View max6661 datasheet:

19-2337; Rev 0; 1/02 Remote Temperature-Controlled Fan-Speed Regulator with SPI-Compatible Interface General Description Features The MAX6661 is a remote temperature sensor and fan- Integrated Thermal Measurement and Fan speed regulator that provides complete closed-loop fan Regulation control. The remote temperature sensor is typically a Programmable Fan Threshold Temperature common-collector PNP, such as a substrate PNP of a microprocessor, or a diode-connected transistor, typi- Programmable Temperature Range for Full-Scale cally a low-cost, easily mounted 2N3904 NPN type or Fan Speed 2N3906 PNP type. Accurate Closed-Loop Fan-Speed Regulation The device also incorporates a closed-loop fan con- troller that regulates fan speed with tachometer feed- On-Chip Power De