Datasheet

Operation descriptions VIPER27
20/31 Doc ID 15133 Rev 5
7.9 About CONT pin
Referring to the
Figure 27
, through the CONT pin, the below features can be implemented:
1. Current limit set point
2. Over voltage protection on the converter output voltage
The
Table 9 on page 20
referring to the
Figure 27
, lists the external components needed to
activate one or plus of the CONT pin functions.
Figure 27. CONT pin configuration
7.10 Feed-back and overload protection (OLP)
The VIPER27 is a current mode converter: the feedback pin controls the PWM operation,
controls the burst mode and actives the overload protection.
Figure 28 on page 22
and
Figure 29
show the internal current mode structure.
With the feedback pin voltage between
V
FBbm
and V
FBlin
, (see
Table 8 on page 7
) the drain
current is sensed and converted in voltage that is applied to the non inverting pin of the
PWM comparator. See
Figure 2 on page 3
.
This voltage is compared with the one on the feedback pin through a voltage divider on
cycle by cycle basis. When these two voltages are equal, the PWM logic orders the switch
off of the power MOSFET. The drain current is always limited to I
Dlim
value.
In case of overload the feedback pin increases in reaction to this event and when it goes
higher than
V
FBlin
, the PWM comparator is disabled and the drain current is limited to I
Dlim
by
the OCP comparator, see
Figure 2 on page 3
.
Table 9. CONT pin configurations
Function / component R
LIM
(1)
1. R
LIM
has to be fixed before of R
OVP
R
OVP
D
AUX
I
Dlim
reduction See
Figure 16
No No
OVP
80 k
Ω See
Equation 4
Ye s
I
Dlim
reduction + OVP See
Figure 16
See
Equation 4
Ye s
+
-
OVP
R
OV P
SOFT
START
CONT
Daux
R
LIM
From R
SENSE
Auxiliary
winding
to GATE driver
OVP
LOGIC
OCP
BLOCK
OCP