FAN3506 Datasheet, PDF (5/16 Pages) Fairchild Semiconductor – PC SMPS Secondary Side Control IC

Block Description & Application Hints
1. OVP Block
PT1 13
PT2 14
V33
V5
V12
1
2
3
R1
R3
R5
VCC
+
+
+
SET of Latch
+
R2
R4
R6
_+ _COMP1
1.26V
FAN3506
OVP function is simply realized by connecting Pin1, Pin2, Pin3 to each secondary output. R1,2,3,4,5,6 are internal resistors of
the IC. Each OVP level is determined by resistor ratio and the typical values are 4V/6.1V/14.2V.
- OVP Detecting voltage for +3.3V
. Vovp33 = (R1+R2)/R2 * 1.26V = 4.1V(Typ)
- OVP Detecting voltage for +5V
. Vovp5 = (R3+R4)/R4 * 1.26V = 6.1V(Typ)
- OVP Detecting voltage for +12V
. Vovp12 = (R5+R6)/R6 * 1.26V = 14.3V(Typ)
Especially, Pin13 & Pin14 are prepared for extra OVP inputs or another protection signal, respectively. That is, if you want
over voltage protection of extra output voltage, then you can make a function with two external resistors.
- Threshold Voltage of Protection Input 1 : Vpt1 = 1.26V
- Threshold Voltage of Protection Input 2 : Vpt1 = 1.26V
OVP function operates without delay time. In the case of OVP, system designer should know a fact that the main power can be
dropped after a little time because of system delay, even if FPO is triggered by OVP.
So when the OVP level is tested with a set, you should check the secondary outputs(+3.3V/+5V/+12V) and FPO(Pin5)
simultaneously. you can know the each OVP level as checking each output voltage in just time that FPO is triggered from low
to high.
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