FA5331P(M)/FA5332P(M)
3. Current error amplifier and overcurrent limiting circuit
The current error amplifier forms a current loop to change the
input circuit current into sinusoidal waves. As Fig. 3 shows, the
multiplier output is connected to pin 2 (IIN –) through a resistor
(RA) to input the reference current signal. Pin 16 (IDET) is a
current input terminal. Design the circuit so that the voltage at
pin 16 will be within the range from 0 (GND potential) to –1.0V.
Connect a phase correction resistor and capacitors across pin
1 (amplifier output) and pin 2. See Fig. 4 for the expected gain
characteristics of the circuit shown in Fig. 3.
MUL
Vm
RA
R5
1
2
C3
10k
C2
CURR.AMP
_
A2
+
PWM
com parator
VREF
5V
RC
5k
RB
16
4
15k
Here,
Cn
Rn
1
Z =
C2
OPC
.................................................. (5)
F.F
2π R5 • C3
Vocp
1
Current
detection
p =
............................................. (6)
2π R5 • C
Fig. 3 Current error amplifier and overcurrent limiting circuit
C2 • C3
C =
C2 + C3
The voltage gain (G1) between Z and P of the circuit (gain
between pins 16 and 1) is given as follows:
Voltage gain
(dB)
R5
RA
G1 = 20 • log10 { 0.75 (
+ 1) }
....................(7)
Ensure an adequate phase margin by selecting C1 and C2 so
that the p/z ratio is about 10. The current error amplifier output
is used as an input to the comparator for PWM.
G1
The overcurrent detection comparator (C2) limits an
Z
P
Frequency
overcurrent. The threshold voltage for overcurrent detection at
pin 16 is –1.15V for FA5331 and –1.10V for FA5332. Connect
noise filters Rn and Cn to prevent the voltage at pin 16 from
fluctuating due to noise, causing the comparator to malfunction.
For Rn, select a resistor of up to 100Ω for FA5331 and up to
27Ω for FA5332. (See P64, 4. No-load operation )
Fig. 4 Voltage gain-frequency
CURR.AMP(A2) output Vc
Oscillator output Va
4. Comparator for PWM
Figure 5 shows the comparator for PWM. When the oscillator
output (Va) is smaller than the current error amplifier output
(Vc), the comparator output is high and the output ON signal is
generated at pin 8. Pin 11 (CS) is a terminal for soft start. This
terminal charges capacitor C4 with the internal constant current
(10µA) for a soft start. Priority is given to Vb and Vc whichever
is lower.
C3
CS
C4
11
Vb
PWM com parator
10µA
Fig. 5 PWM comparator
5. Multiplier
VIN
The multiplier generates a reference current signal. Input a
fully rectified sinusoidal signal voltage into pin 3 (VDET).
Design the circuit to keep the peak voltage at pin 3 within a
range from 0.65V to 2V for FA5331 and 0.65V to 2.4V for
FA5332. The multiplier output voltage (Vm) is roughly given as
follows (see Fig. 6):
ER.AMP(A1) output
R7
Ve
Vm
Vm = 1.25 – (Ve –1.55) • Vs.................................... (8)
MUL
As Fig. 3 shows Vm is internally connected to pin 2 (IIN–) of the
current error amplifier A2 through a 10kΩ resistor. (See the
characteristic curve, page 66 for the input and output
characteristics of the multiplier.)
3
Vs
R6
Fig. 6 Multiplier
6
FUJI [ FUJI ELECTRIC ]
