Datasheet

Operation description VIPER37
24/35 Doc ID 022218 Rev 1
8.8 Overvoltage protection (OVP)
The device can monitor the converter output voltage. This operation is done by the CONT
pin during Power MOSFET OFF-time, when the voltage generated by the auxiliary winding
tracks the converter's output voltage, through turn ratio (see
Figure 28 ).
In order to perform the output voltage monitor, the CONT pin must be connected to the aux.
winding through a resistor divider made up of R
LIM
and R
OVP
(see Figure 21 or Figure 29 ). If
the voltage applied to the CONT pin exceeds the internal reference V
OVP
(see Ta bl e 8 ) for
four consecutive times, the controller recognizes an overvoltage condition. This special
feature uses an internal counter; that is to reduce sensitivity to noise and prevent the latch
from being erroneously activated (see
Figure 28 ). The counter is reset every time the OVP
signal is not triggered in one oscillator cycle.
Referring to
Figure 21 , the resistors’ divider ratio k
OVP
is given by:
Equation 2
Equation 3
where:
● V
OVP
is the OVP threshold (see Tabl e 8 )
● V
OUT OVP
is the converter output voltage value to activate the OVP set by the user
● N
AUX
is the auxiliary winding turns
● N
SEC
is the secondary winding turns
● V
DSEC
is the secondary diode forward voltage
● V
DAUX
is the auxiliary diode forward voltage
● R
OVP
together with R
LIM
make up the output voltage divider.
Then, once the R
LIM
value is fixed
,
according to the desired I
Dlim
, the R
OVP
can be
calculated by:
Equation 4
N
AUX
N
SEC
--------------
k
OVP
V
OVP
N
AUX
N
SEC
--------------
V
OUTOVP
V
DSEC
+()V
DAUX
–⋅
-----------------------------------------------------------------------------------------------------=
k
OVP
R
LIM
R
LIM
R
OVP
+
----------------------------------=
R
OVP
R
LIM
1k
OVP
–
k
OVP
-----------------------
×=