THMC40, THMC41

VARIABLE SPEED 12-VDC BRUSHLESS FAN MOTOR DRIVERS

SLIS097 – MARCH 2000

6

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

general overview

The THMC40 and THMC41 are 2-phase dc brushless fan motor drivers with PWM speed control intended

primarily for applications requiring a wide speed control range and an open-drain tachometer output signal

comparator/signal conditioner, a low power sleep-state mode, locked rotor protection with automatic restart

after a locked rotor condition, and over-temperature protection. The tachometer signal (THMC40) can be used

to monitor the

health of the fan or to close an external loop based on fan RPM. The THMC40 and THMC41

provide a more efficient drive solution to fan RPM control than external linear voltage control. This solution is

also considerably more efficient than controlling dc brushless fan RPM using external PWM drive.

low-side motor phase winding driver outputs (PHA, PHB)

The PHA and PHB outputs provide low-side drive of the motor’s two stator phase windings (see block diagram

energy when a phase drive switches off. The outputs also have global thermal shutdown to prevent device

failure.

Drive commutation of PHA and PHB outputs is controlled according to rotor position monitored by a Hall-effect

position sensor. Discussion of this function is found in the following section, and the relationship between PHA

and PHB outputs to Hall input signal is shown in Figure 2.

Locked Rotor

Detection and

Auto-Restart

VOUT

TACH

(THMC40)

N

S

S

N

Tachometer

Output Driver

VZCLAMP

VZCLAMP

Low-Side

Gate Drive

Control

Logic, and

Global

Thermal

Shutdown

Naked

Hall

Sensor

VPWR

Hall Sensor

Comparator

RD

(THMC41)

H+

H–

PHA

PHB

Figure 1. Low-Side Gate Drive Block Diagram