L6566ATR datasheet(35/51 Pages) STMICROELECTRONICS

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L6566A

Application information

35/51

5.11

OVP block

The OVP function of the L6566A monitors the voltage on the ZCD pin (11) in MOSFET’s

OFF-time, during which the voltage generated by the auxiliary winding tracks converter’s

output voltage. If the voltage on the pin exceeds an internal 5V reference, a comparator is

triggered, an overvoltage condition is assumed and the device is shut down. An internal

current generator is activated that sources 1mA out of the VFF pin (15). If the VFF voltage is

allowed to reach 2 Vbe over 5V, the L6566A will be latched off. See Section 5.9: Latched

disable function on page 32

for more details on IC’s behavior under these conditions. If the

impedance externally connected to pin 15 is so low that the 5+2VBE threshold cannot be

reached or if some means is provided to prevent that, the device will be able to restart after

the Vcc has dropped below 5V. Refer to the “Application examples and Ideas” section

(Table 8 on page 45) for additional hints.

Figure 22. OVP Function: internal block diagram

Fault

2-bit

counter

R Q1

Monostable

STROBE

OVP

COUT

2µs

0.5 µs

Monostable

5 V

ZCD

Counter

reset

40k

Ω

5pF

PWM latch

to triggering

block

L6566A

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