Datasheet

 2002-2013 Microchip Technology Inc. DS21756C-page 23
TC642B/TC647B
Step 2: Selecting the Fan Controller.
The requirements for the fan controller are that it have
minimum speed capability at 20ºC and also indicate a
fan fault condition. No over-temperature indication is
necessary. Based on these specifications, the proper
selection is the TC647B device.
Step 3: Setting the PWM Frequency.
The fan is rated at 4200 RPM with a 12V input. Since
the goal is to run to a 40% duty cycle (roughly 40% fan
speed), which equates to approximately 1700 RPM,
we can assume one full fan revolution occurs every
35 msec. The fan being used is a four-pole fan that
gives four current pulses per revolution. Knowing this
and viewing test results at 40% duty cycle, two fan cur-
rent pulses were always seen during the PWM on time
with a PWM frequency of 30 Hz. For this reason, the C
F
value is selected to be 1.0 F.
Step 4: Setting the V
IN
Voltage.
From the design criteria, the desired duty cycle at 20ºC
is 40%, while full fan speed should be reached at 40ºC.
Based on a V
IN
voltage range of 1.20V to 2.60V, which
represents 0% to 100% duty cycle, the 40% duty cycle
voltage can be found using the following equation:
EQUATION
Using the above equation, the V
IN
values are
calculated to be:
-V
IN
(40%) = 1.76V
-V
IN
(100%) = 2.60V
Using these values in combination with the thermistor
resistance values calculated earlier, the R
1
and R
2
resistor values can now be calculated using the
following equation:
EQUATION
R
TEMP
is the parallel combination of R
1
and the therm-
istor. V(T1) represents the V
IN
voltage at 20ºC and
V(T2) represents the V
IN
voltage at 40ºC. Solving the
equations simultaneously yields the following values
(V
DD
= 5V):
-R
1
= 238,455
-R
2
= 45,161
Using standard 1% resistor values, the selected R
1
and
R
2
values are:
-R
1
= 237 k
-R
2
= 45.3 k
A graph of the V
IN
voltage, thermistor resistance and
R
TEMP
resistance versus temperature for this
configuration is shown in Figure 5-10.
FIGURE 5-10: Thermistor Resistance, V
IN
,
and R
TEMP
vs. Temperature.
Step 5: Setting the Minimum Speed Voltage (V
MIN
).
Setting the voltage for the minimum speed is accom-
plished using a simple resistor voltage divider. The cri-
teria for the voltage divider in this design is that it draw
no more than 100 µA of current. The required minimum
speed voltage was determined earlier in the selection
of the V
IN
voltage at 40% duty cycle, since this was also
set at the temperature which minimum speed is to
occur (20ºC).
-V
MIN
= 1.76V
Given this desired setpoint, and knowing the desired
divider current, the following equations can be used to
solve for the resistor values for R
3
and R
4
:
EQUATION
Using the equations above, the resistor values for R
3
and R
4
are found to be:
-R
3
= 32.4 k
-R
4
= 17.6 k
V
IN
= (DC * 1.4V) + 1.20V
DC = Desired Duty Cycle
VT1
V
DD
R
2

R
TEMP
T1R
2
+
------------------------------------------=
VT2
V
DD
R
2

R
TEMP
T2R
2
+
------------------------------------------=
0
50
100
150
200
250
300
350
400
0 102030405060708090
Temperature (ºC)
Network Resistance (k
:
)
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
V
IN
(V)
V
IN
NTC Thermistor
100 k: @ 25ºC
R
TEMP
V
MIN
=
5V*R
4
R
3
+ R
4
I
DIV
=
5V
R
3
+ R
4