TC647B Microchip Technology Inc., TC647B Datasheet - Page 18

• 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
Example: The following design goals are desired:
• Duty Cycle = 50% (V
• Duty Cycle = 100% (V
Using a 100 k
thermistor values at the desired temperatures:
• R
• R
Substituting these numbers into the given equations
produces the following numbers for R
• R
• R
FIGURE 5-2:
V
Figure 5-2 graphs R
and V
above.
5.3
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.
- Thermometrics
- Ametherm
- U.S. Sensors™
- Advanced Thermal
DS21756B-page 18
IN
Temperature (T1) = 30°C
Temperature (T2) = 60°C
Products™
, and R
T
T
1
2
(T1) = 79428
(T2) = 22593
= 34.8 k
= 14.7 k
140
120
100
IN
80
60
40
20
0
20
Thermistor Selection
versus temperature for the example shown
R
TEMP
TEMP
®
30
thermistor (25°C value), look up the
40
®
Vary With Temperature.
V
Temperature (ºC)
T
@ 30°C
@ 60°C
IN
50
, R
Voltage
How Thermistor Resistance,
IN
IN
TEMP
60
= 1.90 V) with
= 2.60 V) with
- Quality Thermistor™
- Sensor Scientific™
- Vishay
- muRata
NTC Thermistor
100 k
70
(R
@ 25ºC
1
80
in parallel with R
®
1
®
90
and R
100
4.000
3.500
3.000
2.500
2.000
1.500
1.000
0.500
0.000
2
.
T
)
5.4
The FanSense Network (comprised of R
C
detect commutation of the fan motor. R
the fan current into a voltage. C
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
resistors be used. Using a 0.1 µF capacitor results in
reasonable values for R
typical SENSE network.
FIGURE 5-3:
The required value of R
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
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
allowing the voltage waveforms to be monitored to
ensure the proper levels are being reached.
Table 5-1 shows values of R
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.
SENSE
SENSE
SENSE
Note: See Table 5-1 for R
SENSE
) allows the TC642B and TC647B devices to
. Smaller values will require that larger sense
and C
FanSense Network
(R
V
OUT
SENSE
SENSE
and C
(0.1 µF typical)
values to be easily changed while
Typical Sense Network.
SENSE
C
SENSE
SENSE
2003 Microchip Technology Inc.
715
SENSE
R
ISO
SENSE
will change with the cur-
. Figure 5-3 illustrates a
SENSE
SENSE
SENSE
FAN
)
R
values.
according to the
SENSE
AC couples this
SENSE
, the operating
SENSE
converts
and