AN2852

Operating waveforms

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Figure 11.

HICCUP mode

2.4

Overvoltage protection

Since it is impossible to sense the output voltage from the primary side in all load conditions,

the OVP senses such voltage directly on the output. A Zener diode (D25) is used as the

threshold to activate the protection. The information is passed to the controller using

optocoupler U5 that increases the disable pin voltage over the intervention threshold of

4.5 V. In Figure 12 a loop failure is simulated by shorting R93. The overvoltage protection is

invoked and the output voltage reaches a maximum voltage of 14.8 V.

Since this protection uses the disable pin, it is latched. Hence, after PWM is stopped, the HV

generator is invoked to keep Vcc voltage between 14 V and 13.5 V. Diode D27 brings the

PFC_OK pin voltage over 2.5 V, so the L6563 is also shut down and its consumption goes

almost to the startup level. The PWM_LATCH goes high which also keeps the disable pin

high. The latched operation is shown in Figure 13.

AM01826v1

Ch1: SS pin voltage (yellow)

Ch2: COMP pin voltage (blue)

Ch3: FGND pin voltage (purple)

Ch4: PFC_STOP pin voltage (green)