Datasheet

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SLUS542F − OCTOBER 2003 − REVISED JULY 2009
15
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FUNCTIONAL DESCRIPTION
V
CLAMP
V
IN
V
SS
V
OFF
T
SW
OUT
AUX
VCLAMP, MAX
Figure 3. Line Undervoltage Shutdown Waveforms, P−Channel
To eliminate this potential hazard the UCC289x controllers safely discharge the clamp capacitor during power
down. The AUX and OUT output continues switching while the soft-start capacitor C
SS
is being slowly
discharged. Notice that the AUX and OUT pulse width gradually decreases as the clamp voltage decreases
never applying the high voltage across the transformer for extended period of time. From this, the function of
soft stop is achieved.
Line Overvoltage Protection
When the line overvoltage protection is triggered in the UCC2892 and UCC2894 controllers, the gate drive
signals are immediately disabled. At the same time, the slow discharge of C
SS
is initiated. While the soft-start
capacitor is discharging the gate drive signals remains disabled. Once V
SS
= 0.5 V and the overvoltage
disappears from the input of the power supply, operation resumes through a regular soft-start of the converter
as it is demonstrated in Figure 4.