The TC649B is a new version of the existing TC649 fan speed controller. This device is a switch mode fan speed controller, which incorporates a new fan auto restart function. Temperature proportional speed control is accomplished using pulse width modulation. A thermistor (or other voltage output temperature sensor) connected to the VIN input supplies the required control voltage of 1.20V to 2.60V (typical) for 0% to 100% PWM duty cycle. The auto-shutdown speed is set by a simple resistor divider on the VAS input. An integrated Start-Up Timer ensures reliable fan motor start-up at turn-on, coming out of shutdown mode, auto-shutdown mode, or following a transient fault. A logic low applied to VIN, pin 1, causes fan shutdown.

The TC649B also features Microchip Technology's proprietary FanSense™ technology for increasing system reliability. In normal fan operation, a pulse train is present at SENSE, pin 5. A missing-pulse detector monitors this pin during fan operation. A stalled, open, or unconnected fan causes the TC649B device to turn the VOUT output on full (100% duty cycle). If the fan fault persists (a fan current pulse is not detected within a 32/F period), the FAULT output goes low. Even with the FAULT output low, the VOUT output is on full during the fan fault condition in order to try and restart the fan.