US890 / US891

Two-Coil Fan Driver – High Output Current

3901000890

Page 9 of 12

Data Sheet

Rev 002

Sep/07

13 Application Information

13.1 Typical application circuit

13.2 Recommended circuit for ground

disconnection protection

14 Application Comments

The application 13.1 shows the typical application including a diode D1 for additional reverse voltage

protection. Without this diode, the reverse current in each coil is equal to the fan peak current, as it is only

limited by its coil resistances. In some fan construction where the peak current is not so big, this situation

may be acceptable by both IC and coils for short period of time. However, a long exposure may seriously

affect the reliability or damage each of them by overheating.

The diode D1 connected in series between the supply voltage and the common point of the coils prevents

this reverse current to flow. It prevents such reverse voltage damage and best protects both IC and coils.

The application 13.2 is given in case the ground connection might be disconnected while the fan operates. At

this moment, the current flowing through the coils may circulate from the output (OUT1 or OUT2) to the

FG/RD output driver in reverse direction. High reverse current may damage the logic output driver.

A bypass capacitor connected between the coil common node and the device ground should prevent such

damage occurring by providing a close path to this current.

The value of the capacitor needs to be tuned with the motor characteristics. Higher inductance may require

higher capacitor value.

For 2-wire fan application (tachometer or alarm logical output not required), the FG/RD pin (#1) should be

connected to the device ground pin (#4).