Datasheet

UCC2897A
SLUS829D -- AUGUST 2008 -- REVISED JULY 2009
16
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FUNCTIONAL DESCRIPTION
V
CLAMP
V
IN
V
SS
V
OFF
T
SW
OUT
AUX
VCLAMP, MAX
Figure 3. Line U ndervoltage Shutdown Waveforms, P--Channel
To eliminate this potential hazard the UCC2897A controller safely discharge the clamp capacitor during power
down. The OUT and the AUX output continues switching while the soft-start capacitor C
SS
is being slowly
discharged. Notice that the AUX pulse width gradually increases 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 UCC2897A controller, 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. The pulses of OUT and AUX stop if one of three conditions is met:
1. VDD reaches UVLO off,
2. VSS reaches below 2.5 V,
3. or FB voltage is below 2.5 V.