TC649B datasheet(19/36 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

DS21755B-page 19

TC646B/TC648B/TC649B

data sheet. With the values for the thermistor and the

values for VIN, you now have two equations from which

the values for R1 and R2 can be found.

Example: The following design goals are desired:

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

(T1) = 30°C

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

Temperature (T2) = 60°C

Using a 100 k

Ω thermistor (25°C value), we look up the

thermistor values at the desired temperatures:

•RT (T1) = 79428Ω @ 30°C

•RT (T2) = 22593Ω @ 60°C

Substituting these numbers into the given equations

produces the following numbers for R1 and R2.

•R1 = 34.8 kΩ

•R2 = 14.7 kΩ

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 SENSE network (comprised of RSENSE and

CSENSE) allows the TC646B and TC649B 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 TC646B/

TC649B 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 than

this rating. 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 current probe or using a voltage probe

with a low-value resistor (0.5

Ω). Another good tool for

this exercise is the TC642 Evaluation Board. This

board allows the RSENSE and CSENSE values to be eas-

ily 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. Sensor™

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

Ω

Note:

See Table 5-1 for RSENSE values.

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