LM3670MFX-1.8 [NSC]
Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits; 微型降压型DC -DC转换器，用于超低电压电路
| 型号： | LM3670MFX-1.8 |
| 厂家： | 
National Semiconductor |
| 描述： | Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits |
| 文件： | 总17页 (文件大小：994K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
December 2004
LM3670
Miniature Step-Down DC-DC Converter for Ultra Low
Voltage Circuits
General Description
Features
n VOUT = adj (.7V min), 1.2, 1.5, 1.6, 1.8, 1.875, 2.5, 3.3V
n 2.5V ≤ VIN ≤ 5.5V
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.
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
The device offers superior features and performance for
mobile phones and similar portable applications with com-
plex power management systems. Automatic intelligent
switching between PWM low-noise and PFM low-current
mode offers improved system control. During full-power op-
eration, a fixed-frequency 1 MHz (typ). PWM mode drives
loads from ∼70 mA to 350 mA max, with up to 95% efficiency.
Hysteretic PFM mode extends the battery life through reduc-
tion of the quiescent current to 15 µA (typ) during light
current loads and system standby. Internal synchronous rec-
tification provides high efficiency (90 to 95% typ. at loads
between 1 mA and 100 mA). In shutdown mode (Enable pin
pulled low) the device turns off and reduces battery con-
sumption to 0.1 µA (typ.).
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 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.
Applications
n Mobile phones
n Hand-Held Radios
n Personal Digital Assistants
n Palm-top PCs
n Portable Instruments
n Battery Powered Devices
Typical Application
20075801
FIGURE 1. 1.8V - Fixed Output Voltage - Typical Application Circuit
© 2004 National Semiconductor Corporation
DS200758
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Typical Application (Continued)
20075830
FIGURE 2. 1.2V - Adjustable Output Voltage - Typical Application Circuit
Connection Diagram and Package Mark Information
SOT23-5 Package
NS Package Number XXXX
20075802
Note: The actual physical placement of the package marking will vary from part to part.
FIGURE 3. Top View
Pin Descriptions
Pin #
Name
Description
1
2
3
4
5
VIN
Power supply input. Connect to the input filter capacitor (Figure 1).
Ground pin.
GND
EN
Enable input.
FB
Feedback analog input. Connect to the output filter capacitor (Figure 1).
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.
SW
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Ordering Information
Voltage Option
Order Number
(Level 95)
SPEC
Supplied As
(#/reel)
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
1000
3000
Package Marking
(V)
3.3
LM3670MF-3.3
NOPB
SDEB
LM3670MFX-3.3 NOPB
LM3670MF-3.3
LM3670MFX-3.3
2.5
1.875
1.8 *
LM3670MF-2.5
NOPB
SDDB
SEFB
SDCB
SDBB
S82B
SCZB
SDFB
LM3670MFX-2.5 NOPB
LM3670MF-2.5
LM3670MFX-2.5
LM3670MF-1.875 NOPB
LM3670MFX-1.875 NOPB
LM3670MF-1.875
LM3670MFX-1.875
LM3670MF-1.8
NOPB
LM3670MFX-1.8 NOPB
LM3670MF-1.8
LM3670MFX-1.8
1.6
LM3670MF-1.6
NOPB
LM3670MFX-1.6 NOPB
LM3670MF-1.6
LM3670MFX-1.6
1.5
LM3670MF-1.5
NOPB
LM3670MFX-1.5 NOPB
LM3670MF-1.5
LM3670MFX-1.5
1.2
LM3670MF-1.2
NOPB
LM3670MFX-1.2 NOPB
LM3670MF-1.2
LM3670MFX-1.2
Adjustable *
LM3670MF-ADJ
NOPB
LM3670MFX-ADJ NOPB
LM3670MF-ADJ
LM3670MFX-ADJ
*Released. Samples available.
3
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Rating (Note 3)
Human Body Model:
VIN,SW,FB,GND
EN
2.0kV
500V
200V
VIN Pin: Voltage to GND
EN Pin: Voltage to GND
FB, SW Pin:
−0.2V to 6.0V
−0.2V to 6.0V
(GND−0.2V) to
(VIN + 0.2V)
Machine Model:
Operating Ratings (Notes 1, 2)
Input Voltage Range
2.5V to 5.5V
Junction Temperature (TJ-MAX
Storage Temperature Range
Maximum Lead Temperature
(Soldering, 10 sec.)
)
−45˚C to +125˚C
−45˚C to +150˚C
260˚C
Recommended Load Current
0A to 350 mA
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range
−40˚C to +125˚C
−40˚C to +85˚C
Thermal Properties
Junction-to-Ambient
250˚C/W
Thermal Resistance (θJA
)
(SOT23-5)
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
VIN
Parameter
Condition
Min
2.5
Typ
Max
5.5
Units
V
Input Voltage Range
Fixed Output Voltage
VOUT
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
-1.5
+3.0
%
2.5V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 350 mA
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
−4.5
-1.0
-4.0
+3.0
+2.5
+2.5
%
%
Adjustable Output Voltage
Line Regulation
2.5V ≤ VIN ≤ 5.5V
0 mA ≤ IO ≤ 350 mA
2.5V ≤ VIN ≤ 5.5V
IO = 10 mA
%
0.26
%/V
Load Regulation
150 mA ≤ IO ≤ 350 mA
0.0014
0.5
%/mA
V
VREF
Internal Reference Voltage
Shutdown Supply Current
DC Bias Current into VIN
IQ_SHDN
IQ_PFM
TA=85oC
0.1
1
µA
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
V
2.4
RDSON (P)
RDSON (N)
ILKG (P)
ILKG (N)
ILIM
Pin-Pin Resistance for PFET
Pin-Pin Resistance for NFET
P Channel Leakage Current
N Channel Leakage Current
Switch Peak Current Limit
Efficiency
VIN=VGS=3.6V
VIN=VGS=3.6V
VDS=5.5V
360
250
0.1
0.1
620
91
690
660
1
mΩ
mΩ
µA
VDS=5.5V
1.5
750
µA
400
mA
η
ILOAD = 1 mA
(VIN = 3.6V, VOUT = 1.8V)
ILOAD = 10 mA
ILOAD = 100 mA
ILOAD = 200 mA
ILOAD = 300 mA
ILOAD = 350 mA
94
94
%
94
92
90
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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 (Continued)
Symbol
VIH
Parameter
Condition
Min
1.3
Typ
Max
Units
V
Logic High Input
VIL
Logic Low Input
0.4
1
V
IEN
Enable (EN) Input Current
Internal Oscillator Frequency
0.01
µA
FOSC
PWM Mode
550
1000
1300
kHz
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged
directly into each pin. MIL-STD-883 3015.7
5
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20075832
FIGURE 4. Simplified Functional Diagram
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Typical Performance Characteristics (unless otherwise stated: VIN=3.6V, VOUT=1.8V)
IQ vs. VIN
(output pulled above regulation voltage)
IQ Shutdown vs. Temp
20075805
20075807
20075809
20075804
VOUT vs. VIN
VOUT vs. IOUT
20075806
Efficiency vs. IOUT
Efficiency vs. VIN
20075808
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Typical Performance Characteristics (unless otherwise stated: VIN=3.6V, VOUT=1.8V) (Continued)
RDSON vs. VIN
Frequency vs. Temperature
P & N Channel
20075810
20075811
Line Transient
Line Transient
VIN, VOUT vs. Time
VIN = 2.6V to 3.6V
V
IN, VOUT vs. Time
VIN = 3.6V to 4.6V
(ILOAD = 100 mA)
20075812
20075813
Line Transient
VIN, VOUT vs. Time
VIN = 3.6V to 4.6V
(ILOAD = 100 mA)
Line Transient
V
IN, VOUT vs. Time
VIN = 4.6V to 3.6V
(ILOAD = 100 mA)
20075814
20075815
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Typical Performance Characteristics (unless otherwise stated: VIN=3.6V, VOUT=1.8V) (Continued)
Load Transient
Load Transient
V
OUT, ILOAD vs. Time
VOUT, IINDUCTOR, ILOAD vs. Time
ILOAD = 3mA to 280mA
ILOAD = 0mA to 70mA
20075817
20075816
Load Transient
VOUT, ILOAD vs. Time
ILOAD = 0mA to 280mA
Load Transient
OUT, ILOAD vs. Time
ILOAD = 0mA to 350mA
V
20075818
20075819
Load Transient
VOUT, ILOAD vs. Time
ILOAD = 50mA to 350mA
Load Transient
OUT, ILOAD vs. Time
ILOAD = 100mA to 300mA
V
20075820
20075821
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Typical Performance Characteristics (unless otherwise stated: VIN=3.6V, VOUT=1.8V) (Continued)
PFM Mode
PWM Mode
V
SW, VOUT, IINDUCTOR vs. Time
VSW, VOUT, IINDUCTOR vs. Time
20075822
20075823
Soft Start
VIN, VOUT, IINDUCTOR vs. Time
(ILOAD = 350mA)
20075824
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.
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).
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
There are three modes of operation depending on the cur-
rent required - PWM, PFM, and shutdown. PWM mode
handles current loads of approximately 70 mA or higher.
Lighter output current loads cause the device to automati-
cally 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
(IQ, SHUTDOWN = 0.1 µA typ).
by storing energy in a magnetic field. During the second
portion of each cycle, the controller turns the PFET switch
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B. The peak PMOS switch current drops below the IMODE
level:
Operation Description (Continued)
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
During PFM operation, the converter positions the output
voltage 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 0.8% 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:
The output filter stores charge when the inductor current is
high, and releases it when low, smoothing the voltage across
the load.
PWM OPERATION
During PWM operation the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve excellent load and line regulation.
The DC gain of the power stage is proportional to the input
voltage. To eliminate this dependence, feed forward in-
versely proportional to the input voltage is introduced.
Internal Synchronous Rectification
Once the PMOS power switch is turned off, the NMOS
power switch is turned on until the inductor current ramps to
zero. When the NMOS zero-current condition is detected,
the NMOS power switch is turned off. If the output voltage is
below the ‘high’ PFM comparator threshold (see Figure 5),
the PMOS switch is again turned on and the cycle is re-
peated until the output reaches the desired level. Once the
output reaches the ‘high’ PFM threshold, the NMOS switch is
turned on briefly to ramp the inductor current to zero and
then both output switches are turned off and the part enters
an extremely low power mode. Quiescent supply current
during this ‘sleep’ mode is less than 30 µA, which allows the
part to achieve high efficiencies under extremely light load
conditions. When the output drops below the ‘low’ PFM
threshold, the cycle repeats to restore the output voltage to
∼1.6% above the nominal PWM output voltage.
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 loads, the converter enters PFM mode and
operates with reduced switching frequency and supply cur-
rent to maintain high efficiency.
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 will automatically transition into
fixed-frequency PWM mode.
The party will 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
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Operation Description (Continued)
20075803
FIGURE 5. Operation in PFM Mode and Transfer to PWM Mode
Soft-Start
The minimum input voltage needed to support the output
voltage is
The LM3670 will have a soft-start circuit that limits in-rush
current during start-up. Typical currents and times are:
Current (mA)
Duration (µSec)
0
32
•
•
ILOAD
Load current
70
224
RDSON,
Drain to source resistance of PFET switch
in the triode region
140
280
620
256
256
PFET
•
RINDUCTOR Inductor resistance
until soft start ends
Note 4: The first 32µS are to allow the bias currents to stabilize
LDO - Low Drop Out Operation
The LM3670 can operate at 100% duty cycle (no switching,
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.
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Application Information
OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE
LM3670
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from VOUT
to VFB the to GND. VOUT will be adjusted to make VFB equal
to .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 given the VFB is .5V, then the current
through the resistor feedback network will be 2.5µA ( IFB
=.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.
•
•
•
•
VOUT Output Voltage (V)
Given a peak-to-peak current ripple (IPP) the inductor needs
to be at least
VFB Feedback Voltage (.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 .8V a fre-
quency 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 induc-
tor’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 output voltages between .7 and .8V a pole must also be
placed at 10KHz as well. The lowest output voltage possible
is .7V. At the low voltages the duty cycle is very small. In
addition, as the input voltage increases the duty cycle de-
creases 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. The C1 and C2 act as noise
filters at this point rather than frequency poles and zeroes. If
tghe pole and zero are at the sasme frequency the formula
is:
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:
A pole can be usesd at higher output voltages too. For
example, in the table "Adjustable LM3670 Configurations for
Various VOUT"Table 3 there is an entry for 1.24V with both a
pole and zero at approximately 10KHz for noise rejection.
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor current should not saturate, and the inductor
current ripple is small enough to achieve the desired output
voltage ripple.
There are two methods to choose the inductor current rating.
Method 1:
The total current is the sum of the load and the inductor
ripple current. This can be written as
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Application Information (Continued)
TABLE 1. Suggested Inductors and Their Suppliers
Model
Vendor
Vishay
Phone
FAX
IDC2512NB100M
DO1608C-103
ELL6RH100M
CDRH5D18-100
408-727-2500
847-639-6400
714-373-7366
847-956-0666
408-330-4098
847-639-1469
714-373-7323
847-956-0702
Coilcraft
Panasonic
Sumida
OUTPUT CAPACITOR SELECTION
The output filter capacitor 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 capacitors must be selected with sufficient
capacitance and sufficiently low ESR to perform these func-
tions.
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
LMK432BJ106MM
JMK325BJ106MM
4.7 µF for CIN
Ceramic
Ceramic
Ceramic
Vishay
408-727-2500
847-925-0888
847-925-0888
408-330-4098
847-925-0899
847-925-0899
Taiyo-Yuden
Taiyo-Yuden
VJ1812V475MXJAT
EMK325BJ475MN
C3216X5R0J475M
Ceramic
Ceramic
Ceramic
Vishay
Taiyo-Yuden
TDK
408-727-2500
847-925-0888
847-803-6100
408-330-4098
847-925-0899
847-803-6296
TABLE 3. Adjustable LM3670 Configurations for Various VOUT
VOUT (V)
R1 (KΩ)
R2 (KΩ)
200
200
200
200
200
200
200
150
200
200
200
200
C1 (pF)
200
130
100
82
C2 (pF)
150
L (µH)
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
10
CIN (µF)
4.7
COUT (µF)
0.7
0.8
80.6
120
160
200
240
280
300
221
402
442
487
549
10
none
none
none
none
none
none
120
4.7
10
0.9
4.7
10
1.0
4.7
10
1.1
68
4.7
10
1.2
56
4.7
10
1.24
1.24
1.5
56
4.7
10
75
4.7
10
39
none
none
none
none
4.7
10
1.6
39
10
4.7
10
1.7
33
10
4.7
10
14.7
1.875
30
10
4.7
Note: (10 || 4.7)
22
2.5
806
200
22
82
10
4.7
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14
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.
Application Information (Continued)
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter
design. Poor board layout can disrupt the performance of a
20075831
FIGURE 6. Board Layout Design Rules for the LM3670
Good layout for the LM3670 can be implemented by follow-
ing a few simple design rules, as illustrated in .
per fill as a pseudo-ground plane. Then, connect this to
the ground-plane (if one is used) with several vias. This
reduces ground-plane noise by preventing the switching
currents from circulating through the ground plane. It
also reduces ground bounce at the LM3670 by giving it
a low-impedance ground connection.
1. Place the LM3670 on TBDmil (TBD/1000 in.) pads.
Place the LM3670, inductor and filter capacitors close
together and make the traces short. The traces between
these components carry relatively high switching cur-
rents and act as antennas. Following this rule reduces
radiated noise. Place the capacitors and inductor within
0.2 in. (5 mm) of the LM3670.
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.
2. Arrange the components so that the switching current
loops curl in the same direction. During the first half of
each cycle, current flows from the input filter capacitor,
through the LM3670 and inductor to the output filter
capacitor and back through ground, forming a current
loop. In the second half of each cycle, current is pulled
up from ground, through the LM3670 by the inductor, to
the output filter capacitor and then back through ground,
forming a second current loop. Routing these loops so
the current curls in the same direction prevents mag-
netic field reversal between the two half-cycles and re-
duces radiated noise.
5. Route noise sensitive traces, such as the voltage feed-
back path, away from noisy traces between the power
components. The voltage feedback trace must remain
close to the LM3670 circuit and should be direct but
should be routed opposite to noisy components. This
reduces EMI radiated onto the DC-DC converter’s own
voltage feedback trace.
6. Place noise sensitive circuitry, such as radio IF blocks,
away from the DC-DC converter, CMOS digital blocks
and other noisy circuitry. Interference with noise-
sensitive circuitry in the system can be reduced through
distance.
3. Connect the ground pins of the LM3670, and filter ca-
pacitors together using generous component-side cop-
15
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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.
Application Information (Continued)
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
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16
Physical Dimensions inches (millimeters) unless otherwise noted
5-Lead SOT23-5 Package
NS Package Number MF05A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
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1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
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Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned
Substances’’ as defined in CSP-9-111S2.
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