THMC45

5V AND 3.3V DC BRUSHLESS FAN MOTOR DRIVER

WITH SINGLE WIRE CONTROL

SLIS101A − MAY 2001

6

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

Hall sensor amplifier inputs (H+, H−)

The THMC45 has an internal Hall sensor amplifier with signal conditioning to allow the use of low-cost Hall

sensors requiring no external components for noise filtering. The Hall signal conditioning block receives a

low-level differential voltage from the Hall position sensor and implements a zero differential voltage crossing

detection with a de-glitch time of 25

Figure 1, the OUTA output changes from sourcing current to sinking current after the 25-

µs de-glitch time.

Likewise, the OUTB output changes from sinking current to sourcing current after the 25-

µs de-glitch time. The

OUTA

OUTB

PWM Input

t(de-glitch)

t(de-glitch)

Differential Hall

Signal (H+ − H−)

Conditioned Hall

Amplifier Output

(Internal)

t(de-glitch)

Figure 1. Hall Sensor Signal Conditioning Waveform and OUTA/OUTB Commutation

Illustrated in truth table format, Table 1 shows OUTA and OUTB commutation and PWM.

Table 1. OUTA and OUTB Low-Side Drive Commutation

H+

H−

OUTA

OUTB

H

L

H

PWM

L

H

PWM

H

H-bridge motor drive outputs (OUTA, OUTB)

Using an H-bridge to drive a bipolar wound, two-phase BLM provides several advantages for dc fans over the

unipolar-wound motor commonly driven by two commutated low-side switches. A bipolar-wound motor has only

two connections; hence, the H-bridge drive topology requires only two output terminals and two traces are

needed on the fan PCB. A bipolar-wound stator has a single-wire winding which is simpler to manufacture, and

thus increases reliability and reduces manufacturing time. All factors combine to allow a smaller diameter fan

center hub, and thus higher blade area for increased airflow on a given fan frame size. Generally, an H-bridge

drive method with bipolar-wound stator increases fan motor torque density over a typical unipolar drive method.

The H-bridge drive method also eliminates the need for snubbing inductive energy at commutation transitions

and allows for recirculation of winding current to maintain energy in the motor while PWM switching occurs.