THMC40 datasheet(9/17 Pages) TI

9 / 17 page

THMC40, THMC41

VARIABLE SPEED 12-VDC BRUSHLESS FAN MOTOR DRIVERS

SLIS097 – MARCH 2000

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

charge pump (CP)

THMC40 and THMC41 have an internal charge pump which utilizes an external reservoir capacitor (CP) to

generate the gate-drive voltage for the VOUT output high-side DMOS transistor (see Figure 3). The

recommended value for CP is 0.1

µF, with a minimum rating of 35 WVdc. The charge pump is disabled during

the sleep state (see the sleep/run state section) to minimize current consumption into VPWR. The charge pump

also incorporates internal undervoltage lockout detection used to disable VOUT when the charge pump voltage

does not have an adequate level above VPWR to fully drive the high-side DMOS gate. Thus, the VCP (UVLO),

in conjunction with the charge pump start-up time, delays the VOUT drive for a short time after the device has

transitioned from sleep to run state, until the charge pump voltage reaches the UVLO threshold.

PWM oscillator/triangle waveform generator (COSC)

The PWM oscillator uses source and sink currents switched into an external capacitor (COSC) to set the PWM

frequency and generate a triangle waveform. A PWM oscillator cycle consists of charging COSC with a constant

current source (–180

µA typical) until the COSC voltage ramps up to an upper threshold (2.3 V typical), and then

discharging COSC with a constant current sink (180 µA typical) until the COSC voltage ramps down to a lower

threshold (0.5 V typical). The charge/discharge cycle is repeated each time the 2.3-V or 0.5-V threshold is

reached (see Figure 5).

2.3 V

VREF

VPWR

COSC

VPWM

2200 pF

PWM

–180

µA

2.3 V

0.5 V

180

µA

Time

0.5 V

VCOSC

Figure 5. PWM Triangle Waveform Generator

The following equation can be used to calculate the value of COSC needed for a desired PWM frequency, fPWM:

OSC(max) +

CHARGE(min)

PWM(min)

DISCHARGE *

CHARGE

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