US651 / US661

Two-Coil Low Noise Fan Driver

390109020902

Page 5 of 12

Preliminary Datasheet

Rev 002

Mar/07

8 Output Behaviour vs Magnetic Pole

Parameter

Test Conditions

OUT1

OUT2

FG

North pole

B < Brp

Low

High

Low

South pole

B > Bop

High

Low

High

Table 5: Driver output vs magnetic pole

Note : The magnetic pole is applied facing the branded side of the package

9 Detailed General Description

The US651/661 is a one-chip solution for driving two-coil brushless DC fans. Based on CMOS process, the

chip contains a Hall-effect sensor with dynamic offset correction, logic control and two low-ohmic open-drain

output drivers.

The output drivers OUT1 and OUT2 are fully protected against switching transients. So there is no need of

external zener diode to cut the high voltage spikes induced by the fan coils.

full bridge by setting the outputs OUT1 and OUT2 low and setting the output FG (RD) high.

The FG/RD open drain output has an internal current limit which protects the driver in case of accidentally big

current flow through the logic driver. It could occur if a low-ohmic pull-up resistor is used or if the FG/RD

output is directly short connected to a supply voltage.

The US651 has an open-drain tachometer FG output that follows the Hall signal, thus enabling to determine

the rotation speed of the fan.

In the US661, the open-drain alarm output RD is a safety signal which allows detecting if the fan rotates or

not. It is active low during normal spinning of the motor. It goes high when the magnetic flux switching

frequency drops below nearly:

- 2Hz for 12V application (60RPM for 2 pole-pair fan)

- 1Hz for 5V application (30RPM for 2 pole-pair fan)