TC648B View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
TC648B
Microchip Technology
'TC648B' PDF : 36 Pages View PDF
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 (4.7Ω) 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 5-12:
SENSE pin voltage with
3.0Ω sense resistor.
Since the 3.0Ω 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.
FIGURE 5-11:
SENSE pin voltage with
4.7Ω sense resistor.
2003 Microchip Technology Inc.
DS21755B-page 25
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