AS23xx

Secondary Side Housekeeping Circuit

ASTEC Semiconductor

100

3.1 Delaying Remote OFF: DELAY

In systems which use the OFF and FAULT pins

to provide remote on/off switching, the delay

between the OFF pin going high and the FAULT

signal going high is programmable with a capaci-

tor to ground on the DELAY pin as described in

2.5 above. The POK pin, on the other hand will

go high immediately after the OFF pin is open

circuited or pulled high, giving the system warn-

ing of the impending shutdown. The DELAY pin

provides a 1

µA current source to charge the

cap, and when the cap charges above 2.5 V, the

FAULT pin will go high.

3.2 AC Warning Prior to Primary Drop-out

In systems where the input line voltage is

switched, the AC pin threshold should be set so

that it causes POK to go low before the primary

bulk voltage reaches drop-out and the primary

PWM shuts off. The output of the AC comparator

also causes the UVB pin to pull low, so that the

undervoltage sensing does not trip the FAULT

latch as the outputs fall below spec. Recall that

the AC pin senses a divided down and filtered

representation of the secondary side switching

waveform, which will provide a proportional rep-

resentation of the primary voltage via the turns

ratio of the transformer.

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