't = C

I

'V

= 100 PF

3A

3V

= 100 Ps

13

LM2852

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SNVS325E – JANUARY 2005 – REVISED JANUARY 2016

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Typical Application (continued)

8.2.2.2 Soft-Start Capacitor (CSS)

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 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 varies 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 3-V output, a 100-µF output

capacitance and a 3-A current limit threshold would require a time of 100 µs, seen in Equation 2:

(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 = 1 ms. The DAC resistor, R, is 400 k

Ω so C

can be calculated to be 2.5 nF. A 2.7-nF ceramic capacitor can be chosen to yield approximately a 3 ms start-up

time.

8.2.2.3 Soft-Start Capacitor (CSS) and Fault 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 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 1 nF to 50 nF.

8.2.2.4 Compensation

The LM2852 provides a highly integrated solution to power supply design. The compensation of the LM2852,

which is type-three, is included on-chip. The benefit to integrated compensation is straightforward, simple power

supply design. Since the output filter capacitor and inductor values impact the compensation of the control loop,

the range of L, C and CESR values is restricted in order to ensure stability.