The US90 is a one-chip solution for driving two-coil brushless DC fans. Based on advanced Melexis CMOS process, the IC contains a Hall-effect sensor, dynamic offset correction and low side output drivers. The output drivers OUT1 and OUT2 are fully protected against switching transients. So there is no need for an external Zener-diode to cut the high voltage spikes induced by the fan coils.
The US90A has an open-drain integrated tachometer (FG) output that follows the Hall signal. It goes high when the flux switching frequency becomes too low, which means the motor is blocked.
These features are combined with the Melexis patented no-VDD design to fit the IC in a small 4-pin through-hole package (TO92).