
FA7622CP(E)
4
1. Oscillator section
This section charges and discharges an external capacitor CT.
The charge current is determined by the external resistor RT
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:
fOSC ( kHz )
7.1 10
5
RT ( k
) CT ( pF )
(1)
...................…
2. Error amplifier section
Error amplifier
As Fig. 3 shows, the inverting input of the error amplifier is
connected to the VB 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 VB. The power-supply output VOUTA can be
determined as follows:
VOUTA
R1 + R2
R2
(2)
.................................…
VB
The DC gain of the error amplifier is 40dB (typ.), regardless of
external parts connected to the IC. Correct the phase by
connecting capacitor C1 between the VOUTA 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 VOUTB to the IN2+ pin can be
determined as follows:
VOUTB
( R3 + R4 ) R6
R4 ( R5 + R6 )
(3)
..................……
VREF
The DC gain AV from the VOUTB to FB2 pin is 70dB (min),
when R7 is not connected.
When R7 is connected, the AV can be determined as follows:
AV
R4
R3 + R4
(4)
...........
1 +
R7 (R5 + R6)
R5 R6
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
=
CT
1.0 (V)
RT
RT
1.0V
1.6V
CT pin voltage waveform
O S C
1
REF
CT
CT
20
9
GND
I
V
: 1.0
1.6V
CT
I
V
: 1.6
1.0V
CT
Fig. 2
10
20
6
R3
VOUTB
FB2
ER.AMP2
C2
R4
R6
R5
IN2
REF
Q2
R8
R7
5
4
+
-
(Controlled by Q2)
OUT2
R1
18
17
11
VOUTA
R2
FB1
IN1
ER.AMP1
Q1
C1
+
(Controlled by Q1)
VB
36k
OUT1
s Description of each circuit