A5974AD datasheet(41/49 Pages) STMICROELECTRONICS

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A5974AD

Application information

Doc ID 018762 Rev 1

41/49

MLCCs (multiple layer ceramic capacitor) with values in the range of 10 µF-22 µF and rated

voltages in the range of 10 V-25 V are available today at relatively low cost from many

manufacturers.

These capacitors have very low ESR values (a few m

Ω) and are therefore occasionally used

for the output filter in order to reduce the voltage ripple and the overall size of the

application.

However, the zero given by the output capacitor falls outside the designed bandwidth and so

the system becomes unstable with the standard compensation network.

Figure 30 shows the type III compensation network stabilizing the system with ceramic

capacitors at the output (the optimum components value depends on the application). This

configuration introduces two zeroes and a low frequency pole in the designed bandwidth

and so guarantees a proper phase margin.

Figure 30.

MLCC compensation network circuit

8.11

External soft-start network

At startup, the device can quickly increase the current up to the current limit in order to

charge the output capacitor. If soft ramp-up of the output voltage is required, an external

soft-start network can be implemented, as shown in Figure 31. The capacitor C is charged

up to an external reference through R and the BJT clamps the COMP pin.

This clamps the duty cycle, limiting the slew rate of the output voltage.

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