FAN7554 View Datasheet(PDF) - Fairchild Semiconductor
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Description
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FAN7554 Datasheet PDF : 22 Pages
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FAN7554
Feedback
As shown in figure16, the internal oscillator clock turns on the MOSFET. The feedback comparator operates to turn it off
again, when the MOSFET current reaches a set value proportional to Vfb. The feedback capacitor Cfb is charged by the inter-
nal current sources , 1mA and 5uA, and is discharged by the secondary side photo-coupler to control the output voltage.
Vfb
1mA
Cfb
5uA 5V
Vcc
1
FB
2R Vfb/3
OSC
R
SQ
R
OUT
6
IS
3
FAN7554
Figure 16. Feedback & PWM Circuit
DRIN
Vsense
Rs
Delayed Shutdown
During the normal operation, the feedback voltage is between 0~3V. If the output terminal overloads or an error happens to
the feedback loop, the delayed shutdown circuit operates. When the feedback voltage is less than 3V, the feedback capacitor is
charged by current sources, 1mA and 5uA; when the feedback voltage becomes greater than 3V, the capacitor is charged by the
5uA current source because diode D1 turns off. When the feedback voltage is less than 3V, the charge slope becomes an expo-
nential function and, when it is greater than 3V, the charge slope becomes linear. When the feedback voltage reaches almost
6V, the FAN7554 shuts down. The shut down circuit is configured for
auto-restart, so it automatically restarts when Vcc reaches the under voltage 9V.
FB
1
5uA
Vcc
D1
1mA
Cfb
5V
2R
R
OSC
SQ
R
Over Current
Comparator
S Q Shutdown
R
6V
UVLO - out
OUT
6
DRIN
IS
3
Rs
FAN7554
Figure 17-A . Delayed Shutdown & Feedback Circuit
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