US Patent 7710054 Fan controller with duty cycle comparator

A motor controller for a direct current motor includes a first node receiving first PWM signals having a duty cycle indicative of a desired rotational speed of said motor and an input node receiving second digital signals, the frequency of said second signals indicative of the rotational speed of said motor. A frequency-to-PWM circuit is coupled to the input node to provide second PWM signals having a duty cycle corresponding to the rotational speed of the motor. A duty cycle comparator has a first input coupled to the first node and a second input coupled to the second node to generate a control signal for controlling the rotational speed of the motor.