The MAX6643/MAX6644/MAX6645 monitor temperature and automatically adjust fan speed to ensure optimum cooling while minimizing acoustic noise from the fan. Each device measures two temperature locations. The MAX6643/MAX6644/MAX6645 generate a PWM waveform that drives an external power transistor, which in turn modulates the fan's power supply. The MAX6643/MAX6644/MAX6645 monitor temperature and adjust the duty cycle of the PWM output waveform to control the fan's speed according to the cooling needs of the system. The MAX6643 monitors its own die temperature and an optional external transistor's temperature, while the MAX6644 and MAX6645 each monitor the temperatures of one or two external diode-connected transistors. The MAX6643/MAX6644/MAX6645 are available in H and L versions. The H versions are trimmed for higher temperature trip thresholds than the L versions. The MAX6643 and MAX6644 have nine selectable trip temperatures (in 5C increments). The MAX6645 is factory programmed and is not pin selectable. All versions include an overtemperature output ( OT). OT can be used for warning or system shutdown. The MAX6643 also features a FULLSPD input that forces the PWM duty cycle to 100%. The MAX6643/MAX6644/ MAX6645 also feature a FANFAIL output that indicates a failed fan. See the Selector Guide for a complete list of each device's functions.