AN4345 datasheet(17/28 Pages) STMICROELECTRONICS

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AN4345

Functional check

5.2

Overload protection

In case of overload or short-circuit (see Figure 23), the current across the inductor L2

reaches the IDLIM value. In every cycle where this condition is met, a counter is

incremented; if it is maintained continuously for tOVLtime (50 msec typical, internally set) the

overload protection is tripped, the power section is turned off and the converter is disabled

for a tRESTART time (1 sec typical). When this time has elapsed, the IC resumes switching

and, if the short is still present, the protection occurs indefinitely in the same way (see

Figure 24). This ensures restart attempts of the converter with low repetition rate, so that it

can work safely with extremely low power throughput and avoid the IC overheating in case

of repeated overload events.

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 must wait for the tRESTART period to

elapse before switching is resumed (see Figure 26).

Figure 23. Output short-circuit applied:

OLP tripping

Figure 24. Output short-circuit maintained:

OLP steady-state

AM13828V1

Output is shorted here

Normal

operation

tOVL

tRESTART

AM13829V1

tRESTART

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