Datasheet

TC642B/TC647B
DS21756C-page 18  2002-2013 Microchip Technology Inc.
Example: The following design goals are desired:
• Duty Cycle = 50% (V
IN
= 1.90 V) with
Temperature (T1) = 30°C
• Duty Cycle = 100% (V
IN
= 2.60 V) with
Temperature (T2) = 60°C
Using a 100 k thermistor (25°C value), look up the
thermistor values at the desired temperatures:
•R
T
(T1) = 79428 @ 30°C
•R
T
(T2) = 22593 @ 60°C
Substituting these numbers into the given equations
produces the following numbers for R
1
and R
2
.
•R
1
= 34.8 k
•R
2
= 14.7 k
FIGURE 5-2: How Thermistor Resistance,
V
IN
, and R
TEMP
Vary With Temperature.
Figure 5-2 graphs R
T
, R
TEMP
(R
1
in parallel with R
T
)
and V
IN
versus temperature for the example shown
above.
5.3 Thermistor Selection
As with any component, there are a number of sources
for thermistors. A listing of companies that manufacture
thermistors can be found at www.temperatures.com/
thermivendors.html. This website lists over forty
suppliers of thermistor products. A brief list is shown
here.
5.4 FanSense Network
(R
SENSE
and C
SENSE
)
The FanSense Network (comprised of R
SENSE
and
C
SENSE
) allows the TC642B and TC647B devices to
detect commutation of the fan motor. R
SENSE
converts
the fan current into a voltage. C
SENSE
AC couples this
voltage signal to the SENSE pin. The goal of the sense
network is to provide a voltage pulse to the SENSE pin
that has a minimum amplitude of 90 mV. This will
ensure that the current pulse caused by the fan
commutation is recognized by the TC642B/TC647B
device.
A 0.1 µF ceramic capacitor is recommended for
C
SENSE
. Smaller values will require that larger sense
resistors be used. Using a 0.1 µF capacitor results in
reasonable values for R
SENSE
. Figure 5-3 illustrates a
typical SENSE network.
FIGURE 5-3: Typical Sense Network.
The required value of R
SENSE
will change with the cur-
rent rating of the fan and the fan current waveshape. A
key point is that the current rating of the fan specified
by the manufacturer may be a worst-case rating, with
the actual current drawn by the fan being lower. For the
purposes of setting the value for R
SENSE
, the operating
fan current should be measured to get the nominal
value. This can be done by using an oscilloscope cur-
rent probe or by using a voltage probe with a low value
resistor (0.5). Another good tool for this exercise is
the TC642 Evaluation Board. This board allows the
R
SENSE
and C
SENSE
values to be easily changed while
allowing the voltage waveforms to be monitored to
ensure the proper levels are being reached.
Table 5-1 shows values of R
SENSE
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.
- Thermometrics
®
- Quality Thermistor™
-Ametherm
®
- Sensor Scientific™
- U.S. Sensors™ - Vishay
®
- Advanced Thermal
Products™
- muRata
®
0
20
40
60
80
100
120
140
20 30 40 50 60 70 80 90 100
Temperature (ºC)
Network Resistance (k
:
)
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
V
IN
(V)
NTC Thermistor
100 k: @ 25ºC
V
IN
Voltage
R
TEMP
FAN
R
ISO
R
SENSE
C
SENSE
SENSE
V
OUT
(0.1 µF typical)
715 
Note: See Table 5-1 for R
SENSE
values.