TC654 datasheet(12/38 Pages) MICROCHIP

12 / 38 page

TC654/TC655

DS20001734C-page 12

 2002-2014 Microchip Technology Inc.

FIGURE 4-6:

NTC Thermistor Sensor

Network.

The second method for controlling the duty cycle of the

PWM output (VOUT) is via the SMBus interface. In order

to control the PWM duty cycle via the SMBus, DUTYC

(bit 5<0>) of the Configuration Register (Register 6.3)

must be set to a ‘1’. This tells the TC654/TC655 device

that the duty cycle should be controlled by the Duty

Cycle Register. Next, the Duty Cycle Register must be

programmed to the desired value. The Duty Cycle Reg-

ister is a 4 Bit read/write register that allows duty cycles

from 30% to 100% to be programmed. Table 4-1 shows

the binary codes for each possible duty cycle.

TABLE 4-1:

DUTY-CYCLE REGISTER

(DUTY-CYCLE) 4-BITS,

READ/WRITE

This method of control allows for more sophisticated

algorithms to be implemented by utilizing microcontrol-

lers or microprocessors in the system. In this way, mul-

tiple system temperatures can be taken into account for

determining the necessary fan speed.

As shown in Table 4-1, the duty cycle has more of a

step function look than did the VIN control approach.

Because the step changes in duty cycle are small, they

are rarely audibly noticeable, especially when the fans

are integrated into the system.

4.6

PWM Output (VOUT)

The VOUT pin is designed to drive two low-cost NPN

transistors or N-channel MOSFETs as the low side

power switching elements in the system as is shown in

Figure 4-7. These switching elements are used to turn

the fans on and off at the PWM duty cycle commanded

by the VOUT output.

This output has complementary drive (pull up and pull

down) and is optimized for driving NPN transistors or

N-channel MOSFETs (see Section 2.0 “Typical Per-

formance CURVES” for sink and source current capa-

bility of the VOUT drive stage).

The external device needs to be chosen to fit the volt-

age and current rating of the fan in a particular applica-

tion (Refer to Section 7.5 “Output Drive Device

Selection” Output Drive Device Selection). NPN tran-

sistors are often a good choice for low-current fans. If a

NPN transistor is chosen, a base current-limiting resis-

tor should be used. When using a MOSFET as the

switching element, it is sometimes a good idea to have

a gate resistor to help slow down the turn on and turn

off of the MOSFET. As with any switching waveform,

fast rising and falling edges can sometimes lead to

noise problems.

As previously stated, the VOUT output will go to 100%

duty cycle during power-up and release from shutdown

conditions. The VOUT output only shuts down when

commanded to do so via the Configuration Register

(SDM (bit 0<0>)). Even when a locked rotor condition

is detected, the VOUT output will continue to pulse at

the programmed duty cycle.

4.7

Sensing Fan Operation (SENSE1 &

SENSE2)

The TC654 and TC655 also feature Microchip's propri-

etary FanSense technology. During normal fan opera-

tion, commutation occurs as each pole of the fan is

energized. The fan current pulses created by the fan

commutation are sensed using low value current sense

resistors in the ground return leg of the fan circuit. The

voltage pulses across the sense resistor are then AC

coupled through capacitors to the SENSE pins of the

TC654/TC655 device. These pulses are utilized for cal-

culating the RPM of the individual fans. The threshold

voltage for the SENSE pins is 100 mV (typical). The

Duty-Cycle Register (Duty Cycle)

D(3)

D(2)

D(1)

D(0)

Duty-Cycle

30%

34.67%

39.33% (default for VIN

open and when SMBus

is not selected)

44%

48.67%

53.33%

58%

62.67%

67.33%

72%

76.67%

81.33%

86%

90.67%

95.33%

100%

VIN

+5V

NTC Thermistor

0.01 µF

100 k

 @ 25°C

TC654/

TC655

GND

34.8 k



14.7 k



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