AN3028 datasheet(23/37 Pages) STMICROELECTRONICS

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AN3028

IC features

Doc ID 16135 Rev 1

23/37

IC features

7.1

Soft-start

At startup the current limitation value reaches IDLIM after an internally set time, tSS, whose

typical value is 8.5 msec. This time is divided into 16 time intervals, each corresponding to a

current limitation step progressively increasing. In this way the drain current is limited during

the output voltage increase, thus reducing the stress on the secondary diode.

The soft-start phase is shown in Figure 25.

7.2

Overload protection

In case of overload or short-circuit (see Figure 26 a), the drain current reaches the IDLIM

value (or the one set by the user through the RLIM resistor). Every cycle that this condition is

met, a counter is incremented. If it is maintained continuously for the time tOVL (50 msec

typical, set internally), the overload protection is tripped, the power section is turned off, and

the converter is disabled for a tRESTART time (1 sec typical). After this time has elapsed, the

IC resumes switching and, if the short is still present, the protection occurs indefinitely in the

same way (Figure 26 b). This ensures restart attempts of the converter with low repetition

rate, so that it works safely with extremely low-power throughput and avoids overheating the

IC in case of repeated overload events.

Moreover, every time the protection is tripped, the internal soft-start function is invoked

(Figure 27a), in order to reduce the stress on the secondary diode.

After the short removal, the IC resumes working normally. If the short is removed during tSS

or tOVL, i.e. before the protection tripping, the counter is decremented on a cycle-by-cycle

basis down to zero and the protection is not tripped.

If the short-circuit is removed during tRESTART, the IC waits for the tRESTART period to elapse

before resuming switching (Figure 27b).

Figure 25.

Soft-start

a) soft-start at startup

b) soft-start at startup (zoom)

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