FA551 Datasheet, PDF (12/22 Pages) List of Unclassifed Manufacturers – CMOS IC For Switching Power Supply Control

FA551X
4. Overcurrent limiting circuit
The overcurrent limiting circuit detects the peak value of every
drain current pulse (pulse by pulse method) of the main
switching MOSFET to limit the overcurrent. The detection
threshold voltage is +0.24V for FA5510/11 or –0.17V for
FA5514/15 with respect to the ground as shown in Figure 13
and Figure 14. The drain current of the MOSFET is converted
to voltage by resistor Rs and fed to the IS pin of the IC. If the
voltage exceeds the reference voltage +0.24V (FA5510/11) or
–0.17V (FA5514/15) of comparator C4, comparator C4 works to
set flip-flop output Q to high. The output is immediately turned
off to shut off the current. Flip-flop output Q is reset on the next
cycle to turn on the output again. This operation is repeated to
limit the overcurrent.
If the overcurrent limiting circuit malfunctions due to noise,
place an RC filter between the IS pin and MOSFET as shown in
Figure 13 and Figure 14. (See item 12 in “Design advice.”)
Figure 15 is a timing chart that illustrates overcurrent-limiting
operations.
5. Vcc overvoltage protection circuit
The IC contains a Vcc overvoltage protection circuit to protect
the IC from damage by overvoltage. Figure 16 shows the
overvoltage protection circuit. Figure 17 is a timing chart that
illustrates overvoltage protection operations. Overvoltage is
detected if the supply voltage Vcc rises to 31.8V (Icc=14mA) or
more and current flows in the built-in zener diode. The output of
comparator C5 then goes high and the constant current source
(0.95mA) raises the CS pin voltage. When the CS pin voltage
exceeds 8.5V, the output of comparator C2 goes high to turn off
the 5V REF circuit. The IC then enters the latched mode and
the IC output is put in the off (low voltage) state. When latched
mode, the IC current consumption is 45µA (typ) (Vcc=10V).
This current must be supplied through the startup resistor.
The overvoltage shutdown operation can be reset by lowering
the supply voltage to below 9.0V or forcing the CS pin voltage
below 7.9V. (When you want to enable Vcc overvoltage
shutdown at a desired voltage, see item 6 in “Design advice.”)
FA5510/11
PWM ^{OSC UVLO}
CS pin
FB pin
Oscillator
OUT
Output _pin
circuit
O.C.P.
R
Q
0.24V
S
C4
F.F
Rs
3
IS(+)
C
4 _GND
R
Fig. 13 Overcurrent limiting circuit (FA5510/11)
FA5514/15
PWM ^{OSC UVLO}
CS pin
FB pin
Oscillator
OUT
Output _pin
circuit
O.C.P.
R
Q
-0.17V
S
C4
F.F
Rs
3
IS(-)
C
4 _GND
R
Fig. 14 Overcurrent limiting circuit (FA5514/15)
12
CS pin voltage (4V)
DT voltage
FB pin voltage
Oscillator output
H
OUT pin output
L
Comparator C4
reference voltage
FA5510/11:+0.24V
FA5514/15:-0.17V
IS pin voltage
REF pin voltage ^ON
OFF
Overcurrent limiting
Fig. 15 Overcurrent timing chart
~+
Vin
~-
REF ₇
FB 2
Cs
8 ^CS
VCC
6
5V ^VCC
ENB
5V REF
3.5V
C1 ^0.8/0.68V
C2
15.5V
0.95mA
15.5V
C5
8.5/7.9V
UVLO
0.8V
C3
4V
PWM
DMAX
Output
circuit
OSC
1
RT
Fig. 16 Overvoltage shutdown circuit
Comparator C2
reference voltage
(8.5V)
(4V)
CS pin voltage
DT voltage
FB pin voltage
Osillator output
H
OUT pin voltage
L
REF pin voltage ^ON
OFF
(9.5V)
(0V)
Overvoltage
detection
Shutdown
Fig. 17 Overvoltage shutdown timing chart