The TC654 and TC655 are PWM mode fan speed controllers with FanSense technology for use with brushless DC fans. These devices implement temperature proportional fan speed control which lowers acoustic fan noise and increases fan life. The voltage at VIN (Pin 1) represents temperature and is typically provided by an external thermistor or voltage output temperature sensor. The PWM output (VOUT) is adjusted between 30% and 100%, based on the voltage at VIN. The PWM duty cycle can also be programmed via SMBus to allow fan speed control without the need for an external thermistor. If VIN is not connected, the TC654/TC655 will start driving the fan at a default duty cycle of 39.33%. See Section 4.3, "Fan Startup", for more details.