FA5331P(M)/FA5332P(M)
■ Description of each circuit
13 REF
1. Oscillator section
This section outputs sawtooth waves oscillating between 0.15
and 3.55V using the capacitor charge and discharge
characteristics. Figure 1 shows how to connect the required
external components to this circuit. The oscillation frequency
is determined by the CT and RT values. The relationship
between the CT and RT values is shown in characteristic
curves. Pin 14 (SYNC) is a synchronizing input terminal
whose threshold voltage is about 1V. As Fig. 1 shows, input
rectangular synchronizing signal waves to pin 14 through an
RC circuit. Set the free-running frequency about 10% lower
than the synchronizing signal frequency. Connect a clamp
diode (D1) to prevent an unwanted current inside the IC.
RT
CT
15
CT
OSC
R
14
SYNC
Csy
D1
Fig. 1 Oscillator
2. Voltage error amplifier and overvoltage limiting circuit
The voltage error amplifier forms a voltage feedback loop to
keep the output voltage stable. The positive input terminal of
this amplifier is connected to the reference voltage (Vr). Fig. 2
shows how to connect the required external components to
this circuit.
Vo
C1
5
R2
R1
R4
The output voltage (Vo) is as follows:
ER.AMP
_
R1 + R2
6
...............................................................................(1)
Vo =
• Vr
R3
A1
MUL
R1
+
FA5331: Vr=1.54V(typ.)
FA5332: Vr=1.55V(typ.)
Vr
Connect a resistor and a capacitor in parallel across error
amplifier output pin 5 and error amplifier negative input pin
6 to set the voltage gain (Av).
OVP
C1
4
F.F
Vp
The Av value is as follows:
R4
............................... (2)
Av =
R3 ( 1 + jω C1 • R4 )
Fig. 2 Voltage error amplifier and overvoltage limiting circuit
Error amplifier cutoff frequency (fc) is as follows:
1
fc =
................................................. (3)
2π C1 • R4
If 100 or 120Hz ripples appear at the error amplifier output, the
active filter does not operate stably. To ensure stable
operation, set the fc value to about 1Hz.
An overvoltage detection comparator (C1) is built in to limit the
voltage if the output voltage exceeds the design value. The
reference input voltage (Vp) is as follows:
Vp = α • Vr ............................................................. (4)
α =1.065
The connections shown in Fig. 2 limit the output voltage to α
times the design value.
5
FUJI [ FUJI ELECTRIC ]
