FA7700V Datasheet, PDF (10/13 Pages) List of Unclassifed Manufacturers – For Switching Power Supply Control

FA7700V, FA7701V
5. Soft start function
As described in Fig. 6, RCS is connected between CS terminal
and VCC terminal, and Cs is connected between CS terminal
and GND. The voltage of CS terminal rises when starting the
power supply, because Cs is charged by Vcc through Rcs. The
soft start function starts by charging a capacitor Cs connected
to PWM comparator. To estimate the soft start period, the time
(ts) between the start and the moment when the width of output
pulse reaches 50% is calculated by:
( ) ^{ts [ms] Ô¼ Cs Ï« RCS Ï« 1n}
VCC
VCC – 0.88
Cs : Capacity of Cs [µF]
Rcs : Resistance of Rcs [kΩ]
Vcc : Supply voltage [V]
The maximum current flowing in Rcs should be within the
recommended value (50µA max.).
Vcc – 1.5
50µA + IL
Ͻ Rcs [MΩ] Ͻ
Vcc – 1.5
1µA + IL
(IL: leak current of capacitor Cs)
Note: This IC operates ON/OFF function by the CS terminal (CS < 0.3V
typ. : OFF), then it turns off the internal bias voltage VREF (off
mode). Therefore, you can not connect the resistor “Rcs” between
CS terminal and REF terminal, and can connect the resistor only
to VCC terminal.
6. ON/OFF circuit
The ON/OFF function can be controlled by external signal to
the CS terminal, the IC becomes off mode. When the CS
terminal voltage is below 0.30V(typ.), the output of ON/OFF
comparator C3 is set to LOW, and the internal power source
VREF is shut off, then the IC is switched to the off mode.
The power consumption in the off mode is 40µA(typ.). A
sample circuit is given in Fig. 7.
7. Timer latch short-circuit protection circuit
The short-circuit protection circuit consists of two comparators
C1, C2 (Fig. 6). In steady operation, the output of S.C.DET
comparator C2 is set to High, and the CS terminal is clamped
by the 1.5V Zener diode, because the output of error amplifier
is about 1V. If the converter output voltage drops due to a
short-circuit, when the output voltage of error amplifier rises
excesses 1.5V, the output of S.C.DET comparator C2 is set to
low, and then the clamp of Zener diode is turned off.
As a result, the voltage of CS terminal rises up to the lower
value of either 5.5 V or the voltage of VCC terminal.
If the voltage of CS terminal excesses 2.2V, the output of S.C.P
comparator C1 is set to high, and the circuit shuts down the
output circuit of the IC. When it occurs, the current
consumption of the IC is 0.9mA (typ.) because the IC is set to
OFF latch mode. The period (tp) between the occurrence of a
short-circuit in the converter output and the triggering of the
short-circuit protection function can be calculated by the
following expression:
( ) ^{tp [ms] Ô¼ Cs Ï« RCS Ï« 1n}
Vcc – 1.5
Vcc – 2.2
Cs : Capacitance of Cs [µF]
Rcs : Resistance of Rcs [kΩ]
Vcc : Supply voltage [V]
Note: When the IC is used in a product with low VCC voltage, the
period (tp) of the triggering of the short-circuit protection
described above fluctuates significantly. Therefore, sufficient
care should be taken in such cases.
Example When Rcs=750kΩ, Cs=0.1µF: Vcc=2.5V: tp Լ 90ms
Vcc=3.6V: tp Ô¼ 30ms
10
RCC
Rcs
CS
8
Cs
REF OFF
+
C3
0.3 V
ON/OFF
5.5V
1.5V
+
Output
off
C1
S.C.P
FB
2.2V
+
1.5V
C2
S.C.DET
Fig. 6
Vcc
CS
ON/OFF
Cs
Fig. 7
6
Lower value of either 5.5V or Vcc terminal voltage
5
4
Start-up
3
2.2V
2
1.5V
1
0
tp
Momentary short circuit
Soft start
Time
Fig. 8
Short circuit protection
Short circuit