TI1 LM3670 Miniature step-down dc-dc converter for ultra low voltage circuit Datasheet

LM3670
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SNVS250D – NOVEMBER 2004 – REVISED JANUARY 2006
LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
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FEATURES
1
•
2
•
•
•
•
•
•
•
•
•
•
•
VOUT = Adj (0.7V min), 1.2, 1.5, 1.6, 1.8, 1.875,
2.5, 3.3V
2.5V ≤ VIN ≤ 5.5V
15 µA typical quiescent current
350 mA maximum load capability
1 MHz PWM fixed switching frequency (typ.)
Automatic PFM/PWM mode switching
Available in fixed output voltages as well as an
adjustable version
SOT23-5 package
Low drop out operation - 100% duty cycle
mode
Internal synchronous rectification for high
efficiency
Internal soft start
0.1 µA typical shutdown current
•
•
•
Operates from a single Li-Ion cell or 3 cell
NiMH/NiCd batteries
Only three tiny surface-mount external
components required (one inductor, two
ceramic capacitors)
Current overload protection
APPLICATIONS
•
•
•
•
•
•
Mobile phones
HandHeld
PDAs
Palm-top PCs
Portable Instruments
Battery Powered Devices
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
complex power management systems. Automatic intelligent switching between PWM low-noise and PFM lowcurrent mode offers improved system control. During full-power operation, 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 reduction of the quiescent current to 15 µA (typ) during light current loads and system
standby. Internal synchronous rectification 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 consumption 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.
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2004–2006, Texas Instruments Incorporated
LM3670
SNVS250D – NOVEMBER 2004 – REVISED JANUARY 2006
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Typical Application
VIN
2.5V to 5.5V
L1:10uH
VIN
CIN
4.7uF
1
5
VOUT
SW
COUT
10uF
LM3670
GND
2
EN
FB
3
4
Figure 1. Fixed Output Voltage - Typical Application Circuit
L1: 4.7 µH
or 10 µH *
VIN
2.5V to 5.5V
VIN
1
5
* See Adj table
VOUT
SW
LM3670
CIN: 4.7 µF
GND
2
C1
R1
C2
R2
COUT: 10 µF
FB
EN
3
4
Figure 2. Adjustable Output Voltage - Typical Application Circuit
Connection Diagram and Package Mark Information
SW
5
VIN
1
FB
4
GND
2
EN
3
NOTE: The actual physical placement of the package marking will vary from part to part.
Figure 3. SOT23-5 Package (Top View)
Table 1. Pin Descriptions
Pin #
2
Name
Description
1
VIN
2
GND
Power supply input. Connect to the input filter capacitor (Figure 1).
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.
Ground pin.
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings
(1)
−0.2V to 6.0V
VIN Pin: Voltage to GND
−0.2V to 6.0V
EN Pin: Voltage to GND
FB, SW Pin:
(GND−0.2V) to (VIN + 0.2V)
−45°C to +125°C
Junction Temperature (TJ-MAX)
−45°C to +150°C
Storage Temperature Range
Maximum Lead Temperature
ESD Rating
(Soldering, 10 sec.)
260°C
(2)
Human Body Model:
VIN, SW, FB, EN, GND
2.0kV
Machine Model:
200V
(1)
(2)
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.
The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF
capacitor discharged directly into each pin. MIL-STD-883 3015.7
Operating Ratings
(1) (2)
Input Voltage Range
2.5V to 5.5V
Recommended Load Current
0A to 350 mA
Junction Temperature (TJ) Range
−40°C to +125°C
Ambient Temperature (TA) Range
−40°C to +85°C
(1)
(2)
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.
All voltages are with respect to the potential at the GND pin.
Thermal Properties
Junction-to-Ambient Thermal Resistance (θJA) (SOT23-5)
250°C/W
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Electrical Characteristics
Limits in standard typeface are for TJ = 25°C. Limits in boldface type apply over the full operating junction temperature range
(−40°C ≤ TJ ≤ +125°C). Unless otherwise noted VIN = 3.6V, VOUT = 1.8V, IO = 150mA, EN = VIN
Symbol
Parameter
VIN
Input Voltage Range
VOUT
Fixed Output Voltage: 1.2V
Fixed Output Voltage: 1.5V
Fixed Output Voltage: 1.6V,
1.875V
Fixed Output Voltage: 1.8V
Fixed Output Voltage: 2.5V, 3.3V
Adjustable Output Voltage
(2)
Condition
Max
Units
2.5
5.5
V
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
-2.0
+4.0
%
2.5V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 150 mA
-4.5
+4.0
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
-2.5
+4.0
2.5V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 350 mA
-5.0
+4.0
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
-2.5
+4.0
2.5V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 350 mA
-5.5
+4.0
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
-1.5
+3.0
2.5V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 350 mA
−4.5
+3.0
3.6V ≤ VIN ≤ 5.5V
IO = 10 mA
-2.0
+4.0
3.6V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 350 mA
-6.0
+4.0
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
-2.5
+4.5
2.5V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 150 mA
-4.0
+4.5
(1)
Min
Typ
%
%
%
%
%
Line_reg
Line Regulation
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
Load_reg
Load Regulation
150 mA ≤ IO ≤ 350 mA
VREF
Internal Reference Voltage
IQ_SHDN
Shutdown Supply Current
TA=85ºC
0.1
1
µA
IQ
DC Bias Current into VIN
No load, device is not switching
(VOUT forced higher than
programmed output voltage)
15
30
µA
VUVLO
Minimum VIN below which VOUT
will be disabled
RDSON (P)
Pin-Pin Resistance for PFET
VIN=VGS=3.6V
360
690
mΩ
RDSON (N)
Pin-Pin Resistance for NFET
VIN=VGS=3.6V
250
660
mΩ
ILKG
(P)
P Channel Leakage Current
VDS=5.5V
0.1
1
µA
ILKG
(N)
N Channel Leakage Current
VDS=5.5V
0.1
1.5
µA
620
750
mA
ILIM
Switch Peak Current Limit
η
Efficiency
(VIN = 3.6V, VOUT = 1.8V)
Logic High Input
VIL
Logic Low Input
(1)
(2)
4
%/V
0.0014
%/mA
0.5
V
V
2.4
400
ILOAD = 1 mA
91
ILOAD = 10 mA
94
ILOAD = 100 mA
94
ILOAD = 200 mA
94
ILOAD = 300 mA
92
ILOAD = 350 mA
VIH
0.26
%
90
1.3
V
0.4
V
The input voltage range recommended for the specified output voltages are given below: VIN = 2.5V to 5.5V for 0.7V ≤ VOUT <
1.875VVIN = ( VOUT + VDROP OUT) to 5.5V for 1.875 ≤ VOUT≤ 3.3VWhere VDROP OUT = ILOAD * (RDSON (P) + RINDUCTOR)
Output voltage specification for the adjustable version includes tolerance of the external resistor divider.
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Electrical Characteristics (continued)
Limits in standard typeface are for TJ = 25°C. Limits in boldface type apply over the full operating junction temperature range
(−40°C ≤ TJ ≤ +125°C). Unless otherwise noted VIN = 3.6V, VOUT = 1.8V, IO = 150mA, EN = VIN
Symbol
Parameter
Condition
IEN
Enable (EN) Input Current
FOSC
Internal Oscillator Frequency
PWM Mode
Min
550
Typ
Max
Units
0.01
1
µA
1000
1300
kHz
VIN
EN
SW
Current Limit
Comparator
Ramp
Generator
+
Undervoltage
Lockout
Soft
Start
Ref1
PFM Current
Comparator
+
Thermal
Shutdown
1 MHz
Oscillator
Bandgap
Ref2
PWM Comparator
Error
Amp
+
-
Control Logic
Driver
pfm_low
0.5V
+
-
pfm_hi
+
-
Vcomp
1.0V
+
VREF
Zero Crossing
Comparator
Frequency
Compensation
Adj Version
Fixed Version
GND
FB
Figure 4. Simplified Functional Diagram
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Typical Performance Characteristics
(unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
IQ (Non-switching) vs. VIN
IQ vs. Temp
0.1
TA = 85°C
ISHUTDOWN (PA)
NO LOAD IQUIESCENT (PA)
20
TA = 25°C
15
TA = -40°C
10
2.5
3
3.5
4
4.5
5
0.05
0
-40
5.5
-20
0
VOUT vs. VIN
1.86
1.84
VOUT (V)
VOUT (V)
IOUT = 150 mA
PWM mode
PFM Mode
1.82
PWM Mode
1.8
1.78
1.76
VIN = 5.5V
1.78
VIN = 2.5V
1.74
VIN = 3.6V
1.72
1.7
-20
0
20
40
60
0
80
50
100
Efficiency vs. IOUT
200
250
300
350
Efficiency vs. VIN
100
90
150
ILOAD (mA)
TEMPERATURE (°C)
95
80
1.88
VIN = 3.6V
IOUT = 10 mA
PFM mode
1.79
1.77
-40
60
1.9
1.81
1.80
40
VOUT vs. IOUT
1.83
1.82
20
TEMPERATURE (°C)
VIN (V)
100
VIN = 2.7V
ILOAD = 150 mA
95
EFFICIENCY (%)
EFFICIENCY (%)
85
80
75
VIN = 5.0V
70
65
60
90
ILOAD = 1 mA
85
ILOAD = 300 mA
80
VIN = 3.7V
55
50
75
45
40
-2
10
10
-1
10
0
10
1
10
2
10
3
ILOAD (mA)
6
70
2.5
3
3.5
4
4.5
5
5.5
6
VIN (V)
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Typical Performance Characteristics (continued)
(unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
RDSON vs. VIN
P & N Channel
0.8
1010
1000 ILOAD = 150 mA
990
VIN = 3.6V
980
VIN = 5.5V
970
960
950
VIN = 2.5V
940
930
920
910
900
890
880
870
860
850
840
-40
-20
0 10 20 30 40 50 60 70 80
-30
-10
P FET
N FET
0.7
RDSon - N, P CHANNEL (:)
FREQUENCY (kHz)
Frequency vs. Temperature
TA = 85°C
TA = 25°C
TA = -40°C
0.6
0.5
0.4
0.3
0.2
0.1
2.5
3
3.5
4
4.5
5
5.5
VIN (V)
TEMPERATURE (°C)
Line Transient
(VIN = 2.6V to 3.6V, ILOAD = 100 mA)
Line Transient
(VIN = 3.6V to 4.6V , ILOAD = 100 mA)
IOUT = 100 mA
VIN = 3.6V
VIN = 4.6V
VIN rise time = 10 Ps
VIN = 3.6V
LINE TRANSIENT
LINE TRANSIENT
VIN = 2.6V
VOUT = 1.8V
(20 mV/
Div)
TIME (200 Ps/DIV)
TIME (100 Ps/DIV)
Load Transient
ILOAD = 0mA to 70mA
CURRENT LOAD STEP (0 mA - 70 mA)
Load Transient
ILOAD = 3mA to 280mA
CURRENT LOAD STEP (3 mA - 280 mA)
VOUT = 1.8V
(20 mV/
Div)
VOUT (50 mV/Div)
ILOAD = 280 mA
ILOAD = 3 mA
VOUT (50 mV/Div)
Inductor Current = 200 mA/Div
ILOAD = 70 mA
ILOAD = 0 mA
TIME (100 Ps/DIV)
TIME (100 Ps/DIV)
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Typical Performance Characteristics (continued)
(unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
Load Transient
ILOAD = 0mA to 350mA
CURRENT LOAD STEP (0 mA - 350 mA)
CURRENT LOAD STEP (0 mA - 280 mA)
Load Transient
ILOAD = 0mA to 280mA
VOUT (50 mV/Div)
ILOAD = 280 mA
ILOAD = 0 mA
VOUT (50 mV/Div)
ILOAD = 350 mA
ILOAD = 0 mA
TIME (100 Ps/DIV)
TIME (100 Ps/DIV)
Load Transient
ILOAD = 100mA to 300mA
CURRENT LOAD STEP (100 mA - 300 mA)
CURRENT LOAD STEP (50 mA - 350 mA)
Load Transient
ILOAD = 50mA to 350mA
VOUT (50 mV/Div)
ILOAD = 350 mA
ILOAD = 50 mA
TIME (100 Ps/DIV)
ILOAD = 300 mA
ILOAD = 100 mA
PWM Mode
VSW, VOUT, IINDUCTOR vs. Time
ILOAD = 150 mA
VSWITCH
(5V/Div)
VOUT
(20 mV/Div)
PWM MODE
PFM MODE
Inductor Current = 200 mA/
Div
TIME (100 Ps/DIV)
PFM Mode
VSW, VOUT, IINDUCTOR vs. Time
Inductor Current
(100 mA/Div)
VSWITCH
(5V/Div)
VOUT
(20 mV/Div)
Inductor Current
(200 mA/Div)
TIME (1 Ps/DIV)
TIME (2 Ps/DIV)
8
VOUT (50 mV/Div)
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Typical Performance Characteristics (continued)
(unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
CURRENT LOAD STEP (3 mA - 280 mA)
Soft Start
VIN, VOUT, IINDUCTOR vs. Time
(ILOAD = 350mA)
VIN (2V/Div)
VOUT (1V/Div)
Inductor
Current
(200
mA/
Div)
TIME (100 Ps/DIV)
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 current 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 (IQ = 15 µA typ) and a longer battery life. Shutdown mode turns off the device, offering the lowest
current consumption (ISHUTDOWN = 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
VIN-VOUT
(1)
L
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
-VOUT
(2)
L
The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage
across the load.
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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 inversely 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
current to maintain high efficiency.
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 IMODE level:
IMODE < 26 mA +
VIN
(typ)
(3)
50:
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 IPFM level
set for PFM mode. The peak current in PFM mode is:
IPFM Peak = 117 mA +
VIN
64:
(typ)
(4)
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 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 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.
10
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Z
A
x
i
s
PFM Mode at Light Load
High PFM Threshold
~1.016*Vout
Load current
increases
ZAx
is
High PFM
Voltage
Threshold
reached,
go into
sleep mode
Low2 PFM Threshold,
switch back to PWMmode
Current load
increases,
draws Vout
towards
Low2 PFM
Threshold
Low PFM
Threshold,
turn on
PFET
xis
Z-A
Pfet on
until
Ipfm limit
reached
Nfet on
drains
conductor
current
until
I inductor=0
Low1 PFM Threshold
~1.008*Vout
Low2 PFM Threshold
Vout
PWM Mode at
Moderate to Heavy
Loads
Figure 5. Operation in PFM Mode and Transition to 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)
0
32
70
224
140
256
280
256
620
until soft start ends
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 controlled down to the lowest possible input
voltage.
The minimum input voltage needed to support the output voltage is
VIN,MIN = ILOAD * (RDSON,PFET + RINDUCTOR) + VOUT
(5)
•
ILOAD
Load current
•
RDSON, PFET
Drain to source resistance of PFET switch in the triode region
•
RINDUCTOR
Inductor resistance
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Application Information
OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE LM3670
The output voltage of the adjustable parts can be programmed through the resistor network connected from VOUT
to VFB then to GND. VOUT is adjusted to make VFB equal to 0.5V. The resistor from VFB to GND (R2) should be at
least 100KΩ to 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 R2 is 200KΩ, and VFB is 0.5V, then
the current through the resistor feedback network is 2.5µA (IFB =0.5V/R2). The output voltage formula is:
VOUT = VFB * (
•
•
•
•
R1
R2
+ 1)
(6)
VOUT Output Voltage (V)
VFB Feedback Voltage (0.5V typ)
R1 Resistor from VOUT to VFB (Ω)
R2 Resistor from VOUT 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:
C1 =
1
2 *S * R1 *10 kHz
(7)
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 increases, the duty cycle decreases even further. Since the duty cycle is so low any change due to noise
is an appreciable percentage. In other words, it is susceptible to noise. Capacitors C1 and C2 act as noise filters
rather than frequency poles and zeros. If the pole and zero are at the same frequency the formula is:
C2 =
1
2 *S * R2 *10 kHz
(8)
A pole can also be used at higher output voltages. For example, in the table Table 4, 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 inductor; 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
IMAX = ILOAD +
IRIPPLE
VOUT = ILOAD + (
•
•
•
•
•
2
VIN-VOUT
2 *L
(9)
)(
VOUT
VIN
)(
1
f
)
(10)
ILOAD load current
VIN input voltage
L inductor
f switching frequency
IRIPPLE 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 (IPP) the inductor needs to be at least
12
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L >= (
SNVS250D – NOVEMBER 2004 – REVISED JANUARY 2006
VIN - VOUT
I PP
)*(
VOUT
VIN
1
)*( )
f
(11)
A 10 µH inductor with a saturation current rating of at least 800 mA is recommended for most applications. The
inductor’s resistance should be less than around 0.3Ω for good efficiency. Table 2 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 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:
I RMS = I OUTMAX *
VOUT
VIN
The worst case IRMS is:
IRMS
IRMS =
2
* (1 -
VOUT
VIN
)
(duty cycle = 50%)
(12)
Table 2. 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 functions.
The output ripple current can be calculated as:
Voltage peak-to-peak ripple due to capacitance =
VPP-C =
IPP
f*8*C
(13)
Voltage peak-to-peak ripple due to ESR =
VOUT = VPP-ESR = IPP * RESR
(14)
Voltage peak-to-peak ripple, root mean squared =
VPP-RMS =
VPP-C2 + VPP-ESR2
(15)
Note that the output ripple is dependent on the current ripple and the equivalent series resistance of the output
capacitor (RESR).
Because these two components are out of phase the rms value is used. The RESR is frequency dependent (as
well as temperature dependent); make sure the frequency of the RESR given is the same order of magnitude as
the switching frequency.
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LM3670
SNVS250D – NOVEMBER 2004 – REVISED JANUARY 2006
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Table 3. Suggested Capacitors and Their Suppliers
Model
Type
Vendor
Phone
FAX
10 µF for COUT
VJ1812V106MXJAT
Ceramic2
Vishay3
408-727-25004
408-330-4098 5
LMK432BJ106MM
Ceramic
Taiyo-Yuden
847-925-0888
847-925-0899
JMK325BJ106MM
Ceramic
Taiyo-Yuden
847-925-0888
847-925-0899
4.7 µF for CIN
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 4. Adjustable LM3670 Configurations for Various VOUT
(1)
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
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
2.5
806
200
22
82
10
4.7
22
(1)
(10 || 4.7)
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.
14
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SNVS250D – NOVEMBER 2004 – REVISED JANUARY 2006
The light shaded area is the top surface ground. COUT, CIN, Feedback R
and C grounds all come to this area which is as far away from the SW pin
as possible to avoid the noise created at the SW pin.
Note that the top and bottom GND sides are kept away from the SW pin to
EN,GND,VIN,FB,SW are
the pads for the SOT23-5
package
EN
POST
PIN
avoid picking up noise from the SW pin which swings from GND to VIN.
EN post pin is connected to EN with a bottom side trace to
maintain unbroken ground plane on top of board
VIN
GND
CIN
As many through holes
as possible here to
connect the top and
bottom ground planes
E
N
G
N
D
The VIN, SW, VOUT traces,
CIN, COUT traces & pads
should be thick - they are
high current paths
Bottom surface - the darker
shaded area is all GND EXCEPT
for area around SW to avoid
picking up switch noise.
SW node is switching
R2_fb
between VIN and GND at
1 MHz - VERY NOISY! keep all GNDs and GND
planes away!
C2_fb
COUT
SS
W
W
F
B
R1_fb
C1_fb
VOUT
If possible put the feedback Rs and Cs on the back side so the
COUT GND can move closer to the IC GND
L1
Figure 6. Board Layout Design Rules for the LM3670
Good layout for the LM3670 can be implemented by following 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 currents 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 magnetic field reversal between the two half-cycles and reduces radiated noise.
3. Connect the ground pins of the LM3670, 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 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 feedback 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
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LM3670
SNVS250D – NOVEMBER 2004 – REVISED JANUARY 2006
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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 circuitry is shielded with a
metal pan and power to it is post-regulated to reduce conducted noise, using low-dropout linear regulators.
16
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PACKAGE OPTION ADDENDUM
www.ti.com
17-Nov-2012
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package Qty
Drawing
Eco Plan
Lead/Ball Finish
(2)
MSL Peak Temp
Samples
(3)
(Requires Login)
LM3670MF-1.2
ACTIVE
SOT-23
DBV
5
1000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MF-1.2/NOPB
ACTIVE
SOT-23
DBV
5
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MF-1.5
ACTIVE
SOT-23
DBV
5
1000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MF-1.5/NOPB
ACTIVE
SOT-23
DBV
5
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MF-1.6
ACTIVE
SOT-23
DBV
5
1000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MF-1.6/NOPB
ACTIVE
SOT-23
DBV
5
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MF-1.8
ACTIVE
SOT-23
DBV
5
1000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MF-1.8/NOPB
ACTIVE
SOT-23
DBV
5
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MF-1.875
ACTIVE
SOT-23
DBV
5
1000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MF-1.875/NOPB
ACTIVE
SOT-23
DBV
5
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MF-3.3
ACTIVE
SOT-23
DBV
5
1000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MF-3.3/NOPB
ACTIVE
SOT-23
DBV
5
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MF-ADJ
ACTIVE
SOT-23
DBV
5
1000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MF-ADJ/NOPB
ACTIVE
SOT-23
DBV
5
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MFX-1.2
ACTIVE
SOT-23
DBV
5
3000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MFX-1.2/NOPB
ACTIVE
SOT-23
DBV
5
3000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MFX-1.5
ACTIVE
SOT-23
DBV
5
3000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MFX-1.5/NOPB
ACTIVE
SOT-23
DBV
5
3000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MFX-1.6
ACTIVE
SOT-23
DBV
5
3000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MFX-1.6/NOPB
ACTIVE
SOT-23
DBV
5
3000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MFX-1.8
ACTIVE
SOT-23
DBV
5
3000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MFX-1.8/NOPB
ACTIVE
SOT-23
DBV
5
3000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
Addendum-Page 1
PACKAGE OPTION ADDENDUM
www.ti.com
17-Nov-2012
Orderable Device
Status
(1)
Package Type Package Pins Package Qty
Drawing
Eco Plan
Lead/Ball Finish
(2)
MSL Peak Temp
Samples
(3)
(Requires Login)
LM3670MFX-1.875
ACTIVE
SOT-23
DBV
5
3000
TBD
CU SNPB
Level-1-260C-UNLIM
LM3670MFX-1.875/NOPB
ACTIVE
SOT-23
DBV
5
3000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM3670MFX-ADJ/NOPB
ACTIVE
SOT-23
DBV
5
3000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
Addendum-Page 2
PACKAGE MATERIALS INFORMATION
www.ti.com
17-Nov-2012
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
Package Package Pins
Type Drawing
SPQ
Reel
Reel
A0
Diameter Width (mm)
(mm) W1 (mm)
B0
(mm)
K0
(mm)
P1
(mm)
LM3670MF-1.2
SOT-23
DBV
5
1000
178.0
8.4
LM3670MF-1.2/NOPB
SOT-23
DBV
5
1000
178.0
LM3670MF-1.5
SOT-23
DBV
5
1000
178.0
LM3670MF-1.5/NOPB
SOT-23
DBV
5
1000
LM3670MF-1.6
SOT-23
DBV
5
LM3670MF-1.6/NOPB
SOT-23
DBV
LM3670MF-1.8
SOT-23
DBV
LM3670MF-1.8/NOPB
SOT-23
W
Pin1
(mm) Quadrant
3.2
3.2
1.4
4.0
8.0
Q3
8.4
3.2
3.2
1.4
4.0
8.0
Q3
8.4
3.2
3.2
1.4
4.0
8.0
Q3
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
DBV
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MF-1.875
SOT-23
DBV
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MF-1.875/NOPB
SOT-23
DBV
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MF-3.3
SOT-23
DBV
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MF-3.3/NOPB
SOT-23
DBV
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MF-ADJ
SOT-23
DBV
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MF-ADJ/NOPB
SOT-23
DBV
5
1000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.2
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.2/NOPB
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.5
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.5/NOPB
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
17-Nov-2012
Device
Package Package Pins
Type Drawing
SPQ
Reel
Reel
A0
Diameter Width (mm)
(mm) W1 (mm)
B0
(mm)
K0
(mm)
P1
(mm)
W
Pin1
(mm) Quadrant
LM3670MFX-1.6
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.6/NOPB
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.8
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.8/NOPB
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.875
SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-1.875/NOPB SOT-23
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
DBV
5
3000
178.0
8.4
3.2
3.2
1.4
4.0
8.0
Q3
LM3670MFX-ADJ/NOPB
SOT-23
*All dimensions are nominal
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
LM3670MF-1.2
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.2/NOPB
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.5
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.5/NOPB
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.6
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.6/NOPB
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.8
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.8/NOPB
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.875
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-1.875/NOPB
SOT-23
DBV
5
1000
203.0
190.0
41.0
Pack Materials-Page 2
PACKAGE MATERIALS INFORMATION
www.ti.com
17-Nov-2012
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
LM3670MF-3.3
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-3.3/NOPB
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-ADJ
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MF-ADJ/NOPB
SOT-23
DBV
5
1000
203.0
190.0
41.0
LM3670MFX-1.2
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.2/NOPB
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.5
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.5/NOPB
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.6
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.6/NOPB
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.8
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.8/NOPB
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.875
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-1.875/NOPB
SOT-23
DBV
5
3000
206.0
191.0
90.0
LM3670MFX-ADJ/NOPB
SOT-23
DBV
5
3000
206.0
191.0
90.0
Pack Materials-Page 3
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LM3670MF-1.2 LM3670MF-1.2EV LM3670MF-1.5 LM3670MF-1.6 LM3670MF-1.875 LM3670MF-1.8EV
LM3670MF-ADJ LM3670MFX-1.2 LM3670MFX-1.5 LM3670MFX-1.6 LM3670MFX-1.8 LM3670MFX-1.875