Patent ID: 7471125

Claim:
A sawtooth wave generating circuit for use in a sawtooth wave generating apparatus, the sawtooth wave generating apparatus comprising: a base frequency generating section and a frequency generation section which generates a reference signal having a predetermined frequency; a sawtooth wave forming section which generates a sawtooth wave based on the reference signal; a voltage comparator which compares a voltage value of the sawtooth wave generated by the sawtooth wave forming section with a predetermined voltage value; a phase comparator which compares a phase of the reference signal from the frequency generation section with a phase of an output signal outputted from the voltage comparator; and a low-pass filter which cuts out a high frequency component of an output signal from the phase comparator and feeds back the resulting signal to the sawtooth wave forming section, wherein the sawtooth wave generating circuit is provided in the sawtooth wave forming section for outputting a sawtooth wave output signal to generate the sawtooth wave, the sawtooth wave generating circuit including: (a) a constant current circuit which includes: a voltage source which supplies a predetermined voltage; a second FET having a gate electrode connected to the voltage source and a drain electrode which forms an output terminal of the constant current circuit; and a first FET having a source electrode connected to both the voltage source and the gate electrode of the second FET, a gate electrode connected to the low-pass filter so that an amount of a current outputted from the drain electrode of the second FET is adjusted in response to a control signal from the low-pass filter, and a drain electrode connected to a source electrode of the second FET; (b) a resistor having one end connected to the drain electrode of the second FET; (c) a capacitor having one end connected to the drain electrode of the second FET and the other end connected to ground; and (d) an IGFET having one terminal connected to the other end of the resistor, a gate electrode which is turned on or off in response to a predetermined signal from the base frequency generating section, and the other terminal connected to ground, wherein when the IGFET is turned off, a predetermined current flows into the capacitor to charge the capacitor to thereby output the sawtooth wave output signal.