4

FA531X series

s Description of each circuit

1. Oscillator (See block diagram)

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 resistance and capacitance shown in figure 1, and

approximately given by the following equation:

The recommended oscillation range is between 5k and

600kHz.

The oscillator output is connected to a PWM comparator.

2. Feedback pin circuit

Figure 2 gives an example of connection in which an

optocoupler is used to couple the feedback signal to the FB pin.

It is designed to be strong against noise and will not create

parasitic oscillation so much, because the output impedance at

the FB pin is as low as 4k to 5k. If this circuit causes power

supply instability, the frequency gain can be decreased by

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

voltage 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 set by DT voltage { under the

condition set by feedback signal Ž, to stabilize the output

voltage.

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

Fig. 1 Oscillator

Fig. 2 Configuration with optocoupler (FB pin input)

Fig. 3 PWM comparator

Fig. 4 PWM comparator timing chart