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FA5316P Datasheet(PDF) 4 Page - Fuji Electric |
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FA5316P Datasheet(HTML) 4 Page - Fuji Electric |
4 / 17 page 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 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 voltage , and DT voltage {. The lowest of three inputs , , 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 , to stabilize the output voltage. f (kHZ) = 106 .........(1) Fig. 1 Oscillator Fig. 2 Configuration with optocoupler (FB pin input) Fig. 3 PWM comparator Fig. 4 PWM comparator timing chart 4RT (k Ω) • CT (pF) |
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