THMC45

5V AND 3.3V DC BRUSHLESS FAN MOTOR DRIVER

WITH SINGLE WIRE CONTROL

SLIS101A − MAY 2001

8

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

PWM input (PWM)

The PWM input provides several functions:

commutation

and allowing the THMC45 to return to a normal run state on the next rising edge of the PWM input signal

The THMC45 requires a TTL level PWM input signal from another device, such as a Super I/O device. This

signal, along with the Hall sensor input, is used to PWM the OUTA and OUTB outputs according to truth table,

Table 1.

It is recommended that the frequency of the PWM input signal be between 18 kHz and 60 kHz. A PWM frequency

of 18 kHz or higher, being above the audible range, ensures quiet fan operation. Frequencies above 18 kHz

also promote efficient fan speed control by keeping the PWM period below the electrical L/R time constant of

the motor. This allows continuous current in the fan windings (see Figure 3). Keeping the PWM frequency below

60 kHz minimizes switching losses. Switching losses, typically observed at higher frequencies, decreases

overall efficiency.

The speed of the cooling fan can be varied by adjusting the duty cycle of the PWM input signal. The higher the

duty cycle of the PWM input signal, the higher the current in the fan windings, and thus results in faster fan speed.

I(MOTOR)

PWM

I(DRIVE)

PWM Period

I(RECIRCULATE)

tOFF

tON

Figure 3. Motor Current Waveform