TC642B_13 datasheet(18/36 Pages) MICROCHIP

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TC642B/TC647B

DS21756C-page 18

 2002-2013 Microchip Technology Inc.

Example: The following design goals are desired:

• Duty Cycle = 50% (VIN = 1.90 V) with

Temperature (T1) = 30°C

• Duty Cycle = 100% (VIN = 2.60 V) with

Temperature (T2) = 60°C

Using a 100 k

 thermistor (25°C value), look up the

thermistor values at the desired temperatures:

•RT (T1) = 79428 @ 30°C

•RT (T2) = 22593 @ 60°C

Substituting these numbers into the given equations

produces the following numbers for R1 and R2.

•R1 = 34.8 k

•R2 = 14.7 k

FIGURE 5-2:

How Thermistor Resistance,

VIN, and RTEMP Vary With Temperature.

Figure 5-2 graphs RT, RTEMP (R1 in parallel with RT)

and VIN versus temperature for the example shown

above.

5.3

Thermistor Selection

As with any component, there are a number of sources

for thermistors. A listing of companies that manufacture

thermistors can be found at www.temperatures.com/

thermivendors.html. This website lists over forty

suppliers of thermistor products. A brief list is shown

here.

5.4

FanSense Network

(RSENSE and CSENSE)

The FanSense Network (comprised of RSENSE and

CSENSE) allows the TC642B and TC647B devices to

detect commutation of the fan motor. RSENSE converts

the fan current into a voltage. CSENSE AC couples this

voltage signal to the SENSE pin. The goal of the sense

network is to provide a voltage pulse to the SENSE pin

that has a minimum amplitude of 90 mV. This will

ensure that the current pulse caused by the fan

commutation is recognized by the TC642B/TC647B

device.

A 0.1 µF ceramic capacitor is recommended for

CSENSE. Smaller values will require that larger sense

resistors be used. Using a 0.1 µF capacitor results in

reasonable values for RSENSE. Figure 5-3 illustrates a

typical SENSE network.

FIGURE 5-3:

Typical Sense Network.

The required value of RSENSE will change with the cur-

rent rating of the fan and the fan current waveshape. A

key point is that the current rating of the fan specified

by the manufacturer may be a worst-case rating, with

the actual current drawn by the fan being lower. For the

purposes of setting the value for RSENSE, the operating

fan current should be measured to get the nominal

value. This can be done by using an oscilloscope cur-

rent probe or by using a voltage probe with a low value

resistor (0.5

). Another good tool for this exercise is

the TC642 Evaluation Board. This board allows the

RSENSE and CSENSE values to be easily changed while

allowing the voltage waveforms to be monitored to

ensure the proper levels are being reached.

Table 5-1 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.

- Thermometrics®

- Quality Thermistor™

-Ametherm®

- Sensor Scientific™

- U.S. Sensors™

- Vishay®

- Advanced Thermal

Products™

- muRata®

100

120

140

100

Temperature (ºC)

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

NTC Thermistor

100 k: @ 25ºC

VIN Voltage

RTEMP

FAN

RISO

RSENSE

CSENSE

SENSE

VOUT

(0.1 µF typical)

715



Note: See Table 5-1 for RSENSE values.

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