FA7703 Datasheet, PDF (13/21 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7703/04
FA7703/7704
9. Description of each circuit
(1)Reference Voltage Circuit
The reference voltage circuit of FA7703/04
generates the reference voltage (VREF) of
1.00V±1% compensated in temperature from
VCC voltage, and the regulated voltage (VREG)
of 2.2V ±2% for internal controlling. These
voltages start to output when the undervoltage
lockout protection (UVLO) is cancelled, and
they stabilize after the supply voltage (VCC)
reaches up to approx. 2.4V or higher. The
reference voltage (VREF) is connected to the
non-inverting input of Error Amplifier 1 and
serves as the reference voltage of Error
Amplifier 1. Because of Error Amplifiers have
offset voltage then, the precision of voltage in
practical use is 1.00V±2%. The voltage (VREF)
outputs externally from REF terminal, therefore,
it can serve as a stabilized power source. When
using it, be sure to set the output current 1mA
or below.
The regulated voltage (VREG) for internal
controlling serves as the stabilized power
source for maximum output duty setting or the
like. Be sure to set the output current 2mA or
below in operation in this case. This voltage
also serves as the control power source of all
the internal circuits of FA7703/04. A capacitor
for stabilization (CREG) is needed to be
connected to the VREG terminal. See
recommended operating conditions to
determine capacitance.
(2)Oscillator
The oscillator of FA7703/04 generates
triangular waveforms by charging and
discharging the built-in capacitor. Any desired
oscillation frequency can be obtained by setting
the value of the resistor connected to RT
terminal (Fig. 1).
The voltage oscillates between approximately
0.65V and 1.10V in charging and discharging
with almost the same gradients (Fig. 2). Your
desired oscillation frequency can be determined
by changing the gradient using the resistor (RT)
connected to RT terminal. (Large RT: Low
frequency, small RT: High frequency) The
waveforms of oscillator cannot be observed from
the outside because a terminal for this purpose
is not provided.
Fuji Electric Systems Co., Ltd.
AN-057E Rev.1.0
Jun-2010
Approximately DC 1V is output to RT terminal.
The oscillator output is connected to PWM
comparator.
OSC
Fig.1
1.10V
1 _RT
RT
RT value: small RT value: large
0.65V
Fig.2
(3)Error Amplifier Circuit
Error Amplifier 1 of FA7703/04 has the inverting
input IN1(-) terminal (Pin13). The non-Inverting
input is internally connected to the reference
voltage (VB) of 1.00V±2% at 25°C. Because
error Amplifier 2 of FA7703/04 has the inverting
input IN2(-) terminal (Pin5) and non-inverted
input IN2(+) terminal (Pin4) outputting externally,
various circuit can be designed by kinds of
external circuit structures. FB terminals (Pin6,
Pin12) are the outputs of Error Amplifiers.
Voltage Gain and phase compensation can be
set by connecting a capacitor (C) and a resistor
(R) between FB terminal and IN(-) terminal.(Fig.
3) For more information about the connection for
each output voltage of power supply, refer to
Design Advice.
RFB1 CFB1
Vout1
R1
IN1-
13
R2
14
REF
Er.Amp.1
VB
(1.0V)
FB1
12
Comp
Vout2
R4
R3
IN2+
4
5
IN2-
Er.Amp.2 Comp
FB2
6
RFB2 CFB2
Fig.3
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