Programmable electro-magnetic-interference (EMI) reduction with enhanced noise immunity and process tolerance

Abstract

A frequency dithering circuit reduces emissions that cause Electro-Magnetic Interference (EMI) by spreading the spectrum of a clock. The clock sequences a counter that drives a digital count value to a digital-to-analog converter (DAC). The DAC outputs a sawtooth wave with a wide voltage swing. A subtractor scales down the voltage swing to produce a reduced-swing sawtooth wave which is used as an upper limit voltage. Comparators trigger a set-reset latch to toggle the clock when current pumps charge and discharge a capacitor beyond voltage limits. Since the upper limit voltage is the reduced sawtooth wave from the subtractor, the amount of time to charge the capacitor varies, dithering the period of the clock. The degree of dithering can be adjusted by programming the feedback resistance in the subtractor. The subtractor reduces the sensitivity of dithering to errors in the DAC, allowing for an inexpensive, less precise DAC.

Description

FIELD OF THE INVENTION[0001]This invention relates to Electro-Magnetic Interference (EMI) reduction, and more particularly to circuits with frequency dithering for reduced EMI.BACKGROUND OF THE INVENTION[0002]Power converters may include a large transistor that is rapidly switched on and off. This rapid switching can cause noise on the resulting power-supply voltage, and upon other nearly signals. Downstream and upstream devices from the power converter can generate emissions that create disturbances in other electronic equipment. For example, a noisy switching power supply driving a portable computer may emit radiation that interferes with a television receiver. Intentional receivers, such as televisions, cellular phones, pagers, and wireless devices, are often affected by unintentional transmitters that emit electromagnetic radiation. As both types of electronic equipment become more common, interference becomes more noticeable to the consumer.[0003]Electromagnetic interference (E...

AI Technical Summary

Problems solved by technology

This rapid switching can cause noise on the resulting power-supply voltage, and upon other nearly signals.

Downstream and upstream devices from the power converter can generate emissions that create disturbances in other electronic equipment.

For example, a noisy switching power supply driving a portable computer may emit radiation that interferes with a television receiver.

Intentional receivers, such as televisions, cellular phones, pagers, and wireless devices, are often affected by unintentional transmitters that emit electromagnetic radiation.

As both types of electronic equipment become more common, interference becomes more noticeable to the consumer.

Improving technology also worsens the EMI problem.

Higher resolution monitors and displays require that more pixels be transferred to the screen for each screen refresh period; thus a higher clock rate and more interference results.

Coax wires and shielded cables are effective at containing radiation, but are expensive, heavy, bulky, and inflexible.

The weight and bulk of shielded cables make them undesirable for portable devices.

Of course, filters require additional capacitors, resistors, or inductors, raising the cost.

These bulky components are undesirable and costly and defeat integration.

The high intensity of the spike can cause interference in a receiver.

However, often such frequency dithering requires switching precise components such as precision capacitors to generate small steps or adjustments in frequency.

Matching of these precision capacitors or resistors is difficult.

Such small currents are themselves subject to noise interference and leakages and are thus undesirable.

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