FAN7680 Datasheet, PDF (12/16 Pages) Fairchild Semiconductor – PC Power Supply Outputs Monitoring IC

FAN7680
during 75ms to avoid turn-on failure. At turn-off, there is an additional delay of 2.3ms from PSON to FPO.
Power should be delivered to the rails only when the PSON signal is held at ground potential, thus the FPO becomes a low
state after a debounce of 38ms. The FPO pin can be connected to +5V(or up to +15V) through a pull-up resistor.
Under Voltage Protection(UVP)
The FAN7680 provides undervoltage protection(UVP) for the +3.3V, +5V, and +12V power rails. When an undervoltage con-
dition appears at one of the VS33(+3.3V), VS5(+5V), or Vcc(+12V) input pins for more than 73us, the PGO goes low and
FPO output goes high. Also, this fault condition will be latched until the PSON is toggled from low to high or the Vcc falls
below a minimum operating voltage.
When the power supply is turned on by the AC input or PSON, an internal UVP delay is 75ms. But at normal state an UVP
delay time is a 73us debounce time. The need for undervoltage protection is often overlooked in off-line switching power sup-
ply system design. But it is very important in battery powered or hand-held equipment since the TTL or CMOS logic often
malfunctions under UVP condition.
Over Voltage Protection(OVP)
The overvoltage protection(OVP) of the FAN7680 monitors +3.3V, +5V, and +12V(the +12V output is sensed via the Vcc
pin). When an overvoltage condition appears at one of the +3.3V, +5V, or +12V input pins for more than 73us, the FPO output
goes high and the PGO goes low. Also, this fault condition will be latched until the PSON is toggled from low to high or Vcc
drops below a minimum operating voltage. During fault conditions, most power supplies have the potential to deliver higher
output voltages than those normally specified or required. In unprotected equipment, it is possible for output voltages to be
high enough to cause internal or external damage to the system. To protect the system under these abnormal conditions, it is
common practice to provide overvoltage protection within the power supply.
Because TTL and CMOS circuits are very vulnerable to overvoltage, it is becoming industry standard to provide overvoltage
protection on all +3.3V, +5V, and +12V outputs. Therefore, not only the +3.3V and +5V rails for the logic circuits on the moth-
erboard need to be protected, but also the +12V peripheral devices such as the hard disk, flopply disk, and CD-ROM players
etc., need to be protected.
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