L6566B View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

L6566B

Multi-mode controller for SMPS

STMicroelectronics 

L6566B

Application information

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 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 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+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

PWM latch

RQ

SQ

to triggering

block

ZCD

11

5V

40kΩ

-

+

5pF

COUT

L6566B

Monostable

M1

2 µs

Monostable STROBE

M2

0.5 µs

OVP

FF

R Q1

S

2-bit

counter

Fault

Counter

reset

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

k OVP

=

5

Vout OVP

Ns

Naux

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.

35/51

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