NSC LM3670MF-3.3

LM3670
Miniature Step-Down DC-DC Converter for Ultra Low
Voltage Circuits
General Description
Features
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).
n VOUT = Adj (0.7V min), 1.2, 1.5, 1.6, 1.8, 1.875, 2.5,
3.3V
n 2.5V ≤ VIN ≤ 5.5V
n 15 µA typical quiescent current
n 350 mA maximum load capability
n 1 MHz PWM fixed switching frequency (typ.)
n Automatic PFM/PWM mode switching
n Available in fixed output voltages as well as an
adjustable version
n SOT23-5 package
n Low drop out operation - 100% duty cycle mode
n Internal synchronous rectification for high efficiency
n Internal soft start
n 0.1 µA typical shutdown current
n Operates from a single Li-Ion cell or 3 cell NiMH/NiCd
batteries
n Only three tiny surface-mount external components
required (one inductor, two ceramic capacitors)
n Current overload protection
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 low-current
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 surfacemount components, an inductor and two ceramic capacitors,
are required.
Applications
n
n
n
n
n
n
Mobile phones
HandHeld
PDAs
Palm-top PCs
Portable Instruments
Battery Powered Devices
Typical Application
20075801
FIGURE 1. Fixed Output Voltage - Typical Application Circuit
© 2006 National Semiconductor Corporation
DS200758
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LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
January 2006
LM3670
Typical Application
(Continued)
20075830
FIGURE 2. Adjustable Output Voltage - Typical Application Circuit
Connection Diagram and Package Mark Information
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
Pin Descriptions
Pin #
Name
1
VIN
2
GND
3
EN
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.
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Description
Power supply input. Connect to the input filter capacitor (Figure 1).
Ground pin.
Enable input.
2
Voltage Option
(V)
3.3
Order Number
(Level 95)
SPEC
LM3670MF-3.3
NOPB
LM3670MFX-3.3
NOPB
Package Marking
Supplied As
(#/reel)
SDEB
1000
3000
LM3670MF-3.3
1000
LM3670MFX-3.3
2.5
3000
LM3670MF-2.5
NOPB
LM3670MFX-2.5
NOPB
SDDB
3000
LM3670MF-2.5
1000
LM3670MFX-2.5
1.875
3000
LM3670MF-1.875
NOPB
LM3670MFX-1.875
NOPB
SEFB
LM3670MF-1.875
1000
3000
LM3670MF-1.8
NOPB
LM3670MFX-1.8
NOPB
SDCB
1000
LM3670MFX-1.8
1.5
3000
LM3670MF-1.6
NOPB
LM3670MFX-1.6
NOPB
SDBB
3000
1000
LM3670MFX-1.6
3000
LM3670MF-1.5
NOPB
LM3670MFX-1.5
NOPB
S82B
1000
3000
1000
LM3670MFX-1.5
3000
LM3670MF-1.2
NOPB
LM3670MFX-1.2
NOPB
SCZB
1000
3000
LM3670MF-1.2
1000
LM3670MFX-1.2
Adjustable
1000
LM3670MF-1.6
LM3670MF-1.5
1.2
1000
3000
LM3670MF-1.8
1.6
1000
3000
LM3670MFX-1.875
1.8
1000
3000
LM3670MF-ADJ
NOPB
LM3670MFX-ADJ
NOPB
SDFB
1000
3000
LM3670MF-ADJ
1000
LM3670MFX-ADJ
3000
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LM3670
Ordering Information
LM3670
Absolute Maximum Ratings (Note 1)
ESD Rating (Note 3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Human Body Model:
VIN Pin: Voltage to GND
EN Pin: Voltage to GND
FB, SW Pin:
VIN, SW, FB, EN, GND
2.0kV
Machine Model:
200V
−0.2V to 6.0V
−0.2V to 6.0V
Operating Ratings (Notes 1, 2)
(GND−0.2V) to
(VIN + 0.2V)
Input Voltage Range
2.5V to 5.5V
Junction Temperature (TJ-MAX)
−45˚C to +125˚C
Recommended Load Current
0A to 350 mA
Storage Temperature Range
−45˚C to +150˚C
Junction Temperature (TJ) Range
−40˚C to +125˚C
Maximum Lead Temperature
(Soldering, 10 sec.)
260˚C
Ambient Temperature (TA) Range
−40˚C to +85˚C
Thermal Properties
Junction-to-Ambient
Thermal Resistance (θJA)
(SOT23-5)
250˚C/W
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
Max
Units
VIN
Symbol
Input Voltage Range
Parameter
(Note 5)
2.5
5.5
V
VOUT
Fixed Output Voltage: 1.2V
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
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
(Note 4)
Condition
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
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Min
Typ
%
%
%
%/V
0.0014
%/mA
0.1
4
%
0.26
0.5
TA=85oC
%
V
1
µA
Symbol
Parameter
IQ
DC Bias Current into VIN
VUVLO
Minimum VIN below which VOUT
will be disabled
Condition
Min
No load, device is not switching
(VOUT forced higher than
programmed output voltage)
Typ
Max
Units
15
30
µA
V
2.4
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
ILIM
Switch Peak Current Limit
η
Efficiency
(VIN = 3.6V, VOUT = 1.8V)
400
Logic High Input
VIL
Logic Low Input
IEN
Enable (EN) Input Current
FOSC
Internal Oscillator Frequency
1.5
µA
750
mA
ILOAD = 1 mA
91
ILOAD = 10 mA
94
ILOAD = 100 mA
94
ILOAD = 200 mA
94
ILOAD = 300 mA
92
ILOAD = 350 mA
VIH
0.1
620
%
90
1.3
V
0.4
PWM Mode
550
V
0.01
1
µA
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 kΩ resistor 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 = ( VOUT + VDROP OUT) to 5.5V for 1.875 ≤ VOUT≤ 3.3V
Where VDROP OUT = ILOAD * (RDSON (P) + RINDUCTOR)
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LM3670
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 (Continued)
LM3670
20075832
FIGURE 4. Simplified Functional Diagram
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LM3670
Typical Performance Characteristics
(unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
IQ (Non-switching) vs. VIN
IQ vs. Temp
20075805
20075804
VOUT vs. VIN
VOUT vs. IOUT
20075806
20075807
Efficiency vs. IOUT
Efficiency vs. VIN
20075808
20075809
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LM3670
Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
(Continued)
RDSON vs. VIN
P & N Channel
Frequency vs. Temperature
20075810
20075811
Line Transient
(VIN = 3.6V to 4.6V , ILOAD = 100 mA)
Line Transient
(VIN = 2.6V to 3.6V, ILOAD = 100 mA)
20075812
20075813
Load Transient
ILOAD = 0mA to 70mA
Load Transient
ILOAD = 3mA to 280mA
20075817
20075816
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Load Transient
ILOAD = 0mA to 280mA
(Continued)
Load Transient
ILOAD = 0mA to 350mA
20075818
20075819
Load Transient
ILOAD = 100mA to 300mA
Load Transient
ILOAD = 50mA to 350mA
20075820
20075821
PWM Mode
VSW, VOUT, IINDUCTOR vs. Time
PFM Mode
VSW, VOUT, IINDUCTOR vs. Time
20075822
20075823
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LM3670
Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
LM3670
Typical Performance Characteristics (unless otherwise stated: VIN= 3.6V, VOUT= 1.8V)
(Continued)
Soft Start
VIN, VOUT, IINDUCTOR vs. Time
(ILOAD = 350mA)
20075824
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
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.
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 voltagemode 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.
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
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.
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10
LM3670
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
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 fixedfrequency PWM mode.
B. The peak PMOS switch current drops below the IMODE
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 IPFM level set for PFM mode. The peak current
in PFM mode is:
20075803
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
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
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LM3670
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 controlled down to the lowest possible input voltage.
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.
The minimum input voltage needed to support the output
voltage is
INDUCTOR SELECTION
• ILOAD
• RDSON,
PFET
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.
Load current
Drain to source resistance of PFET switch
in the triode region
There are two methods to choose the inductor current rating.
Method 1:
• RINDUCTOR Inductor resistance
The total current is the sum of the load and the inductor
ripple current. This can be written as
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:
• 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
• 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:
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 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.
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:
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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
12
LM3670
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 functions.
Voltage peak-to-peak ripple, root mean squared =
The output ripple current can be calculated as:
Voltage peak-to-peak ripple due to capacitance =
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.
Voltage peak-to-peak ripple due to ESR =
TABLE 2. Suggested Capacitors and Their Suppliers
Model
Type
Vendor
Phone
FAX
10 µF for COUT
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 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 3. Adjustable LM3670 Configurations for Various VOUT
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
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LM3670
Application Information
(Continued)
TABLE 3. Adjustable LM3670 Configurations for Various VOUT (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
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806
200
22
82
14
10
4.7
22
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.
(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
20075831
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 .
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 noisesensitive 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,
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
15
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LM3670
Application Information
LM3670
Application Information
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 lowdropout linear regulators.
(Continued)
arrange the CMOS digital circuitry around it (since this also
generates noise), and then place sensitive preamplifiers and
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16
inches (millimeters) unless otherwise noted
5-Lead SOT23-5 Package
NS Package Number MF05A
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LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
Physical Dimensions