PWM controller with temperature regulation of switching frequency

1. A circuit for controlling the switching frequency of an oscillator for a PWM controller for a switching mode power supply, comprising:

a first stage providing a signal which is related to the temperature of the controller for the switching mode power supply; and

an oscillator having an oscillation frequency and being responsive to the signal from the first stage for changing the oscillation frequency such that the oscillation frequency decreases as the temperature increases and the oscillation frequency increases as the temperature decreases.

2. The circuit of claim 1 , wherein the first stage comprises:

a first circuit responsive to the temperature of the controller and providing a current proportional to the absolute temperature of the controller; and

a second circuit responsive to the current proportional to the absolute temperature from the first circuit and providing a second current inversely proportional to the absolute temperature and wherein the current inversely proportional to the absolute temperature is provided to the oscillator circuit such that as the temperature increases, the oscillation frequency of the oscillator reduces.

3. The circuit of claim 2 , wherein the first circuit comprises:

at least one bipolar transistor responsive to the absolute temperature of the controller; and

wherein the second circuit comprises an amplifier stage having an inverting input and a non-inverting input with said current proportional to the absolute temperature being fed through a resistor coupled to said non-inverting input to provide a voltage proportional to the absolute temperature, and whereby said amplifier produces an output comprising said current inversely proportional to the absolute temperature.

4. The circuit of claim 3 , wherein said current inversely proportional to the absolute temperature charges a timing capacitor, said timing capacitor being coupled to an input of a comparator, the comparator providing an output when the voltage across said capacitor exceeds a predetermined reference voltage coupled to said comparator.

5. The circuit of claim 4 , wherein the oscillator circuit comprises at least one delay stage and a feedback line for discharging said capacitor after a delay time implemented by said delay stage.

6. The circuit of claim 5 , wherein the oscillator comprises a controlled switch controlled by said feedback line for discharging said capacitor.

7. A method for controlling the switching frequency of an oscillator for a PWM controller for a switching mode power supply, comprising:

providing a first signal which is related to the temperature of the controller for the switching mode power supply; and

controlling an oscillator having an oscillation frequency in response to the first signal by changing the oscillation frequency such that the oscillation frequency reduces as the temperature increases and as the temperature decreases, the oscillator frequency of the oscillator increases.

8. The method of claim 7 , wherein the step of providing a first signal comprises:

providing a current proportional to the absolute temperature of the controller; and

responsive to the current proportional to the absolute temperature, providing a second current inversely proportional to the absolute temperature and providing the current inversely proportional to the absolute temperature to the oscillator such that as the temperature increases, the oscillation frequency of the oscillator decreases.

9. The method of claim 8 , wherein the step of providing a current proportional to the absolute temperature comprises:

providing at least one bipolar transistor responsive to the absolute temperature of the controller; and

providing an amplifier stage having an inverting input and a non-inverting input with said current proportional to the absolute temperature being fed through a resistor coupled to said non-inverting input to provide a voltage proportional to the absolute temperature, and whereby said amplifier produces an output comprising said current inversely proportional to the absolute temperature.

10. The method of claim 9 , further comprising charging a timing capacitor with said current inversely proportional to the absolute temperature, and providing a voltage across said timing capacitor to an input of a comparator for providing an output when the voltage across said capacitor exceeds a predetermined reference voltage coupled to said comparator.