FAN7585
13
5. Power Good Signal Generator
Power Good Signal Generator circuit generates "ON or OFF" signal depending on the status of output voltage to prevent the
malfunctions of following systems like microprocessor, etc. caused by the output instability at power on or off .
At power on, it produces PG "High" signal after some delay time(about 260ms with C
TPG
=2.2uF) for stabilizing output volt-
age. At power off, it produces PG "Low" signal without delay time by sensing the status of power source for protecting follow-
ing systems. Vcc detection point(Pin9) can be calculated by following equation. Recommended values of R11, R12 are
determined by the following equation.
The COMP3 creates PG "Low" without delay when +5V output falls to less than 4.0V to prevent some malfunction at transient
status, thus it improves system stability.
When Remote On/Off signal is high, it generates PG "Low" signal without delay. It means that PG becomes "Low" before
main power is grounded.
PG delay time(T
PG
) is determined by capacitor value(C
TPG
), threshold voltage of COMP3 and the charging current and its
euqation is as following.
Considering the lightning surge and noise, there are two types of protections. One is a few time delay between TPG and PG for
safe operation and another is some noise margin of Pin10.
Noise_Margin_of_T
PG
= V
Pin10
(max)- Vth(L) = 2.9V - 0.6V = 2.3V
Vcc
Q2
PG
DET
Ichg
12
Vref
COMP3
T
PG
C
PG
2.2 uF
11
R15
1kΩ
Q3
16
+5V
9
0.6 V
1.8V
1.2 5V
COMP1
COMP2
Vref
Vref
R13
R12
R14
Remote
ON/OFF
10
R11
60kΩ
PG COMP
4.7kΩ
DET 1.25V= 1
R11
R12
-----------+
× 17.2V=
T
PG
C
TPG
∆V×
Ichg
-----------------------------=
C
TPG
Vth×
Ichg
------------------------------
≈
2.2uF 1.8V×
15uA
-----------------------------------= 260msec≈
