US90A_14 Datasheet, PDF (4/12 Pages) Melexis Microelectronic Systems – 24V Hall IC Fan Driver

US90A / US91A
24V Hall IC Fan Driver
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
The absence of a VDD pin enables the two low side output drivers and FG/RD signal output to fit in a four-pin VK package.
The lack of a VDD pin decreases also the probability to damage the chip due to reverse voltage 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.
3901090263
Rev 008
Page 4 of 12
Data Sheet
Jan/14