AN4165 datasheet(21/37 Pages) STMICROELECTRONICS

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AN4165

Functional check

Doc ID 023661 Rev 1

21/37

Functional check

10.1

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, therefore reducing the stress on the secondary diode. The soft-

start phase is shown in

Figure 24 and 25.

10.2

Overload protection

In case of overload or short-circuit (see

Figure 26), 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 the fault 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 s 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 27). This ensures restart attempts of the converter with

low repetition rate, so that it works safely with extremely low power throughput and avoids

overheating of the IC in case of repeated overload events.

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

Figure 25) is

implemented, 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 is tripped, 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 29).

Figure 24.

Soft-start at startup

Figure 25.

Soft-start at startup (zoom)

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