TC654 datasheet(24/38 Pages) MICROCHIP

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TC654/TC655

DS20001734C-page 24

 2002-2014 Microchip Technology Inc.

7.4

FanSense Network (RSENSE &

CSENSE)

The network comprised of RSENSE and CSENSE allows

the TC654/TC655 devices to detect commutation of the

fan motor. RSENSE converts the fan current into a volt-

age. CSENSE AC couples this voltage signal to the

SENSE pins (SENSE1 and SENSE2). The goal of the

SENSE network is to provide a voltage pulse to the

SENSE pin that has a minimum amplitude of 120 mV.

This will ensure that the current pulse caused by the

fan commutation is recognized by the TC654/TC655

device.

A 0.1 µF ceramic capacitor is recommended for

CSENSE. Smaller values will require larger sense resis-

tors be used. Using a 0.1 µF capacitor results in rea-

sonable values for RSENSE. Figure 7-5 illustrates a

typical SENSE network.

FIGURE 7-5:

Typical Sense Network.

The value of RSENSE will change with the current rating

of the fan. A key point is that the current rating of the

fan specified by the manufacturer may be a worst case

rating. The actual current drawn by the fan may be

lower than this rating. For the purposes of setting the

value for RSENSE, the operating fan current should be

measured.

Table 7-1 shows values of RSENSE according to the

nominal operating current of the fan. The fan currents

are average values. If the fan current falls between two

of the values listed, use the higher resistor value.

TABLE 7-1:

RSENSE VS. FAN CURRENT

Figure 7-6 shows some typical waveforms for the fan

current and the voltage at the Sense pins.

FIGURE 7-6:

Typical Fan Current and

Sense Pin Waveforms.

7.5

Output Drive Device Selection

The TC654/TC655 is designed to drive two external

NPN transistors or two external N-channel MOSFETs

as the fan speed modulating elements. These two

arrangements are shown in Figure 7-7. For lower cur-

rent fans, NPN transistors are a very economical

choice for the fan drive device. It is recommended that,

for higher-current fans (500 mA and above), MOSFETs

be used as the fan drive device. Table 7-2 provides

some possible part numbers for use as the fan drive

element.

When using an NPN transistor as the fan drive ele-

ment, a base current limiting resistor must be used.

This is shown in Figure 7-7.

When using MOSFETs as the fan drive element, it is

very easy to turn the MOSFETs on and off at very high

rates. Because the gate capacitances of these small

FAN

RISO1

RISO2

RSENSE2

RSENSE1

CSENSE2

CSENSE1

SENSE2

SENSE1

VOUT

(0.1 µF typical)

715



715



Note:

See Table 7-1 for RSENSE1 and RSENSE2 values.

RISO2

Nominal Fan Current

(mA)

RSENSE (ohm)

9.1

100

4.7

150

3.0

200

2.4

250

2.0

300

1.8

350

1.5

400

1.3

450

1.2

500

1.0

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