Block Diagram

20127012

Applications Information

The LM2852 is a DC-DC synchronous buck regulator be-

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

Fast switching of large currents in the buck converter places

a heavy demand on the voltage source supplying PVIN. The

input capacitor, C

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

current rating and the voltage rating of the C

therefore important in the selection of C

specification can be approximated by:

where D is the duty cycle, V

filtering of the supply. Trace resistance and inductance de-

grade the benefits of the input capacitor, so C

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

ceramic capacitor is typically sufficient for C

with the large input capacitance a smaller capacitor may be

added such as a 1µF ceramic for higher frequency filtering.

SOFT-START CAPACITOR (C

The DAC that sets the reference voltage of the error amp

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 200k

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 20pF 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 3A

current limit threshold would require a time of 100 µs:

Since it is undesirable for the power supply to start up in

current limit, a soft-start capacitor must be chosen to force

the LM2852 to start up in a more controlled fashion based on

the charging of the soft-start capacitance. In this example,

supposea3ms start time is desired. Three time constants

are required for charging the soft-start capacitor to 95% of

the final reference voltage. So in this case RC=1ms. The

DAC resistor, R, is 400 k

Ω so C can be calculated to be

2.5nF. A 2.7nF ceramic capacitor can be chosen to yield

approximately a 3ms start-up time.

SOFT-START CAPACITOR (C

CONDITIONS

Various fault conditions such as short circuit and UVLO of

the LM2852 activate internal circuitry designed to control the

voltage on the soft-start capacitor. For example, during a

short circuit current limit event, the output voltage typically

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