LM2853MH-1.2 datasheet(7/12 Pages) NSC

Nome de Peças	LM2853MH-1.2

Descrição Electrónicos	3A 550 kHz Synchronous SIMPLE SWITCHER Buck Regulator

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Fabricante Electrônico	NSC [National Semiconductor (TI)]

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Block Diagram

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Applications Information

The LM2853 is a DC-DC buck regulator belonging to Na-

tional Semiconductor’s synchronous SIMPLE SWITCHER®

family. Integration of the PWM controller, power switches

and compensation network greatly reduces the component

count required to implement a switching power supply. A

typical application requires only four components: an input

capacitor, a soft-start capacitor, an output filter capacitor and

an output filter inductor.

INPUT CAPACITOR (C

IN)

Fast switching of large currents in the buck converter places

a heavy demand on the voltage source supplying PVIN. The

input capacitor, C

IN, supplies extra charge when the switcher

needs to draw a burst of current from the supply. The RMS

current rating and the voltage rating of the C

IN capacitor are

therefore important in the selection of C

IN. The RMS current

specification can be approximated by:

where D is the duty cycle, V

OUT/VIN.CIN also provides

filtering of the supply. Trace resistance and inductance de-

grade the benefits of the input capacitor, so C

IN should be

placed very close to PVIN in the layout. A 22 µF or 47 µF

ceramic capacitor is typically sufficient for C

IN. In parallel

with the large input capacitance a smaller capacitor should

be added such asa1µF ceramic for higher frequency

filtering. Ceramic capacitors with high quality dielectrics such

as X5R or X7R should be used to provide a constant capaci-

tance across temperature and line variations. For improved

load regulation and transient performance, the use of a small

1 µF ceramic capacitor is also recommended as a local

bypass for the AVIN pin.

SOFT-START CAPACITOR (C

SS)

The DAC that sets the reference voltage of the error ampli-

fier sources a current through a resistor to set the reference

voltage. The reference voltage is one half of the output

voltage of the switcher due to the 200 k

Ω divider connected

to the SNS pin. Upon start-up, the output voltage of the

switcher tracks the reference voltage with a two to one ratio

as the DAC current charges the capacitance connected to

the reference voltage node. Internal capacitance of 20 pF is

permanently attached to the reference voltage node which is

also connected to the soft start pin, SS. Adding a soft-start

capacitor externally increases the time it takes for the output

voltage to reach its final level. The charging time required for

the reference voltage can be estimated using the RC time

constant of the DAC resistor and the capacitance connected

to the SS pin. Three RC time constant periods are needed

for the reference voltage to reach 95% of its final value. The

actual start up time will vary with differences in the DAC

resistance and higher-order effects.

If little or no soft-start capacitance is connected, then the

start up time may be determined by the time required for the

current limit current to charge the output filter capacitance.

The capacitor charging equationI=C

∆V/∆t can be used to

estimate the start-up time in this case. For example, a part

with a 3V output, a 100 µF output capacitance and a 5A

current limit threshold would require a time of 60 µs:

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