't = C

I

'V

= 100 PF

3A

3V

= 100 Ps

D(1-D)

LM2852

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SNVS325D – JANUARY 2005 – REVISED APRIL 2013

APPLICATIONS INFORMATION

The LM2852 is a DC-DC synchronous buck regulator belonging to Texas Instrument’s 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.

Fast switching of large currents in the buck converter places a heavy demand on the voltage source supplying

where

•

(1)

input capacitance a smaller capacitor may be added such as a 1µF ceramic for higher frequency filtering.

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 equation I = C

ΔV/Δt

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:

(2)

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, suppose a 3 ms 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.

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 falls to a low voltage. During this time, the soft-start voltage is forced to track the output so that

once the short is removed, the LM2852 can restart gracefully from whatever voltage the output reached during

the short circuit event. The range of soft-start capacitors is therefore restricted to values 1nF to 50nF.

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