The TC646B/TC648B/TC649B devices are new versions of the existing TC646/TC648/TC649 fan speed controllers. These devices are switch-mode fan speed controllers that incorporate 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 threshold/temperature is set by a simple resistor divider on the VAS input. An integrated Start-Up Timer ensures reliable fan motor startup at turn-on, coming out of shutdown mode, autoshutdown mode or following a transient fault. A logic low applied to VIN (pin 1) causes fan shutdown.