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AN487 APPLICATION NOTE

OSCILLATOR and FEDFORWARD

The oscillator block (fig.4) generates the sawtooth waveform that sets the switching frequency of the sys-

tem. The signal, compared with the output voltage of the error amplifier, generates the PWM signal to be

sent to the power output stage. The oscillator features a voltage feed-forward technique which is com-

pletely integrated and doesn’t require any external component. Feed-forward function works in the supply

voltage range 15-45V. The rate of increase of the sawtooth wavw-form is directly proportional to the input

voltage Vcc. As Vcc increases, the output pulse-width (transistor on-time) decreases in such a manner as

to provide a constant "volt-second’ product to the inductance(fig.5).

Figure 4. Oscillator circuit

From fig.5 it is shown that the duty cycle changes due to the ramp increase when Vcc increases. The error

amplifier output doesn’t have to change to keep the loop in regulation. This feature increases significantly

the line regulation performance.

Figure 5. Voltage Feeforward waveform.