THMC40, THMC41

VARIABLE SPEED 12-VDC BRUSHLESS FAN MOTOR DRIVERS

SLIS097 – MARCH 2000

13

POST OFFICE BOX 655303

open-drain tachometer output (TACH)—THMC40 Only

The THMC40 TACH output is an open-drain output activated by the Hall sensor comparator output (see Figure

9). When the Hall sensor comparator output is high, the TACH output floats high. When the Hall sensor amplifier

output is low, the TACH output is pulled low. The resulting output signal has two pulses per revolution on a

four-pole motor.

The TACH output can be used to monitor and measure actual fan speed. This output can also be used as part

of a closed-loop speed control system.

NOTE:

It is recommended that the fan manufacturer not place a pullup resistor for this terminal on the fan

circuit board. Leaving the output as open-drain allows the end user to pull up this terminal with an

external resistor to the supply voltage of their choice (that is, 3.3 V or 5 V).

open-drain locked rotor detection output—THMC41 only

The THMC41 RD output is an open-drain output pulled low during normal fan operation and allowed to float

during a locked rotor condition (see Figure 9). This signal can be used to alert the system of a locked rotor

condition.

The RD output can also be used as a

fan present or fan OK signal by using a general-purpose input terminal

on a PC Super I/O chip to detect the logic state of this terminal. When this input is high, the signal indicates that

the fan has been disconnected or is in a locked rotor condition.

NOTE:

It is recommended that the fan manufacturer not place a pullup resistor for this terminal on the fan

circuit board. Leaving the output as open-drain allows the end user to pull up this terminal with an

external resistor to the supply voltage of their choice (that is, 3.3V or 5V).

should be placed as close to this terminal as the layout permits. Additional bulk capacitance of 2.2

µF to 10 µF

on this terminal is highly recommended to reduce current spikes on the supply line during motor commutation,

thus reducing radiated emissions from the fan. See Application Information for further details.