US90A / US91A

24V Hall IC Fan Driver

390109026302

Page 5 of 12

Data Sheet

Rev 002

Apr/07

9 Detailed General Description

The US90A/91A 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.

In the US91A, the open-drain rotation detection output RD is active low during normal spinning of the motor.

It goes high when the flux switching frequency becomes too low, which means the motor is blocked.

10 Unique Features

connection, using the coil resistance to limit the reverse current. In this condition, the total reverse current is

twice the peak current value of the fan used.

The built-in locked-rotor protection will automatically shut off the coil current when the rotor is mechanically

blocked, or the rotational speed drops below 60 RPM (4-pole rotor magnet). The fan will try to restart every

1.5 seconds until the obstruction is clear. The On / Off cycling reduces the average stall current to 1/7 normal;

this is enough to prevent overheating or damage to most fans. Both the US90A and US91A have this feature.