L6566ATR datasheet(35/51 Pages) STMICROELECTRONICS

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L6566A

Application information

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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 5 V reference, a comparator is

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

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

is allowed to reach 2 Vbe over 5 V, 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+2 VBE 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 5 V. Refer to the “Application examples and Ideas” section

(Table 7 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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