TC647BEOATR datasheet(24/34 Pages) MICROCHIP

24 / 34 page

TC642B/TC647B

DS21756B-page 24

 2003 Microchip Technology Inc.

Using standard 1% resistor values yields the following

values:

-R3 = 32.4 kΩ

-R4 = 17.8 kΩ

Step 6: Selecting the Fan Drive Device (Q1).

Since the fan operating current is below 200 mA, a

transistor or MOSFET can be used as the fan drive

device. In order to reduce component count and cur-

rent draw, the drive device for this design is chosen to

be a N-channel MOSFET. Selecting from Table 5-2,

there are two MOSFETs that are good choices: the

MGSF1N02E and the SI2302. These devices have the

same pinout and are interchangeable for this design.

Step 7: Selecting the RSENSE and CSENSE Values.

The goal again for selecting these values is to ensure

that the signal at the SENSE pin is 90 mV in amplitude

under all operating conditions. This will ensure that the

pulses are detected by the TC642B/TC647B device

and that the fan operation is detected.

The fan current waveform is shown in Figure 5-8 and,

as discussed previously, with a waveform of this shape,

the current sense resistor values shown in Table 5-1 are

good reference values. Given that the average fan oper-

ating current was measured to be 120 mA, this falls

between two of the values listed in the table. For refer-

ence purposes, both values have been tested and

these results are shown in Figures 5-11 (4.7

Ω) and 5-12

(3.0

Ω). The selected CSENSE value is 0.1 µF as this pro-

vides the appropriate coupling of the voltage to the

SENSE pin.

FIGURE 5-11:

SENSE pin voltage with

4.7

Ω sense resistor.

FIGURE 5-12:

SENSE pin voltage with

3.0

Ω sense resistor.

Since the 3.0

Ω value of sense resistor provides the

proper voltage to the SENSE pin, it is the correct choice

for this solution as it will also provide the lowest power

dissipation and the most voltage to the fan. Using the

RMS fan current that was measured previously, the

power dissipation in the resistor during a fan fault con-

dition is 71 mW (Irms2 * RSENSE). This number will set

the wattage rating of the resistor that is selected. The

selected value will vary depending upon the derating

guidelines that are used.

Now that all the values have been selected, the sche-

matic representation of this design can be seen in

Figure 5-13.

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