Patent ID: 7495407

Claim:
A circuit for controlling rotational speed of a fan, comprising: a sensor detecting an ambient temperature and generating a corresponding temperature signal; a first chipset connected to the fan and the sensor, the first chipset providing a pulse-width modulation signal to the fan for driving the fan, and receiving a rotational speed signal which indicates an actual rotational speed of the fan from the fan; and a second chipset connected to the first chipset, the second chipset defining a controlling relationship between the rotational speed of the fan and the temperature, wherein the temperature is partitioned into a plurality of temperature ranges and each of the temperature ranges corresponds to a rotational speed range of the fan; wherein the temperature signal is transmitted from the sensor to the second chipset, so that the second chipset determines a desired rotational speed range according to the controlling relationship, and controls the first chipset to adjust the pulse-width modulation signal until the actual rotational speed of the fan is in the desired rotational speed range.