FAN3506 Datasheet, PDF (6/16 Pages) Fairchild Semiconductor – PC SMPS Secondary Side Control IC

FAN3506
2. PSON & ON /OFF Delay Block
Toff
Ton
FPO
5
PSON
FPO
OVP
S
Q
R
VCC
lil
3.2V
VCC
VCC
+
_
COMP2
1.4V
PG
lon
VCC
_
+
1.26V
COMP3
1.5V ~ 0.6V
VCC
+
_
Q1
VCC
loff
7
PSON
8
TPSON
C1
103
11
Rd
R1
27K
PSON & On/Off Delay Block is controlled by a Microprocessor.
If a high signal is supplied to the PSON(Pin7), the output of COMP2 becomes high status. The output signal is transferred to
On/Off delay block and PG block.
If no signal is supplied to Pin7, Pin7 maintains high status(=3.2V) for the internal pull-up resistor.
When PSON is high, it produces FPO(Pin5) "High" signal after OFF delay time (about 8ms) for stabilizing system.
Then, all outputs (+3.3V, +5V, +12V) are grounded.
When PSON is changed to "Low", it produces FPO "Low" signal after ON delay time (about 26ms) for stabilizing the
system.
If PSON is low, then FPO is low. That means the main SMPS is Active(operational). When PSON is high, FPO is high and the
main SMPS is turned-off.
On/Off Delay Time can be calculated by following equation. 0.22uF Capacitor is recommended for following equations.
- Ton = (Ctpson*VthH)/Ion=(0.22uF*1.8V)/16uA = 26ms
- Toff = [Ctpson*(V8max-VthL)] / Ioff = [Ctpson*(V8max-VthL)] / (VRd/Rd)
Because Ion current for charging is fixed by internal current source, On delay time is varied by the capacitor value. On the con-
trary, Off Delay time is decided by the Rd value. If the Rd is 27K(Recommended) and the Delay capacitor valuse is 0.22uF,
Toff is 8ms(Typ).
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