FA551X
11
3.2 Overload shutdown function
Figure 9 shows the overload shutdown circuit, and Figure 10 is
a timing chart that illustrates overload shutdown operation.
If the output voltage drops due to an overload or short circuit,
the FB pin output voltage rises. If the FB pin voltage exceeds
the reference voltage (3.5V) of comparator C3, the output of
comparator C3 goes low to turn off the switch. With the switch
off, the CS pin voltage clamped at 4.0V by zener diode in
normal operation is unclamped, and the constant current
source (5.2µA) begins to charge capacitor Cs again and the CS
pin voltage rises. When the CS pin voltage exceeds the
reference voltage (8.5V) of comparator C2, the output of
comparator C2 toggles to turn off the 5V REF circuit. The IC
then enters the latched mode and shuts down the output. IC
current consumption for shutdown is 45µA (typ) (Vcc=10V).
This current must be supplied through the startup resistor. The
IC enters output off (low voltage) state.
The overload shutdown operation can be reset by lowering the
supply voltage Vcc to below the OFF threshold voltage (9.0V)
or forcing the CS pin voltage below 7.9V.
The period tOL from the time the output is short-circuited to the
time the output circuit goes off is given by the following
equation:
When you want to disable the overload shutdown function, see
item 9 in “Design advice”
3.3 Output ON/OFF control function
The IC can be turned on or off via an external signal applied to
the CS pin. Figure 11 shows the output on/off control circuit,
and Figure 12 is a timing chart. The IC is turned off when the
CS pin voltage is externally made to drop below 0.68V (typ).
The output of comparator C1 goes high to turn the 5V REF
circuit off. This shuts the output down. The IC enters output off
(low voltage) state. Required IC current consumption during
shutdown is 80µA (typ) (Vcc=17V). This current must be
supplied through the startup resistor. The IC goes on when the
CS pin is opened and the CS pin voltage exceeds 0.80V (typ).
This turns the 5V REF circuit on and results in automatic soft
start. The power supply then restarts operation.
Fig. 9
Overload shutdown circuit
PWM
Output
circuit
UVLO
REF
RT
CS
FB
VCC
OSC
5V VCC
ENB
5V REF
4V
0.8/0.68V
8.5/7.9V
5.2
µA
DMAX
Cs
C3
3.5V
8
6
1
2
7
C1
C2
Comparator C2
reference voltage
(8.5V)
CS pin voltage
DT voltage
Oscillator output
OUT pin voltage
REF pin voltage
FB pin voltage
(4V)
(9.5V)
H
L
ON
OFF
Overload detection
Overload shutdown
(0V)
Fig. 10
Overload shutdown timing chart
Fig. 11
External output ON/OFF control circuit
Fig. 12
Output ON/OFF control circuit timing chart
PWM
Output
circuit
UVLO
REF
RT
CS
FB
VCC
OSC
5V VCC
ENB
5V REF
0.8/0.68V
8.5/7.9V
5.2
µA
DMAX
Cs
8
6
1
2
7
C1
C2
ON/OFF control
CS pin voltage
DT voltage
Oscillator output
OUT pin output
REF pin voltage
FB pin voltage
(4V)
H
L
ON
OFF
ON mode
OFF mode
Comparator C1
reference voltage
(0.8/0.68V)
..................................................... (4)
Cs: Soft start capacitor [µF]
tOL [ms]
870
CS [ms]
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