FA7622CP(E)
4
1. Oscillator section
This section charges and discharges an external capacitor C
T
.
The charge current is determined by the external resistor R
T
connected to the IC. By charging and discharging the
capacitor, this section provides a 1.0 to 1.6V triangle wave at
the CT pin. The oscillation frequency can be set between
50kHz to 1MHz. The frequency can be
calculated approximately as follows:
f
OSC
( kH
z
)
5
7.1 10
5
R
T
( k
) C
T
( pF )
(1)
...................…
2. Error amplifier section
Error amplifier
As Fig. 3 shows, the inverting input of the error amplifier is
connected to the V
B
reference voltage (0.858V typ.). The non-
inverting input IN1+ and output FB1 connect to external
terminals.
During ordinary operation, the IN1+ terminal voltage is almost
equal to V
B
. The power-supply output V
OUTA
can be
determined as follows:
V
OUTA
5
R
1
+ R
2
R
2
(2)
.................................…
V
B
The DC gain of the error amplifier is 40dB (typ.), regardless of
external parts connected to the IC. Correct the phase by
connecting capacitor C
1
between the V
OUTA
and FB1 pins.
Error amplifier
Voltage step-up or step-down chopper circuit
As Fig. 4 shows, the non-inverting input IN2+, inverting input
IN2–, and output FB2 of the error amplifier are connected to
external terminals.
The feedback voltage V
OUTB
to the IN2+ pin can be
determined as follows:
V
OUTB
5
( R
3
+ R
4
) R
6
R
4
( R
5
+ R
6
)
(3)
..................……
V
REF
The DC gain A
V
from the V
OUTB
to FB2 pin is 70dB (min),
when R
7
is not connected.
When R
7
is connected, the A
V
can be determined as follows:
A
V
5
R
4
R
3
+ R
4
(4)
...........
1 +
R
7
(R
5
+ R
6
)
R
5
R
6
Fig. 3
To correct the phase, connect the resistor R8 and capacitor C2
in series between the IN2– and FB2 pins.
Fig. 4
Fig. 1 Oscillator
1
2
RT
CT
C
T
V =1.0 (V)
RT
I =
w
CT
1.0 (V)
R
T
R
T
1.0V
1.6V
CT pin voltage waveform
O S C
1
REF
CT
C
T
20
9
GND
I
V : 1.0
1.6V
CT
I
V : 1.6
1.0V
CT
Fig. 2
10
20
1
2
6
R3
V
OUTB
FB2
ER.AMP2
C2
R4
R6
R5
IN2
IN2
REF
Q2
R8
R7
5
4
+
-
(Controlled by Q2)
OUT2
R1
18
17
11
1
2
V
OUTA
R2
FB1
IN1
ER.AMP1
Q1
C1
+
(Controlled by Q1)
V
B
36k
OUT1
I
Description of each circuit