOUT = 1.5V)
(PFM Mode 0.5mA to 50mA)
20108414
Load Transient Response (VOUT = 1.5V)
(PFM Mode 50mA to 0.5mA)
20108415
Load Transient Response (VOUT = 2.5V)
(PFM Mode 0.5mA to 50mA)
20108452
Load Transient Response (VOUT = 2.5V)
(PFM Mode 50mA to 0.5mA)
20108453
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LM3671
Mode Change by Load Transients
VOUT = 1.5V (PFM to PWM)
20108420
Mode Change by Load Transients
VOUT = 1.5V (PWM to PFM)
20108421
Mode Change by Load Transients
VOUT = 2.5V (PFM to PWM)
20108454
Mode Change by Load Transients
VOUT = 2.5V (PWM to PFM)
20108455
Start Up into PWM Mode
VOUT = 1.5V (Output Current= 300mA)
20108424
Start Up into PWM Mode
VOUT = 2.5V (Output Current= 300mA)
20108456
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LM3671
Start Up into PFM Mode
VOUT = 1.5V (Output Current= 1mA)
20108419
Start Up into PFM Mode
VOUT = 2.5V (Output Current= 1mA)
20108457
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LM3671
Operation Description
DEVICE INFORMATION
The LM3671, a high efficiency step down DC-DC switching
buck converter, delivers a constant voltage from a single Li-
Ion battery and input voltage rails from 2.7V to 5.5V to
portable devices such as cell phones and PDAs. Using a volt-
age mode architecture with synchronous rectification, the
LM3671 has the ability to deliver up to 600 mA depending on
the input voltage, output voltage, ambient temperature and
the inductor chosen.
There are three modes of operation depending on the current
required - PWM (Pulse Width Modulation), PFM (Pulse Fre-
quency Modulation), and shutdown. The device operates in
PWM mode at load current of approximately 80 mA or higher.
Lighter load current cause the device to automatically switch
into PFM for reduced current consumption (IQ = 16 µA typ)
and a longer battery life. Shutdown mode turns off the device,
offering the lowest current consumption
(ISHUTDOWN = 0.01 µA typ).
Additional features include soft-start, under voltage protec-
tion, current overload protection, and thermal shutdown pro-
tection. As shown in Figure 1, only three external power
components are required for implementation.
The part uses an internal reference voltage of 0.5V. It is rec-
ommended to keep the part in shutdown until the input voltage
is 2.7V or higher.
CIRCUIT OPERATION
During the first portion of each switching cycle, the control
block in the LM3671 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 (VIN–VOUT)/L, by storing en-
ergy 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 - VOUT/L.
The output filter stores charge when the inductor current is
high, and releases it when inductor current is low, smoothing
the voltage across the load.
The output voltage is regulated by modulating the PFET
switch on time to control the average current sent to the load.
The effect is identical to sending a duty-cycle modulated rect-
angular wave formed by the switch and synchronous rectifier
at the SW pin to a low-pass filter formed by the inductor and
output filter capacitor. The output voltage is equal to the av-
erage voltage at the SW pin.
PWM OPERATION
During PWM operation the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve good load and line regulation. The
DC gain of the power stage is proportional to the input voltage.
To eliminate this dependence, feed forward inversely propor-
tional to the input voltage is introduced.
While in PWM mode, the output voltage is regulated by
switching at a constant frequency and then modulating the
energy per cycle to control power to the load. At the beginning
of each clock cycle the PFET switch is turned on and the in-
ductor current ramps up until the comparator trips and the
control logic turns off the switch. The current limit comparator
can also turn off the switch in case the current limit of the
PFET is exceeded. Then the NFET switch is turned on and
the inductor current ramps down. The next cycle is initiated
by the clock turning off the NFET and turning on the PFET.
20108423
FIGURE 7. Typical PWM Operation
Internal Synchronous Rectification
While in PWM mode, the LM3671 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 LM3671 to protect itself and
external components during overload conditions. PWM mode
implements current limiting using an internal comparator that
trips at 1020 mA (typ.). If the output is shorted to ground the
device enters a timed current limit mode where the NFET is
turned on for a longer duration until the inductor current falls
below a low threshold. This allows the inductor current more
time to decay, thereby preventing runaway.
PFM OPERATION
At very light load, the converter enters PFM mode and oper-
ates with reduced switching frequency and supply current to
maintain high efficiency.
The part automatically transitions into PFM mode when either
of two conditions occurs for a duration of 32 or more clock
cycles:
A.The NFET current reaches zero.
B.The peak PMOS switch current drops below the IMODE
level, (Typically IMODE < 30mA + VIN/42 Ω ).
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LM3671
20108422
FIGURE 8. Typical PFM Operation
During PFM operation, the converter positions the output volt-
age slightly higher than the nominal output voltage during
PWM operation, allowing additional headroom for voltage
drop during a load transient from light to heavy load. The PFM
comparators sense the output voltage via the feedback pin
and control the switching of the output FETs such that the
output voltage ramps between ~0.6% and ~1.7% 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 reaches the
‘high’ PFM threshold or the peak current exceeds the IPFM
level set for PFM mode. The typical peak current in PFM mode
is: IPFM = 112mA + VIN/27Ω .
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 be-
low the ‘high’ PFM comparator threshold (see Figure 9), the
PMOS switch is again turned on and the cycle is repeated
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 ex-
tremely low power mode. Quiescent supply current during this
‘sleep’ mode is 16µA (typ.), which allows the part to achieve
high efficiency under extremely light load conditions.
If the load current should increase during PFM mode (see
Figure 9) causing the output voltage to fall below the ‘low2’
PFM threshold, the part will automatically transition into fixed-
frequency PWM mode. When VIN =2.7V the part transitions
from PWM to PFM mode at ~35mA output current and from
PFM to PWM mode at ~85mA , when VIN=3.6V, PWM to PFM
transition happens at ~50mA and PFM to PWM transition
happens at ~100mA, when VIN =4.5V, PWM to PFM transition
happens at ~65mA and PFM to PWM transition happens at
~115mA.
20108403
FIGURE 9. Operation in PFM Mode and Transfer to PWM Mode
SHUTDOWN MODE
Setting the EN input pin low (<0.4V) places the LM3671 in
shutdown mode. During shutdown the PFET switch, NFET
switch, reference, control and bias circuitry of the LM3671 are
turned off. Setting EN high (>1.0V) enables normal operation.
It is recommended to set EN pin low to turn off the LM3671
during system power up and undervoltage conditions when
the supply is less than 2.7V. Do not leave the EN pin floating.
SOFT START
The LM3671 has a soft-start circuit that limits in-rush current
during start-up. During start-up the switch current limit is in-
creased in steps. Soft start is activated only if EN goes from
logic low to logic high after Vin reaches 2.7V. Soft start is im-
plemented by increasing switch current limit in steps of 70mA,
140mA, 280mA and 1020mA (typical switch current limit). The
start-up time thereby depends on the output capacitor and
www.national.com 16
LM3671
load current demanded at start-up. Typical start-up times with
a 10µF output capacitor and 300mA load is 400 µs and with
1mA load is 275µs.
LDO - LOW DROP OUT OPERATION
The LM3671-ADJ can operate at 100% duty cycle (no switch-
ing; PMOS switch completely on) for low drop out support of
the output voltage. In this way the output voltage will be con-
trolled down to the lowest possible input voltage. When the
device operates near 100% duty cycle, output voltage ripple
is approximately 25 mV.
The minimum input voltage needed to support the output volt-
age is
VIN, MIN = ILOAD * (RDSON, PFET + RINDUCTOR) + VOUT
ILOAD Load current
RDSON, PFET Drain to source resistance of PFET
switch in the triode region
RINDUCTOR Inductor resistance
Application Information
OUTPUT VOLTAGE SELECTION FOR LM3671-ADJ
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from VOUT
to FB, then to GND. VOUT is adjusted to make the voltage at
FB equal to 0.5V. The resistor from FB to GND (R2) should
be 200 k to keep the current drawn through this network well
below the 16 µA quiescent current level (PFM mode) but large
enough that it is not susceptible to noise. If R2 is 200 k, and
VFB is 0.5V, the current through the resistor feedback network
will be 2.5 µA. The output voltage of the adjustable parts
ranges from 1.1V to 3.3V.
The formula for output voltage selection is:
VOUT: output voltage (volts)
VFB : feedback voltage = 0.5V
R1: feedback resistor from VOUT to FB
R2: feedback resistor from FB to GND
For any output voltage greater than or equal to 1.1V, a zero
must be added around 45 kHz for stability. The formula for
calculation of C1 is:
For output voltages higher than 2.5V, a pole must be placed
at 45 kHz as well. If the pole and zero are at the same fre-
quency the formula for calculation of C2 is:
The formula for location of zero and pole frequency created
by adding C1 and C2 is given below. By adding C1, a zero as
well as a higher frequency pole is introduced.
See the "LM3671-ADJ configurations for various VOUT" table.
17 www.national.com
LM3671
LM3671-ADJ Configurations For Various VOUT (Circuit of Figure 2)
(Refer to Note 11 for VIN requirements)
VOUT(V) R1(kΩ) R2 (kΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT(µF)
0.90 160 200 22 none 2.2 4.7 10
1.1 240 200 15 none 2.2 4.7 10
1.2 280 200 12 none 2.2 4.7 10
1.3 320 200 12 none 2.2 4.7 10
1.5 357 178 10 none 2.2 4.7 10
1.6 442 200 8.2 none 2.2 4.7 10
1.7 432 178 8.2 none 2.2 4.7 10
1.8 464 178 8.2 none 2.2 4.7 10
1.875 523 191 6.8 none 2.2 4.7 10
2.5 402 100 8.2 none 2.2 4.7 10
2.8 464 100 8.2 33 2.2 4.7 10
3.3 562 100 6.8 33 2.2 4.7 10
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple should be small enough to achieve the desired output
voltage ripple. Different saturation current rating specifica-
tions are followed by different manufacturers so attention
must be given to details. Saturation current ratings are typi-
cally specified at 25°C. However, ratings at the maximum
ambient temperature of application should be requested from
the manufacturer. The minimum value of inductance to
guarantee good performance is 1.76µH at ILIM (typ.) dc
current over the ambient temperature range. Shielded in-
ductors radiate less noise and should be preferred.
There are two methods to choose the inductor saturation cur-
rent rating.
Method 1:
The saturation current should be greater than the sum of the
maximum load current and the worst case average to peak
inductor current. This can be written as
IRIPPLE: average to peak inductor current
IOUTMAX: maximum load current (600mA)
VIN: maximum input voltage in application
L : min inductor value including worst case tolerances
(30% drop can be considered for method 1)
f : minimum switching frequency (1.6Mhz)
VOUT: output voltage
Method 2:
A more conservative and recommended approach is to
choose an inductor that has a saturation current rating greater
than the maximum current limit of 1150mA.
A 2.2 µH inductor with a saturation current rating of at least
1150 mA is recommended for most applications.The
inductor’s resistance should be less than 0.3 for good effi-
ciency. Table 1 lists suggested inductors and suppliers. For
low-cost applications, an unshielded bobbin inductor could be
considered. For noise critical applications, a toroidal or shield-
ed-bobbin inductor should be used. A good practice is to lay
out the board with overlapping footprints of both types for de-
sign flexibility. This allows substitution of a low-noise shielded
inductor, in the event that noise from low-cost bobbin models
is unacceptable.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 µF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible to
the VIN pin of the device. A larger value may be used for im-
proved input voltage filtering. Use X7R or X5R types; do not
use Y5V. DC bias characteristics of ceramic capacitors must
be considered when selecting case sizes like 0805 and 0603.
The minimum input capacitance to guarantee good per-
formance is 2.2µF at 3V dc bias; 1.5µF at 5V dc bias
including tolerances and over ambient temperature
range. The input filter capacitor supplies current to the PFET
switch of the LM3671 in the first half of each cycle and re-
duces voltage ripple imposed on the input power source. A
ceramic capacitor’s low ESR provides the best noise filtering
of the input voltage spikes due to this rapidly changing cur-
rent. Select a capacitor with sufficient ripple current rating.
The input current ripple can be calculated as:
www.national.com 18
LM3671
TABLE 1. Suggested Inductors and Their Suppliers
Model Vendor Dimensions LxWxH(mm) D.C.R (max)
DO3314-222MX Coilcraft 3.3 x 3.3 x 1.4 200 m
LPO3310-222MX Coilcraft 3.3 x 3.3 x 1.0 150 m
ELL5GM2R2N Panasonic 5.2 x 5.2 x 1.5 53 m
CDRH2D14NP-2R2NC Sumida 3.2 x 3.2 x 1.55 94 m
OUTPUT CAPACITOR SELECTION
A ceramic output capacitor of 10 µF, 6.3V is sufficient for most
applications. Use X7R or X5R types; do not use Y5V. DC bias
characteristics of ceramic capacitors must be considered
when selecting case sizes like 0805 and 0603. DC bias char-
acteristics vary from manufacturer to manufacturer and dc
bias curves should be requested from them as part of the ca-
pacitor selection process.
The minimum output capacitance to guarantee good per-
formance is 5.75µF at 1.8V dc bias including tolerances
and over ambient temperature range. The output filter ca-
pacitor smoothes out current flow from the inductor to the
load, helps maintain a steady output voltage during transient
load changes and reduces output voltage ripple. These ca-
pacitors must be selected with sufficient capacitance and
sufficiently low ESR to perform these functions.
The output voltage ripple is caused by the charging and dis-
charging of the output capacitor and by the RESR and can be
calculated as:
Voltage peak-to-peak ripple due to capacitance can be ex-
pressed as follow:
Voltage peak-to-peak ripple due to ESR can be expressed as
follow:
VPP-ESR = (2 * IRIPPLE) * RESR
Because these two components are out of phase the rms (root
mean squared) value can be used to get an approximate val-
ue of peak-to-peak ripple.
The peak-to-peak ripple voltage, rms value can be expressed
as follow:
Note that the output voltage ripple is dependent on the induc-
tor current ripple and the equivalent series resistance of the
output capacitor (RESR).
The RESR is frequency dependent (as well as temperature
dependent); make sure the value used for calculations is at
the switching frequency of the part.
TABLE 2. Suggested Capacitors and Their Suppliers
Model Type Vendor Voltage Rating Case Size
Inch (mm)
4.7 µF for CIN
C2012X5R0J475K Ceramic, X5R TDK 6.3V 0805 (2012)
JMK212BJ475K Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012)
GRM21BR60J475K Ceramic, X5R Murata 6.3V 0805 (2012)
C1608X5R0J475K Ceramic, X5R TDK 6.3V 0603 (1608)
10 µF for COUT
GRM21BR60J106K Ceramic, X5R Murata 6.3V 0805 (2012)
JMK212BJ106K Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012)
C2012X5R0J106K Ceramic, X5R TDK 6.3V 0805 (2012)
C1608X5R0J106K Ceramic, X5R TDK 6.3V 0603 (1608)
Micro SMD PACKAGE ASSEMBLY AND USE
Use of the Micro SMD package requires specialized board
layout, precision mounting and careful re-flow techniques, as
detailed in National Semiconductor Application Note 1112.
Refer to the section "Surface Mount Technology (SMD) As-
sembly Considerations". For best results in assembly, align-
ment ordinals on the PC board should be used to facilitate
placement of the device. The pad style used with Micro SMD
package must be the NSMD (non-solder mask defined) type.
This means that the solder-mask opening is larger than the
pad size. This prevents a lip that otherwise forms if the solder-
mask and pad overlap, from holding the device off the surface
of the board and interfering with mounting. See Application
Note 1112 for specific instructions how to do this. The 5-Bump
package used for LM3671 has 300 micron solder balls and
requires 10.82 mils pads for mounting on the circuit board.
The trace to each pad should enter the pad with a 90° entry
angle to prevent debris from being caught in deep corners.
Initially, the trace to each pad should be 7 mil wide, for a sec-
tion approximately 7 mil long or longer, as a thermal relief.
Then each trace should neck up or down to its optimal width.
The important criteria is symmetry. This ensures the solder
bumps on the LM3671 re-flow evenly and that the device sol-
ders level to the board. In particular, special attention must be
paid to the pads for bumps A1 and A3, because VIN and GND
19 www.national.com
LM3671
are typically connected to large copper planes, inadequate
thermal relief can result in late or inadequate re-flow of these
bumps.
The Micro SMD package is optimized for the smallest possi-
ble size in applications with red or infrared opaque cases.
Because the Micro SMD package lacks the plastic encapsu-
lation characteristic of larger devices, it is vulnerable to light.
Backside metallization and/or epoxy coating, along with front-
side shading by the printed circuit board, reduce this sensi-
tivity. However, the package has exposed die edges. In
particular, Micro SMD devices are sensitive to light, in the red
and infrared range, shining on the package’s exposed die
edges.
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter de-
sign. 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, re-
sulting in poor regulation or instability.
Good layout for the LM3671 can be implemented by following
a few simple design rules below. Refer to Figure 9 for top layer
board layout.
1. Place the LM3671, inductor and filter capacitors close
together and make the traces short. The traces between
these components carry relatively high switching
currents and act as antennas. Following this rule reduces
radiated noise. Special care must be given to place the
input filter capacitor very close to the VIN and GND pin.
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 LM3671 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 LM3671 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 magnetic
field reversal between the two half-cycles and reduces
radiated noise.
3. Connect the ground pins of the LM3671 and filter
capacitors together using generous component-side
copper 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 LM3671 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
feedback path, away from noisy traces between the
power components. The voltage feedback trace must
remain close to the LM3671 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. A good approach is to route the
feedback trace on another layer and to have a ground
plane between the top layer and layer on which the
feedback trace is routed. In the same manner for the
adjustable part it is desired to have the feedback dividers
on the bottom layer.
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, 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 cir-
cuitry is shielded with a metal pan and power to it is post-
regulated to reduce conducted noise, using low-dropout
linear regulators.
20108449
FIGURE 10. Top layer board layout for SOT23-5
www.national.com 20
LM3671
21 www.national.com
LM3671
Physical Dimensions inches (millimeters) unless otherwise noted
5-Lead SOT23-5 Package
NS Package Number MF05A
5-Bump (Large) Micro SMD Package, 0.5mm Pitch
NS Package Number TLA05CBA
The dimensions for X1, X2, and X3 are as given:
X1 = 1.057 mm +/- 0.030mm
X2 = 1.387 mm +/- 0.030mm
X3 = 0.600 mm +/- 0.075mm
www.national.com 22
LM3671
6-Pin LLP Package, 0.5mm Pitch
NS Package Number LCA06B
The dimensions for X1, X2, and X3 are as given:
X1 = 2.000 mm +/- 0.100mm
X2 = 2.000 mm +/- 0.100mm
X3 = 0.600 mm +/- 0.075mm
23 www.national.com
LM3671
Notes
LM3671 2MHz, 600mA Step-Down DC-DC Converter
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