FA7622CP Datasheet, PDF (5/8 Pages) Fuji Electric – Bipolar IC For Switching Power Supply Control

FA7622CP(E)
• Inverting chopper circuit
According to the circuit shown in Fig. 5, the power output
voltage VOUTB can be determined as follows:
REF
20
R11
VOUTB = –
• VREF .............................. ⁽⁵⁾
R10
The AV between the VOUTB and FB2 pins can be determined
as follows:
–R11
AV Ï©
................................................. ⁽⁶⁾
R12
R10
IN2⁺
R9
IN2 - 4
5
R11
R12
R13
C3
6
FB2
Ï©
Ϫ
ER.AMP2
V OUTB (Controlled by Q3)
Fig. 5
To correct the phase, connect the resistor R13 and capacitor
C3 in series between the IN2– and FB2 pins.
By using this circuit, invert the output polarity of OUT2 with an
external transistor to drive a P-channel MOSFET (or PNP
transistor).
DT1(DT2)
CT
FB1(FB2)
V CC1
Q3
10
OUT2
3. PWM comparator section
As Fig. 6 shows, a PWM comparator has three input
terminals. PWM comparator 1 determines the duty cycle of
the output from the OUT1 pin. This comparator compares the
CT oscillator Voltage (Pin 1) with the FB1 voltage (Pin 17) or
the DT1 voltage (Pin 16), whichever is greater. When the
highest of these voltages is lower than the CT voltage, the
PWM output is high. When it is higher than CT, the PWM
output is low.
PWM comparator 2 determines the duty cycle of the output
from the OUT2 pin. To determine the PWM output, this
comparator compares the CT oscillator voltage (Pin 1) with the
FB2 voltage (Pin 6) or the DT2 voltage (Pin 7) whichever is
higher.
During ordinary operation, the OUT1 and OUT2 pin voltages
have the same polarity as the output from each comparator.
When the power supply is turned on, the pulse width
gradually increases. The time constant for soft-start is
determined by the external resistor and capacitor across pins
16 and 7. In Figures 7 and 8, the time ts required for the pulse
width (duty-cycle) to reach about 30% after start-up can be
determined as follows:
(Units: µF for Cs and kΩ for Rs, Rs1, and Rs2)
Fig.7:
tS (mS) = 0.54CS • RS ................................. ⁽⁷⁾
Fig.8:
RS1 • RS2
RS1
tS (mS) = CS
• ln
……(8)
RS1 Ï© RS2
0.417RS1 – 0.583 RS2
Where, RS1 / RS2 > 0.716
Please connect enough large capacitance between REF and
GND pins in order to prevent irregular output pulse caused by
minus voltage at DT1 or DT2 pin when IC is shut down.
PWM output
CT
DT1(DT2)
FB1(FB2)
Ï©
Ϫ
Ϫ
PWM1
(PWM2)
Fig. 6
PWM output
Time
CT
1
CS
DT1(DT2)
RS
FB1(FB2)
REF
20
Ï©
PWM output
Ϫ
Ϫ
PWM1
(PWM2)
Fig. 7
CS RS1
CT
1
DT1(DT2)
RS2
FB1(FB2)
REF
20
Ï©
PWM output
Ϫ
Ϫ
PWM1
(PWM2)
Fig. 8
5