TC664 datasheet(24/36 Pages) MICROCHIP

24 / 36 page

TC664/TC665

DS21737A-page 24

 2002 Microchip Technology Inc.

7.4

FanSense Network (RSENSE &

CSENSE)

The network comprised of RSENSE and CSENSE allows

the TC664/TC665 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 pin. The goal of the SENSE network is to pro-

vide a voltage pulse to the SENSE pin that has a mini-

mum amplitude of 120 mV. This will ensure that the

current pulse caused by the fan commutation is recog-

nized by the TC664/TC665 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 purpose 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 TC664/TC665 devices are 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 current fans, NPN transistors are a very econom-

ical choice for the fan drive device. It is recommended

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

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

vides 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

RISO

RSENSE

CSENSE

SENSE

VOUT

(0.1 µF typical)

715

Ω

Note:

See Table 7-1 for RSENSE value.

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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