Datasheet

DocID023409 Rev 5 23/34
HVLED815PF Device description
34
Figure 19. Feedforward compensation: internal schematic
During MOSFET's ON-time the current sourced from DMG pin is mirrored inside the
“Feedforward Logic” block in order to provide a feedforward current, I
FF
.
Such “feedforward current” is proportional to the input voltage according to Equation 9:
Equation 9
Where m is the primary-to-auxiliary turns ratio.
According to the schematic in Figure 19, the voltage on the non-inverting comparator will be:
Equation 10
The offset introduced by feedforward compensation will be:
Equation 11
As R
FF
>> R
SENSE
, the previous one can be simplified as:
Equation 12
This offset is proportional to V
IN
and it is used to compensate the current overshoot,
according to Equation 13:
AM13569v1
CS
.
IFF
Feedf orward
Logic
+
-
CC
Rff
Rsense
PWM
LOGIC
DMG
CC
Block
Aux
Rfb
Rdmg
SOURCE
DRAIN