Datasheet

HVLED805 Application information
Doc ID 18077 Rev 1 21/29
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 the formula:
Equation 9
Where m is the primary-to-auxiliary turns ratio.
According to the schematic, 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
Figure 17. Feedforward compensation: internal schematic
.
CC
Bl ock
Aux
Rdmg
Rf b
IF F
Rs en s e
RFF
+
-
CC
Feedforward
Logic
PW M
LOGIC
DMG
DRAIN
SO URCE
dmg
IN
FF
Rm
V
I
⋅
=
(
)
(-)
SENSE D FF FF SENSE
V=R I+I R+R⋅⋅
()
SENSEFF
dmg
IN
OFFSET
RR
Rm
V
V +⋅
⋅
=
dmg
FFIN
OFFSET
Rm
RV
V
⋅
⋅
=