TC649B View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
TC649B
Microchip Technology
'TC649B' PDF : 36 Pages View PDF
TC646B/TC648B/TC649B
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 CF value is selected to be 1.0 µF.
Step 4: Setting the VIN 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 VIN 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
VIN = (DC * 1.4V) + 1.20V
DC = Desired Duty Cycle
Using the above equation, the VIN values are
calculated to be:
- VIN (40%) = 1.76V
- VIN (100%) = 2.60V
Using these values along with the thermistor resistance
values calculated earlier, the R1 and R2 resistor values
can now be calculated using the following equation:
EQUATION
V(T1) = ----------V----D---D-----×-----R----2----------
RTEMP(T1) + R2
V(T2) = ----------V----D---D-----×-----R----2----------
RTEMP(T2) + R2
RTEMP is the parallel combination of R1 and the ther-
mistor. V(T1) represents the VIN voltage at 20ºC and
V(T2) represents the VIN voltage at 40ºC. Solving the
equations simultaneously yields the following values
(VDD = 5V):
- R1 = 238,455 Ω
- R2 = 45,161 Ω
Using standard 1% resistor values, the selected R1 and
R2 values are:
- R1 = 237 kΩ
- R2 = 45.3 kΩ
A graph of the VIN voltage, thermistor resistance and
RTEMP resistance versus temperature for this
configuration is shown in Figure 5-10.
400
5.00
350
4.50
300
VIN
4.00
3.50
250
3.00
200
2.50
150
2.00
NTC Thermistor
100
100 k: @ 25ºC
1.50
1.00
50
RTEMP
0.50
0
0.00
0 10 20 30 40 50 60 70 80 90
Temperature (ºC)
FIGURE 5-10:
Thermistor Resistance, VIN
and RTEMP vs. Temperature
Step 5: Setting the Auto-Shutdown Voltage (VAS).
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
VIN voltage at 40% duty cycle, since this was also set
at the temperature that auto-shutdown is to occur
(20ºC).
- VAS = 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 R3 and R4:
EQUATION
5V
IDIV = R3 + R4
VAS =
5V * R4
R3 + R4
Using the equations above, the resistor values for R3
and R4 are found to be:
- R3 = 32.4 kΩ
- R4 = 17.6 kΩ
Using standard 1% resistor values yields the following
values:
- R3 = 32.4 kΩ
- R4 = 17.8 kΩ
DS21755B-page 24
2003 Microchip Technology Inc.
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