AN4116 datasheet(21/38 Pages) STMICROELECTRONICS

21 / 38 page

AN4116

Functional check

Doc ID 023220 Rev 1

21/38

Functional check

6.1

Soft-start

At startup the current limitation value reaches IDLIM after an internally fixed 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 28 and 29.

6.2

Overload protection

In the case of overload or short-circuit (see

Figure 38), the drain current reaches the IDLIM

value (or the one set by the user through the RLIM resistor). In every cycle where this

condition is met, a counter is incremented; if it is maintained continuously for the time tOVL

(50 msec typical, internally fixed), the overload protection is tripped, the power section is

turned off and the converter is disabled for a tRESTART time (1sec 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 31). This ensures restart attempts of the converter with

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

the IC overheating in the case of repeated overload events.

Furthermore, every time the protection is tripped, the internal soft startup function is invoked

(

Figure 32), in order to reduce the stress on the secondary diode.

After the short removal, the IC resumes normal working. 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 (

Figure 33).

Figure 28.

Soft-start at startup

Figure 29.

Soft-start at startup (zoom)

AM13094v1

AM13095v1

AN4116 датащи(PDF) 21 Page - STMicroelectronics

AN4116 датащи(HTML) 21 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

датащи скачать

ссылки URL