Low EMI producing switch-mode power supply within an intelligent electronic device

1. An apparatus for varying the timing of a switch-mode power supply (SMPS), the apparatus comprising:

a microcontroller configured to output a first pulse width modulated (PWM) signal; and

a converter coupled to the microcontroller and configured to receive the first PWM signal and output an analog signal to a SMPS timing capacitor coupled to a SMPS, wherein the SMPS timing capacitor is configured to discharge at a rate that determines a switching frequency of the SMPS;

wherein the converter is configured to generate the analog signal to vary the discharge time of the timing capacitor to vary switching frequency of the SMPS.

2. The apparatus of claim 1 , wherein the converter comprises a low-pass filter.

3. The apparatus of claim 1 , wherein the converter comprises a class D amplifier with an average time constant of approximately 100 ms.

4. The apparatus of claim 1 , wherein the analog signal comprises a sawtooth signal.

5. The apparatus of claim 1 , wherein the analog signal is a periodic signal, the period of the analog signal being less than 100 ms.

6. The apparatus of claim 1 , wherein the variation of the SMPS switching frequency has a bandwidth equal to the difference between a maximum switching frequency and a minimum switching frequency, the bandwidth of the switching frequency variation being greater than 9 kHz;

and wherein the period of the analog signal is less than 160 ms.

7. The apparatus of claim 1 , wherein the controller is configured to output the first PWM signal according to a bitstream comprising:

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8. The apparatus of claim 1 , further comprising an impedance coupled to the converter, the impedance configured to receive the analog signal and output to the timing capacitor an attenuated analog signal.

9. The apparatus of claim 1 , wherein the converter is a digital to analog converter.

10. A method of varying the timing of a switch-mode power supply (SMPS), the method comprising:

discharging a SMPS timing capacitor to set a switching frequency of the SMPS;

outputting a pulse width modulated (PWM) signal to a converter;

at the converter, converting the PWM signal to an analog signal;

receiving the analog signal at the SMPS timing capacitor;

varying the discharge time of the SMPS timing capacitor by varying the charge on the SMPS timing capacitor with the analog signal; and

causing the switching frequency of the SMPS to be modified by the varying of the discharge time of the SMPS timing capacitor.

11. The method of claim 10 , wherein converting the PWM signal comprises filtering the PWM signal with a low-pass filter.

12. The method of claim 11 , wherein the low-pass filter comprises a class D amplifier and wherein the average time constant of the low pass filter is approximately 100 ms.

13. The method of claim 10 , wherein the analog signal is a sawtooth signal.

14. The method of claim 10 , wherein the analog signal is a periodic signal, the period of the analog signal being less than 100 ms.

15. The method of claim 10 , wherein the variation of the SMPS switching frequency has a bandwidth equal to the difference between a maximum switching frequency and a minimum switching frequency, the bandwidth of the switching frequency variation being greater than 9 kHz;

and wherein the period of the analog signal is less than 160 ms.

16. The method of claim 10 , wherein the PWM signal is generated by a microcontroller and is represented by a bitstream comprising:

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17. The method of claim 10 , further comprising attenuating the analog signal with an impedance.

18. The method of claim 10 , wherein converting the PWM signal comprises using a digital to analog converter.

19. A system comprising:

a microcontroller configured to output a first pulse width modulated (PWM) signal;

a converter coupled to the microcontroller and configured to receive the first PWM signal and output an analog signal;

a timing capacitor coupled to the converter and responsive to the analog signal, wherein the converter is configured to output the analog signal to the timing capacitor to vary the discharge time of the timing capacitor by varying the charge on the timing capacitor; and

a switch-mode power supply (SMPS) coupled to the timing capacitor configured to monitor the charge on the timing capacitor and vary the frequency of a second PWM signal in response to the charge on the timing capacitor.

20. The system of claim 19 , wherein:

the converter comprises a low pass filter having a time constant of approximately 100 ms; and

the analog signal is a sawtooth signal.

21. The system of claim 19 , wherein the analog signal is a periodic signal, the period of the analog signal being less than 100 ms.

22. The system of claim 19 , wherein the variation of the SMPS switching frequency has a bandwidth equal to the difference between a maximum switching frequency and a minimum switching frequency, the bandwidth of the switching frequency being greater than 9 kHz and the period of the switching frequency dithering is less than 160 ms.

23. The system of claim 19 , wherein the first PWM signal is produced by a function generating a bit stream comprising:

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24. The system of claim 19 , further comprising an impedance configured to attenuate the analog signal.