THMC40, THMC41

VARIABLE SPEED 12-VDC BRUSHLESS FAN MOTOR DRIVERS

SLIS097 – MARCH 2000

8

POST OFFICE BOX 655303

phase windings using an internally generated PWM signal. The PWM duty cycle controls the effective drive

of 400 m

Ω at 25°C and a 1-A continuous current.

PWM

CP

COSC

Charge Pump,

HS Gate Drive,

Synchronous

Rectification,

and

Global Thermal

Shutdown

Triangle Wave

Generator

0.5 V to 2.3 V

PUC

Synchronous

Low-Side

Switch

High-Side

Driver

0.1

µF

VPWR

VOUT

VPWM

2200 pF

COMP

Figure 3. High-Side PWM Drive

The frequency of the PWM drive (typically 23 kHz) is such that the L/R time constant of the motor phase winding

DMOS is switched off, the phase winding inductive energy is recirculated, and the current decreases. To

minimize the voltage drop and the resulting energy loss during recirculation, a low-side DMOS synchronous

switch is provided, as shown in Figure 3. This low-side DMOS device has a 2-A minimum current limit to prevent

IRECIRCULATE

IMOTOR

tON

tOFF

IDRIVE

tPWM

PWM

Time

Figure 4. Motor Current Waveform