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PDF LM2671N-12 Data sheet ( Hoja de datos )

Número de pieza LM2671N-12
Descripción SIMPLE SWITCHER Power Converter High Efficiency 500mA Step-Down Voltage Regulator with Features
Fabricantes National Semiconductor 
Logotipo National Semiconductor Logotipo



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September 1998
LM2671
SIMPLE SWITCHER® Power Converter High Efficiency
500mA Step-Down Voltage Regulator with Features
General Description
The LM2671 series of regulators are monolithic integrated
circuits built with a LMDMOS process. These regulators pro-
vide all the active functions for a step-down (buck) switching
regulator, capable of driving a 500mA load current with ex-
cellent line and load regulation. These devices are available
in fixed output voltages of 3.3V, 5.0V, 12V, and an adjustable
output version.
Requiring a minimum number of external components, these
regulators are simple to use and include patented internal
frequency compensation (Patent Nos. 5,382,918 and
5,514,947), fixed frequency oscillator, external shutdown,
soft-start, and frequency synchronization.
The LM2671 series operates at a switching frequency of
260 kHz, thus allowing smaller sized filter components than
what would be needed with lower frequency switching regu-
lators. Because of its very high efficiency (>90%), the cop-
per traces on the printed circuit board are the only heat sink-
ing needed.
A family of standard inductors for use with the LM2671 are
available from several different manufacturers. This feature
greatly simplifies the design of switch-mode power supplies
using these advanced ICs. Also included in the datasheet
are selector guides for diodes and capacitors designed to
work in switch-mode power supplies.
Other features include a guaranteed ±1.5% tolerance on
output voltage within specified input voltages and output
load conditions, and ±10% on the oscillator frequency. Ex-
ternal shutdown is included, featuring typically 50 µA
stand-by current. The output switch includes current limiting,
as well as thermal shutdown for full protection under fault
conditions.
To simplify the LM2671 buck regulator design procedure,
there exists computer design software, LM267X Made
Simple.
Features
n Efficiency up to 96%
n Available in SO-8 and 8-pin DIP packages
n Computer Design Software LM267X Made Simple
n Simple and easy to design with
n Requires only 5 external components
n Uses readily available standard inductors
n 3.3V, 5.0V, 12V, and adjustable output versions
n Adjustable version output voltage range: 1.21V to 37V
n ±1.5% max output voltage tolerance over line and load
conditions
n Guaranteed 500mA output load current
n 0.25DMOS Output Switch
n Wide input voltage range: 8V to 40V
n 260 kHz fixed frequency internal oscillator
n TTL shutdown capability, low power standby mode
n Soft-start and frequency synchronization
n Thermal shutdown and current limit protection
Typical Applications
n Simple High Efficiency (>90%) Step-Down (Buck)
Regulator
n Efficient Pre-Regulator for Linear Regulators
Typical Application (Fixed Output Voltage Versions)
DS100042-1
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation.
Windows® is a registered trademark of Microsoft Corporation.
© 1998 National Semiconductor Corporation DS100042
www.national.com

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LM2671N-12 pdf
Typical Performance Characteristics (Continued)
Drain-to-Source
Resistance
Switch Current Limit
Operating
Quiescent Current
DS100042-6
DS100042-7
Standby
Quiescent Current
DS100042-8
ON/OFF Threshold
Voltage
ON/OFF Pin
Current (Sourcing)
DS100042-9
DS100042-10
DS100042-11
5
www.national.com

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LM2671N-12 arduino
LM2671 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version)
4. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
transients from appearing at the input. This capacitor should
be located close to the IC using short leads. In addition, the
RMS current rating of the input capacitor should be selected to
be at least 12 the DC load current. The capacitor manufacturer
data sheet must be checked to assure that this current rating
is not exceeded. The curves shown in Figure 14 show typical
RMS current ratings for several different aluminum electrolytic
capacitor values. A parallel connection of two or more
capacitors may be required to increase the total minimum RMS
current rating to suit the application requirements.
For an aluminum electrolytic capacitor, the voltage rating
should be at least 1.25 times the maximum input voltage.
Caution must be exercised if solid tantalum capacitors are
used. The tantalum capacitor voltage rating should be twice
the maximum input voltage. The tables in Figure 15 show the
recommended application voltage for AVX TPS and Sprague
594D tantalum capacitors. It is also recommended that they be
surge current tested by the manufacturer. The TPS series
available from AVX, and the 593D and 594D series from
Sprague are all surge current tested. Another approach to
minimize the surge current stresses on the input capacitor is to
add a small inductor in series with the input supply line.
Use caution when using ceramic capacitors for input
bypassing, because it may cause severe ringing at the VIN pin.
5. Boost Capacitor (CB)
This capacitor develops the necessary voltage to turn the
switch gate on fully. All applications should use a 0.01 µF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (CSS - optional)
This capacitor controls the rate at which the device starts up.
The formula for the soft-start capacitor CSS is:
EXAMPLE (Fixed Output Voltage Version)
4. Input Capacitor (CIN)
The important parameters for the input capacitor are the input
voltage rating and the RMS current rating. With a maximum
input voltage of 12V, an aluminum electrolytic capacitor with a
voltage rating greater than 15V (1.25 x VIN) would be needed.
The next higher capacitor voltage rating is 16V.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately 12 the DC load current. In this
example, with a 500 mA load, a capacitor with a RMS current
rating of at least 250 mA is needed. The curves shown in
Figure 14 can be used to select an appropriate input capacitor.
From the curves, locate the 16V line and note which capacitor
values have RMS current ratings greater than 250 mA.
For a through hole design, a 100 µF/16V electrolytic capacitor
(Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
equivalent) would be adequate. Other types or other
manufacturers’ capacitors can be used provided the RMS
ripple current ratings are adequate. Additionally, for a complete
surface mount design, electrolytic capacitors such as the
Sanyo CV-C or CV-BS and the Nichicon WF or UR and the
NIC Components NACZ series could be considered.
For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the
capacitor surge current rating and voltage rating. In this
example, checking Figure 15, and the Sprague 594D series
datasheet, a Sprague 594D 15 µF, 25V capacitor is adequate.
5. Boost Capacitor (CB)
For this application, and all applications, use a 0.01 µF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (CSS - optional)
For this application, selecting a start-up time of 10 ms and
using the formula for CSS results in a value of:
where:
ISS = Soft-Start Current :4.5 µA typical.
tSS = Soft-Start Time :Selected.
VSSTH = Soft-Start Threshold Voltage :0.63V typical.
VOUT = Output Voltage :Selected.
VSCHOTTKY = Schottky Diode Voltage Drop :0.4V typical.
VIN = Input Voltage :Selected.
If this feature is not desired, leave this pin open.
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