NCP1028
www.onsemi.com
17
The bridge power dissipation is 330
2
/3.018 Meg =
36 mW in nominal high−line operation. Figure 29b
simulation result confirms our calculations.
Figure 30 describes signal variations during a brown−out
condition. Please note that output pulses only reappear
when V
CC
reaches V
CC(ON)
, ensuring a clean startup
sequence. As in fault mode conditions, the startup source
is activated on and off and self−supplies the controller in a
Dynamic Self−Supply (DSS) mode.
Figure 30. Signal Evolution During a Brown−Out Condition
Depending on input surge tests, it might be necessary to
wire a filtering capacitor between BO and GND (close to
the circuit) to avoid adversely triggering the internal latch
(unless this is a wanted feature) when the pulse train
appears.
Latchoff Protection
There are some situations where the converter shall be
fully turned−off and stay latched. This can happen in the
presence of a secondary overvoltage (the feedback loop is
drifting) or when an overtemperature is detected.
Secondary monitoring is usually implemented when the
coupling between auxiliary and power windings does not
lead to a precise primary detection. Due to the addition of
a comparator on the BO pin, a simple external circuit can
lift up this pin above VLATCH and permanently disable
pulses. The V
CC
needs to be cycled down below 3.5 V
typically to reset the controller.
