DC-DC converter with reduced electromagnetic interference

1. A DC-DC switching power supply, comprising:

a power train;

a variable frequency oscillator generating a variable frequency output as a function of a pseudo random number and comprising:

a pseudo random number generator providing the pseudo random number; and

a digital to analog converter that takes the pseudo random number output by the pseudo random number generator and sets a current level as a function of the pseudo random number; and

a control system adapted to control the power train as a function of the variable frequency output of the variable frequency oscillator.

2. The DC-DC switching power supply of claim 1 wherein said switching power supply comprises a boost converter.

3. The DC-DC switching power supply of claim 1 wherein said control system selectively gates a clock signal from the variable frequency oscillator to the power train.

4. The DC-DC switching power supply of claim 1 wherein said variable frequency oscillator further comprises a plurality of capacitors, said capacitors activated as a function of an output of the pseudo random number generator.

5. The DC-DC switching power supply of claim 3 wherein said variable frequency oscillator further comprises a capacitor adapted to charge and discharge as a function of said current level.

6. The DC-DC switching power supply of claim 5 wherein said variable frequency oscillator further comprises a plurality of current mirrors adapted to create a current sink and a current source so that the capacitor may charge and discharge.

7. The DC-DC switching power supply of claim 1 wherein said variable frequency oscillator further comprises a pair of capacitors both selectively charged and discharged in turn.

8. The DC-DC switching power supply of claim 1 wherein said switching power supply comprises a buck converter.

9. A DC-DC switching power supply, comprising:

a power train;

a variable frequency oscillator having an output and comprising:

a pseudo random number generator; and

a pair of digital to analog converters adapted to take a pseudo random number output by the pseudo random number generator and set a current level for charging and discharging a capacitor as a function of the pseudo random number; and

a control system adapted to control the power train as a function of the output of the variable frequency oscillator.

10. A method of operating a DC-DC converter comprising:

generating a variable frequency output with an oscillator as a function of a pseudo random number, generating the variable frequency output comprising:

generating the pseudo random number with the pseudo random number generator; and

providing the pseudo random number to a pair of digital to analog converters to set a current level used to charge and discharge a capacitor, thereby generating the variable frequency output as a function of the pseudo random number; and controlling a power train as a function of the variable frequency output.

11. The method of claim 10 wherein generating the pseudo random number with the pseudo random number generator comprises using a linear feedback shift register.

12. The method of claim 10 wherein controlling the power train as a function of the variable frequency output comprises selectively gating the variable frequency output to the power train.

13. The method of claim 10 further comprising using an output from a flip-flop to trigger generation of a new pseudo random number.

14. The method of claim 13 further comprising dividing the output from the flip-flop by a predetermined number such that a new pseudo random number is generated approximately once every 4 milliseconds.