The MAX6650/MAX6651 fan controllers use an SMBusTM/I2CTM-compatible interface to regulate and monitor the speed of 5VDC/12VDC brushless fans with built-in tachometers. They automatically force the fan's tachometer frequency (fan speed) to match a preprogrammed value in the Fan-Speed Register by using an external MOSFET or bipolar transistor to linearly regulate the voltage across the fan. The MAX6650 regulates the speed of a single fan by monitoring its tachometer output. The MAX6651 also regulates the speed of a single fan, but it contains additional tachometer inputs to monitor up to four fans and control them as a single unit when they are used in parallel.