L6565
It acts on the clamp level of the control voltage V
csx
, that is on the overcurrent setpoint, so that it is a function
of the converter's input voltage sensed through a dedicated pin (#3, VFF): the higher the input voltage, the lower
the setpoint. This is illustrated in the diagram of figure 17a that shows the relationship between the voltage at
the pin VFF and V
csx
(with the error amplifier saturated high in the attempt of keeping output voltage regulation).
The schematic in figure 17b shows also how the function is included in the control loop. With a proper selection
of the external divider R1-R2 it is possible to achieve the optimum compensation described by the lower curve
in the diagram of figure 16.
In applications where this function is not wanted, e.g. because of a narrow input voltage range, the VFF pin can
be simply grounded, thus saving the resistor divider. The overcurrent setpoint will be then fixed at the maximum
value of about 1.4V (1.5V max.).
Line Feedforward is also beneficial to other characteristics of quasi-resonant converters: it improves their input
ripple rejection ability and limits the variation of the power stage's small-signal gain versus the line voltage.
Figure 17. a) Overcurrent setpoint vs. VFF voltage; b) Line Feedforward function block
V
csx
[V]
1.5
V
COMP
= Upper clamp
1
a)
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
V
VFF
[V]
+Vin
R1
R2
COMP
2
3
VFF
CS
4
ZCD
5
ZCD
-
E/A
+
2.5V
2V
+
INV 1
VOLTAGE
FEED
FORWARD
PWM
STARTER
Rs
starter STOP
S
Q
R
(reset-dominant)
7
DRIVER
GD
Hiccup
b)
10/17
+
-
-
DISABLE
L6565
STMICROELECTRONICS [ STMICROELECTRONICS ]
