Switch-mode power supply having reduced audible noise

A power supply that provides output voltage to a load comprising:

• a power converter coupled to the load, said power converter having at least one switch that is operated at a variable switching frequency that decreases towards an audible frequency range as the load decreases, wherein current pulses are produced through the at least one switch in accordance with the variable switching frequency, said variable switching frequency initially being inside the audible frequency range when the load decreases below a first level;
• a controller that regulates the output voltage by controlling said variable switching frequency based on a feedback signal coupled to the load, said controller comprising an error amplifier, a voltage-controlled oscillator, a comparator, a current sense resistor, and a switch timing and drive circuit, said comparator comparing a current sense voltage developed across the current sense resistor by the switch current pulses with a current-sense reference voltage;
• and
• an audible noise reduction circuit that modifies said variable switching frequency of said power converter when said variable switching frequency is inside the audible frequency range, the audible noise reduction circuit configured to operate by:
• increasing the variable switching frequency above a top threshold of he audible frequency range when the load is between the first level and a second level smaller than the first level,
• increasing the variable switching frequency to an upper part of the audible frequency range that is below the top threshold of the audible frequency range when the load is between the second level and a third level smaller than the second level, and
• decreasing the variable switching frequency to a lower part of the audible frequency range that is above a bottom threshold of the audible frequency range when the toad is below the third level.