L6566B datasheet(35/51 Pages) STMICROELECTRONICS

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L6566B

Application information

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5.11

OVP block

The OVP function of the L6566B 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 L6566B 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

The ZCD pin will be connected to the auxiliary winding through a resistor divider RZ1, RZ2

(see Figure 8 on page 20). The divider ratio kOVP = RZ2 / (RZ1 + RZ2) will be chosen equal

to:

Equation 13

where VoutOVP is the output voltage value that is to activate the protection, Ns the turn

number of the secondary winding and Naux the turn number of the auxiliary winding.

Fault

2-bit

counter

R Q1

Monostable

STROBE

OVP

COUT

2 µs

0.5 µs

Monostable

5 V

ZCD

Counter

reset

40k

Ω

5pF

PWM latch

to triggering

block

L6566B

Naux

Vout

OVP

OVP =

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