Datasheet

 2002-2013 Microchip Technology Inc. DS21755C-page 19
TC646B/TC648B/TC649B
data sheet. With the values for the thermistor and the
values for V
IN
, you now have two equations from which
the values for R
1
and R
2
can be found.
Example: The following design goals are desired:
• Duty Cycle = 50% (V
IN
= 1.90V) with Temperature
(T1) = 30°C
• Duty Cycle = 100% (V
IN
= 2.60V) with
Temperature (T2) = 60°C
Using a 100 k thermistor (25°C value), we 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 SENSE network (comprised of R
SENSE
and
C
SENSE
) allows the TC646B and TC649B 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 TC646B/TC649B
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 than
this rating. 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 current probe or 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 eas-
ily 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. Sensor
™
-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.