Application Note AN-1150 - International Rectifier

A standard 1MΩ, 1% tolerance resistor is selected for R FB1 & R FB2 for this
converter. Then, R FB3 is determined based on error amplifier V REF (Typ)=5V and
V OUT =385V converter specification.
R
FB3
V
=
REF
( RFB1
+ RFB
2
( V
out
V
REF
5.0V
(2000k)
R FB 3
= = 26. 3kΩ
(385V - 5.0V)
A standard resistor, R FB3 = 26.1kΩ, 1% tolerance, is selected for this converter.
The new regulation V OUT value based on actual resistor values is then calculated.
V
OUT
( R
=
FB1
+ RFB
2
+ R
R
FB3
)
FB3
)
) ⋅V
( 2000k
+ 26.1k)
⋅ 5.
0V
V OUT
=
= 388. 1V
26.1k
Power dissipation of divider resistors is given by the following.
P
= P
=
2
( Vout
−VREF
)
( R R )
RFB 1 RFB
2
2
FB1
+
( 388.1V
− 5V
)
FB 2
REF
PR = P
mW
FB R FB
=
= 37
1
2
4 × 1000k
VFB is a multi-function pin with the following functionalities:
• The VFB pin is an input to the open-loop comparator that references a V OLP
threshold of 19% of V REF . The IC is restrained in the Stand-by Mode
whenever VFB pin is less than V OLP .
• The VFB pin is also non-inverting input to the overvoltage comparator. This
comparator is designated the OVP(VFB) comparator in the IC Block diagram
in the datasheet. The typical overvoltage set-point is V OVP =106%V REF and the
re-enable set-point is V OVP(RST) =103%V REF .
VOVP = 1 .06⋅VREF
= 5. 30V
VOVP RST
= 1.03 ⋅VREF
5. 15V
( )
=
Using the resister divider determined earlier, it is a straightforward calculation
to obtain the overvoltage trigger/re-enable set-points.
V
V
V
V
OVP
OVP
OVP ( RST )
OVP ( RST )
= 1.06 ⋅V
Out
= 1.06 × 388.1 = 412V
= 1.03 ⋅V
Out
= 1.03 × 388 .1 = 400 V
2
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13