THMC40 datasheet(8/17 Pages) TI

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THMC40, THMC41

VARIABLE SPEED 12-VDC BRUSHLESS FAN MOTOR DRIVERS

SLIS097 – MARCH 2000

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

high-side PWM output driver (VOUT)

Referring to Figure 3, VOUT is a drive output to provide PWM controlled power to the common node of the motor

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

power to the motor, and thus motor speed. The high-side VOUT DMOS output transistor has a typical RDS(ON)

of 400 m

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

PWM

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

filters the current. Referring to Figure 4, during the on-time (tON) of a PWM period, VOUT is driven high forcing

voltage across a phase winding and increasing the current. During tOFF, PWM off-time, the VOUT high-side

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

device failures should a solder bridge occur between the adjacent VPWR and VOUT terminals.

IRECIRCULATE

IMOTOR

tON

tOFF

IDRIVE

tPWM

PWM

Time

Figure 4. Motor Current Waveform

The VOUT circuit is protected by the global thermal shutdown of the THMC40 and THMC41 by turning off both

the high-side and low-side DMOS drivers when an over-temperature condition is detected. VOUT is also held

off when the charge-pump voltage (VCP) is lower than its undervoltage lock-out threshold, VCP(UVLO).

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