TC654 View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
'TC654' PDF : 36 Pages View PDF
TC654/TC655
7.4 FanSense Network (RSENSE &
CSENSE)
The network comprised of RSENSE and CSENSE allows
the TC654/TC655 devices to detect commutation of the
fan motor. RSENSE converts the fan current into a volt-
age. CSENSE AC couples this voltage signal to the
SENSE pins (SENSE1 and SENSE2). The goal of the
SENSE network is to provide a voltage pulse to the
SENSE pin that has a minimum amplitude of 120 mV.
This will ensure that the current pulse caused by the
fan commutation is recognized by the TC654/TC655
device.
A 0.1 µF ceramic capacitor is recommended for
CSENSE. Smaller values will require larger sense resis-
tors be used. Using a 0.1 µF capacitor results in rea-
sonable values for RSENSE. Figure 7-5 illustrates a
typical SENSE network.
TABLE 7-1: RSENSE VS. FAN CURRENT
Nominal Fan Current
(mA)
RSENSE (ohm)
50
9.1
100
4.7
150
3.0
200
2.4
250
2.0
300
1.8
350
1.5
400
1.3
450
1.2
500
1.0
Figure 7-6 shows some typical waveforms for the fan
current and the voltage at the Sense pins.
VOUT
SENSE1
SENSE2
FAN
1
RISO1
FAN
2
715
RISO2
CSENSE1
(0.1 µF typical)
715
RSENSE1
CSENSE2
(0.1 µF typical)
RSENSE2
Note: See Table 7-1 for RSENSE1 and RSENSE2 values.
FIGURE 7-5:
Typical Sense Network.
The value of RSENSE will change with the current rating
of the fan. A key point is that the current rating of the
fan specified by the manufacturer may be a worst case
rating. The actual current drawn by the fan may be
lower than this rating. For the purposes of setting the
value for RSENSE, the operating fan current should be
measured.
Table 7-1 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.
FIGURE 7-6:
Typical Fan Current and
Sense Pin Waveforms.
7.5 Output Drive Device Selection
The TC654/TC655 is designed to drive two external
NPN transistors or two external N-channel MOSFETs
as the fan speed modulating elements. These two
arrangements are shown in Figure 7-7. For lower cur-
rent fans, NPN transistors are a very economical
choice for the fan drive device. It is recommended that,
for higher-current fans (500 mA and above), MOSFETs
be used as the fan drive device. Table 7-2 provides
some possible part numbers for use as the fan drive
element.
When using an NPN transistor as the fan drive ele-
ment, a base current limiting resistor must be used.
This is shown in Figure 7-7.
When using MOSFETs as the fan drive element, it is
very easy to turn the MOSFETs on and off at very high
rates. Because the gate capacitances of these small
DS20001734C-page 24
2002-2014 Microchip Technology Inc.
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