The circuit described in this application note reduces acoustic noise by converting PWM fan drive to linear. Fans are a necessary part of many thermal management strategies, but can cause excessive levels of audible noise when operating at full speed. Adjusting the cooling-fan speed as a function of temperature is a good way to reduce acoustic noise in a system. The standard way to make that adjustment uses an IC that combines temperature sensing with a PWM output, which in turn controls the power supply for a DC brushless fan.