Datasheet

TC646B/TC648B/TC649B
DS21755C-page 24  2002-2013 Microchip Technology Inc.
Step 2: Selecting the Fan Controller.
The requirements for the fan controller are that it have
auto-shutdown capability at 20ºC and also indicate a
fan fault condition. No over-temperature indication is
necessary. From these specifications, the proper
selection is the TC649B device.
Step 3: Setting the PWM Frequency.
The fan is rated at 4200 RPM with a 12V input. The
goal is to run to a 40% duty cycle (roughly 40% fan
speed), which equates to approximately 1700 RPM. At
1700 RPM, one full fan revolution occurs every
35 msec. The fan being used is a four-pole fan that
gives four current pulses per revolution. With this infor-
mation, and viewing test results at 40% duty cycle, two
fan current pulses were always seen during the PWM
on time with a PWM frequency of 30 Hz. For this rea-
son, the C
F
value is selected to be 1.0 µF.
Step 4: Setting the V
IN
Voltage.
From the design criteria, the desired duty cycle at 20ºC
is 40% and 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 along 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 Auto-Shutdown Voltage (V
AS
).
Setting the voltage for the auto-shutdown is done using
a simple resistor voltage divider. The criteria for the
voltage divider in this design is that it draw no more
than 100 µA of current. The required auto-shutdown
voltage was determined earlier in the selection of the
V
IN
voltage at 40% duty cycle, since this was also set
at the temperature that auto-shutdown is to occur
(20ºC).
-V
AS
= 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
Using standard 1% resistor values yields the following
values:
-R
3
= 32.4 k
-R
4
= 17.8 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
I
DIV
=
5V
R
3
+ R
4
V
AS
=
5V * R
4
R
3
+ R
4