US890 Melexis Company, US890 Datasheet - Page 5

• Manufacturer Part Number: US890
• Description: Two-coil Fan Driver ? High Output Curren
• Manufacturer: Melexis Company

The US890/891 is a one-chip solution for driving two-coil brushless DC motors and fans. Based on advanced
Melexis CMOS process, the chip contains a Hall-effect sensor, dynamic offset correction and powerful output
drivers with 1200mA peak output current capability.
The low side output coil drivers are fully protected against switching transients. So an external zener diode is
not needed to cut the high voltage spikes induced by the motor coils.
The US890 has an open-drain integrated tachometer FG output that follows the Hall signal.
In the US891, 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.
Specially designed for driving large fans, both ICs have LRP characteristics optimised for low start-up
voltage. At low V
The motor start-up phase (from zero to maximum rotation speed) is illustrated below. Big fans with large
inertia may have slow start-up causing a longer first output pulse after power-on. If this pulse duration is
longer than the LRP T
The US890/891 provides an adequate and simple solution to prevent this by automatically adjusting the LRP
period at low voltage. It directly improves the motor start-up reliability.
3901000890
Rev 002
9 Detailed General Description
10 Unique Features
DD
, fans typically take longer to start up than at higher V
ON
period, the fan may falsely enter locked rotor condition.
Two-Coil Fan Driver – High Output Current
Page 5 of 12
US890 / US891
DD
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Data Sheet
Sep/07