TC647B Microchip Technology Inc., TC647B Datasheet - Page 24

• Manufacturer Part Number: TC647B
• Description: Pwm Fan Speed Controllers With Minimum Fan Speed, Fan Restart And Fansense Technology For Fault Detection
• Manufacturer: Microchip Technology Inc.

TC642B/TC647B
Using standard 1% resistor values yields the following
values:
Step 6: Selecting the Fan Drive Device (Q
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 R
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 ) and 5-12
(3.0 ). The selected C
vides the appropriate coupling of the voltage to the
SENSE pin.
.
FIGURE 5-11:
4.7
DS21756B-page 24
- R
- R
3
4
sense resistor.
= 32.4 k
= 17.8 k
SENSE
SENSE pin voltage with
SENSE
value is 0.1 F as this pro-
and C
SENSE
1
Values.
).
.
FIGURE 5-12:
3.0
Since the 3.0
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 (Irms
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.
sense resistor.
value of sense resistor provides the
2
SENSE pin voltage with
* R
2003 Microchip Technology Inc.
SENSE
). This number will set