VIPER27 STMicroelectronics, VIPER27 Datasheet - Page 18

• Manufacturer Part Number: VIPER27
• Description: VIPerPlus family: fixed frequency high performance off line high voltage converter
• Manufacturer: STMicroelectronics

Operation descriptions
7.7
7.8
18/31
Current mode conversion with adjustable current limit set
point
The device is a current mode converter: the drain current is sensed and converted in voltage
that is applied to the non inverting pin of the PWM comparator. This voltage is compared
with the one on the feed-back pin through a voltage divider on cycle by cycle basis.
The
the electrical specification, by the R
page 11
The CONT pin has a minimum current sunk needed to activate the I
R
Table 8 on page 7
Overvoltage protection (OVP)
The VIPER27 has integrated the logic for the monitor of the output voltage using as input
signal the voltage V
the voltage from the auxiliary winding tracks the output voltage, through the turn ratio
The CONT pin has to be connected to the auxiliary winding through the diode D
resistors R
the voltage V
on page 7
the auto-restart mode.
In order to bypass the noise immediately after the turn off of the power MOSFET, the voltage
V
the
digital signal and increments the internal counter. The same counter is reset every time the
signal OVP is not triggered in one oscillator cycle.
Referring to the
Equation 2
Equation 3
N
--------------
N
CONT
LIM
AUX
SEC
Figure 26 on page 19
VIPER27
or with high R
is sampled inside a short window after the time T
.
) the overvoltage protection will stop the power MOSFET and the converter enters
OVP
CONT
has a default current limit value, I
and R
Figure 21
).
exceeds, four consecutive times, the reference voltage V
LIM
CONT
LIM
(i.e. 100 kΩ) the current limit is fixed to the default value (see I
as shows the
k
during the OFF time of the power MOSFET. This is the time when
. The sampled signal, if higher than V
, the resistors divider ratio k
OVP
=
Doc ID 15133 Rev 5
-------------------------------------------------------------------------------------------------- -
N
--------------
N
AUX
SEC
LIM
k
OVP
⋅
Figure 27 on page 20
resistor connected to the CONT see
(
V
=
OUTOVP
--------------------------------- -
R
LIM
Dlim
V
R
OVP
+
LIM
+
, that the designer can adjust according
R
V
OVP
DSEC
OVP
STROBE
) V
will be given by:
–
When, during the OFF time,
DAUX
OVP
, see
, trigger the internal OVP
Dlim
Table 8 on page 7
adjustment: without
OVP
Figure 16 on
(see
OVP
VIPER27
and the
Table 8
Dlim
and
,