TC665 datasheet(12/36 Pages) MICROCHIP

12 / 36 page

TC664/TC665

DS21737B-page 12

 2002-2013 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 TC664/TC665 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 microcon-

trollers or microprocessors in the system. In this way,

multiple 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 a low cost NPN tran-

sistor or N-channel MOSFET as the low side switching

element in the system, as is shown in Figure 4-7. The

switching element is used to turn the fan 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 typical characteristic curves

for sink and source current capability 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). NPN transistors are often a good choice for

low current fans. If a NPN transistor is chosen, a base

current limiting resistor 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 switch-

ing 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 (SENSE)

The TC664/TC665 devices also feature Microchip’s

proprietary FanSense technology. During normal fan

operation, commutation occurs as each pole of the fan

is energized. The fan current pulses created by the fan

commutation are sensed using a low value current

sense resistor in the ground return leg of the fan circuit.

The voltage pulses across the sense resistor are then

AC coupled through a capacitor to the SENSE pin of

the TC664/TC665 device. These pulses are utilized for

calculating the RPM of the fan. The threshold voltage

for the SENSE pin is 100 mV (typical). The peak of the

voltage pulse at the SENSE pin must exceed the

100 mV (typical) threshold in order for the pulse to be

counted in the fan RPM measurement.

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

+5 V

NTC Thermistor

0.01 µF

100 k

 @ 25°C

TC664

TC665

GND

34.8 k



14.7 k



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