FA7622CP Datasheet, PDF (4/8 Pages) Fuji Electric – Bipolar IC For Switching Power Supply Control

FA7622CP(E)
s Description of each circuit
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:
7.1 • 10⁵
fOSC ( kHz ) Ï­
...................… ⁽¹⁾
RT ( kΩ ) • CT ( pF )
2. Error amplifier section
Error amplifier Œ
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 Ï­
R1 + R2
•VB .................................… ⁽²⁾
R2
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:
( R3 + R4 ) • R6
VOUTB Ï­
• VREF ..................……(3)
R4 • ( R5 + R6 )
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:
R4
AV Ï­
R3 + R4
R7 • (R5 + R6)
• 1+
........... ⁽⁴⁾
R5 • R6
To correct the phase, connect the resistor R8 and capacitor C2
in series between the IN2– and FB2 pins.
CT pin voltage waveform
1.6V
1.0V
CT
CT
1
RT
2
OSC
Fig. 1 Oscillator
RT
VRT =1.0 (V)
ICT = Ȁ ^{1.0 (V)}
RT
CT
1
C_T
REF
20
I CT
V _CT : 1.0
¡ 1.6V
I _CT
V _CT : 1.6
¡ 1.0V
9
GND
Fig. 2
VOUTA (Controlled by Q1)
FB1
17
C1
R1
IN1+
18
R2
36kΩ
Ï©
Ϫ
VB ER.AMP1
Q1
11
OUT1
Fig. 3
VOUTB (Controlled by Q2)
REF
20
R5
R3
R4
IN2 ⁺
IN2 ^{- 4}
5
R6
R8
R7 _C2
6
FB2
Ï©
Ϫ
ER.AMP2
Q2
10
OUT2
Fig. 4
4