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FA5304AP

Bipolar IC For Switching Power Supply Control

Fuji Electric 

s Description of each circuit

1. Oscillator (See block diagram on page 8.)

The oscillator generates a triangular waveform by charging

and discharging a capacitor. CT pin voltage oscillates

between an upper limit of approx. 3.0V and a lower limit of

approx. 1.0V. The oscillation frequency is determined by a

external capacitance CT connected to CT pin, and

approximately given by the following equation:

f (kHZ) =

4.8 • 104

CT (pF)

..................(1)

The recommended oscillation range is between 5k and

600kHz.

The oscillator output is connected to a PWM comparator.

2. Feedback circuit

Figure 1 gives an example of connection in which built-in error

amplifier is used to couple the feedback signal to IN(-) pin. Let n2

be the number of turns of secondary winding L2 and n3 be the

number of turns of tertiary winding L3. VCC and Vout are given by

Vcc= 2(V)•(R1+R2)/R2....................................(2)

VOUTȃ(n2/n3)•(Vcc+VD3)–VD2........................(3)

(where VD2 and VD3 are the forward voltage drops across diodes D2

and D3 respectively).

Here, the following equation must be satisfied to prevent from

the malfunction of OUT pin at shutdown.

(R1•R2)/ (R1+R2)Œ11kΩ...............................(4)

Figure 2 gives an example of connection in which an

optocoupler is used to couple the feedback signal to the FB

pin. If this circuit causes power supply instability, the frequency

gain can be decreased by connecting R4 and C4 as shown in

figure 2. R4 should be between several tens of ohms to

several kiloohms and C4 should be between several thousand

picofarads to one microfarads.

3. PWM comparator

The PWM comparator has four inputs as shown in Figure 3.

Oscillator output x is compared with CS pin voltage , FB pin

Ž, and DT voltage {. The lowest of three inputs , Ž, and {

is compared with output x. If it is lower than the oscillator

output, the PWM comparator output is high, and if it is higher

than the oscillator output, the PWM comparator output is low

(see Fig. 4).

The IC output voltage is high during when the comparator

output is low, and the IC output voltage is low during when the

comparator output is high.

When the IC is powered up, CS pin voltage  controls soft

start operation. The output pulse then begins to widen

gradually. During normal operation, the output pulse width is

determined within the maximum duty cycle (FA5304A,

FA5305A: 45%) set by DT voltage { under the condition set

by feedback signal Ž, to stabilize the output voltage.

FA5304AP(S)/FA5305AP(S)

Fig. 1 Configuration with error amplifier

Fig. 2 Configuration with optocoupler (FB pin input)

Fig. 3 PWM comparator

Fig. 4 PWM comparator timing chart

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