
FA7622CP(E)
5
Inverting chopper circuit
According to the circuit shown in Fig. 5, the power output
voltage V
OUTB
can be determined as follows:
V
OUTB =
–
R
11
R
10
(5)
..............................
V
REF
The A
V
between the V
OUTB
and FB2 pins can be determined
as follows:
A
V
1
–R
11
R
12
(6)
.................................................
To correct the phase, connect the resistor R
13
and capacitor
C
3
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).
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
C
T
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 C
T
voltage, the
PWM output is high. When it is higher than C
T
, 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 C
T
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:
μ
F for Cs and k
for Rs, Rs1, and Rs2)
Fig. 5
Fig. 8
Fig. 6
1
2
ER.AMP2
C3
Q3
FB2
R10
R9
REF
R11
R12
R13
IN2
IN2
V
CC1
+
-
V
OUTB
(Controlled by Q3)
10
20
6
5
4
OUT2
1
2
2
PWM output
FB1(FB2)
DT1(DT2)
Time
CT
PWM output
PWM1
(PWM2)
DT1(DT2)
FB1(FB2)
CT
Fig. 7
DT1(DT2)
20
1
2
2
1
FB1(FB2)
C
S
CT
R
S
REF
PWM output
PWM1
(PWM2)
20
1
2
2
1
DT1(DT2)
FB1(FB2)
C
S
CT
R
S2
REF
PWM output
PWM1
(PWM2)
R
S1
Fig.8:
t
S
(m
S
) = CS R
R
S1
1
R
S2
R
S1
0.417R
S1
– 0.583 R
S2
(7)
.................................
Fig.7:
t
S
(m
S
) = 0.54C
S
R
S
Where, R
S1
/
R
S2
> 0.716
(8)
……
S1
R
S2
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.