TC647B Microchip Technology Inc., TC647B Datasheet - Page 17

• 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.

5.0
5.1
The PWM frequency of the V
capacitor value attached to the C
quency will be 30 Hz (typical) for a 1 µF capacitor. The
relationship between frequency and capacitor value is
linear, making alternate frequency selections easy.
As stated in previous sections, the PWM frequency
should be kept in the range of 15 Hz to 35 Hz. This will
eliminate the possibility of having audible frequencies
when varying the duty cycle of the fan drive.
A very important factor to consider when selecting the
PWM frequency for the TC642B/TC647B devices is the
RPM rating of the selected fan and the minimum duty
cycle that the fan will be operating at. For fans that have
a full speed rating of 3000 RPM or less, it is desirable
to use a lower PWM frequency. A lower PWM fre-
quency allows for a longer time period to monitor the
fan current pulses. The goal is to be able to monitor at
least two fan current pulses during the on time of the
V
Example: The system design requirement is to operate
the fan at 50% duty cycle when ambient temperatures
are below 20°C. The fan full speed RPM rating is
3000 RPM and has four current pulses per rotation. At
50% duty cycle, the fan will be operating at
approximately 1500 RPM.
EQUATION
If one fan revolution occurs in 40 msec, each fan pulse
occurs 10 msec apart. In order to detect two fan current
pulses, the on time of the V
20 msec. With the duty cycle at 50%, the total period of
one cycle must be at least 40 msec, which makes the
PWM frequency 25 Hz. For this example, a PWM fre-
quency of 20 Hz is recommended. This would define a
C
5.2
As discussed in previous sections, the V
has a range of 1.20V to 2.60V (typical), which repre-
sents a duty cycle range on the V
100%, respectively. The V
as representing temperatures. The 1.20V level is the
low temperature at which the system requires very little
cooling. The 2.60V level is the high temperature, for
which the system needs maximum cooling capability
(100% fan speed).
One of the simplest ways of sensing temperature over
a given range is to use a thermistor. By using an NTC
thermistor, as shown in Figure 5-1, a temperature
variant voltage can be created.
OUT
F
2003 Microchip Technology Inc.
Time for one revolution (msec.)
capacitor value of 1.5 µF.
output.
APPLICATIONS INFORMATION
Setting the PWM Frequency
Temperature Sensor Design
IN
OUT
voltages can be thought of
OUT
pulse must be at least
=
F
OUT
output is set by the
pin. The PWM fre-
60
----------------------- -
1500
output of 0% to
IN
1000
analog input
=
40
FIGURE 5-1:
Circuit.
Figure 5-1 represents a temperature-dependent volt-
age divider circuit. R
while R
a parallel resistor combination that will be referred to as
R
increases, the value of R
R
V
desired relationship for the V
ize the response of the sense network and aids in
obtaining the proper V
perature range. An example of this is shown in
Figure 5-2.
If less current draw from V
thermistor should be chosen. The voltage at the V
can also be generated by a voltage output temperature
sensor device. The key is to get the desired V
to system (or component) temperature relationship.
The following equations apply to the circuit in
Figure 5-1.
EQUATION
In order to solve for the values of R
for V
occur, need to be selected. The variables T1 and T2
represent the selected temperatures. The value of the
thermistor at these two temperatures can be found in
the thermistor data sheet. With the values for the ther-
mistor and the values for V
equations from which the values for R
found.
IN
TEMP
TEMP
increases as temperature increases, giving the
IN
(R
, and the temperatures at which they are to
will decrease with it. Accordingly, the voltage at
1
and R
TEMP
R
TC642B/TC647B
T
V T1
V T2
2
= R
are standard resistors. R
1
T
* R
=
=
is a conventional NTC thermistor,
IN
Temperature Sensing
T
----------------------------------------- -
R
----------------------------------------- -
R
voltages over the desired tem-
/ R
V
t
TEMP
TEMP
DD
decreases and the value of
DD
1
V
V
IN
R
R
+ R
DD
DD
I
is desired, a larger value
1
2
DIV
IN
input. R
T1
T2
, there are now two
T
). As the temperature
R
R
1
2
+
2
+
and R
DS21756B-page 17
R
R
1
1
2
2
helps to linear-
and R
V
1
2
IN
and R
, the values
IN
2
voltage
can be
T
IN
form
pin