6

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.

Here,

Z =

1

2

π R5 • C3

.................................................. (5)

p =

1

2

π R5 • C

............................................. (6)

C =

C2 • C3

C2 + C3

The voltage gain (G1) between Z and P of the circuit (gain

between pins 16 and 1) is given as follows:

R5

RA

+ 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.

The overcurrent detection comparator (C2) limits an

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 100

Ω for FA5331 and up to

27

Ω for FA5332. (See P64, 4. No-load operation )

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

(10

µA) for a soft start. Priority is given to Vb and Vc whichever

is lower.

5. Multiplier

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):

Vm = 1.25 – (Ve –1.55) • Vs .................................... (8)

As Fig. 3 shows Vm is internally connected to pin 2 (IIN–) of the

current error amplifier A2 through a 10k

Ω resistor. (See the

characteristic curve, page 66 for the input and output

characteristics of the multiplier.)

Fig. 6 Multiplier

Fig. 4 Voltage gain-frequency

Fig. 3 Current error amplifier and overcurrent limiting circuit

Fig. 5 PWM comparator

PWM

VREF

_

+

A2

F.F

Vocp

2

1

4

16

Rn

Cn

RC

5k

RB

15k

5V

C2

C3

C2

Vm

RA

10k

R5

comparator

Current

detection

MUL

CURR.AMP

OPC

G1

(dB)

Voltage gain

ZP

Frequency

R7

Ve

ER.AMP(A1) output

Vs

Vm

R6

3

MUL

C4

Vb

10

µA

CS

C3

Oscillator output Va

PWM comparator

CURR.AMP(A2) output Vc

11