Datasheet
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 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 V
BE
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
(
Table7 on page45) for additional hints.
Figure 22. OVP function: internal block diagram
The ZCD pin will be connected to the auxiliary winding through a resistor divider R
Z1
, R
Z2
(see
Figure 8 on page 20). The divider ratio k
OVP
= R
Z2
/ (R
Z1
+ R
Z2
) will be chosen equal
to:
Equation 13
where Vout
OVP
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
S
Monostable
M1
STROBE
OVP
COUT
2 µs
0.5 µs
Monos t abl e
M2
-
+
5 V
ZCD
Counter
reset
FF
40k
Ω
5pF
PWM latch
Q
QS
R
11
to triggering
block
L6566B
Naux
Ns
Vout
5
k
OVP
OVP
=