NCP1027

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15

Fault Condition – Output Too Low

This particular mode of operation occurs when the

feedback is ensured by a two−loop control imposing either

constant output voltage (CV) or constant output current

(CC), for instance in a battery charger. In CC mode, the

output voltage falls down below the original target but the

feedback loop is kept closed by the CC controller. For that

reason, the controller becomes un−able to detect a real

output short−circuit since Ipflag will never be asserted.

Due to a good winding coupling, the primary side auxiliary

collapsing will ensure a proper fault detection via the

UVLO internal circuit. Figure 29 depicts this operating

way.

Figure 29. In this particular case, the output goes low but the timer is not started since the FB pin is still held