TC646BEOATR datasheet(25/36 Pages) MICROCHIP

25 / 36 page

 2003 Microchip Technology Inc.

DS21755B-page 25

TC646B/TC648B/TC649B

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 TC649B 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 the average fan operating

current was measured to be 120 mA, this falls between

two of the values listed in the table. For reference pur-

poses, both values have been tested and these results

are shown in Figures 5-11 (4.7

Ω) and 5-12 (3.0Ω). The

selected CSENSE value is 0.1 µF, as this provides 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 maximum amount of voltage to the

fan. Using the RMS fan current which was measured

previously, the power dissipation in the resistor during

a fan fault condition 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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