THMC45 Datasheet, PDF (8/16 Pages) Texas Instruments – 5-V AND 3.3-V DC BRUSHLESS FAN MOTOR DRIVER WITH SINGLE WIRE CONTOL

THMC45
5ĆV AND 3.3ĆV DC BRUSHLESS FAN MOTOR DRIVER
WITH SINGLE WIRE CONTROL
SLIS101A − MAY 2001
PRINCIPLES OF OPERATION
PWM input (PWM)
The PWM input provides several functions:
D Input for controlling H-bridge PWM drive frequency and duty cycle
D Output for a tachometer current sink pulse on the first rising edge of the PWM input signal after a
commutation
D Initiating a low-current sleep state when the voltage maintained a logic low level for 2 ms (typical) or longer,
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(DRIVE)
I(MOTOR)
PWM
I(RECIRCULATE)
PWM Period
tOFF
tON
Figure 3. Motor Current Waveform
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