TC648B Microchip Technology Inc., TC648B Datasheet - Page 23

• Manufacturer Part Number: TC648B
• Description: Pwm Fan Speed Controllers With Auto-shutdown, Fan Restart And Fansense Technology For Fault Detection
• Manufacturer: Microchip Technology Inc.

The fan used in the system is a Panasonic
series fan, model number FBA06T12H.
A fault indication is desired when the fan is in a locked-
rotor condition. This signal is used to indicate to the
system that cooling is not available and a warning
should be issued to the user. No fault indication from
the fan controller is necessary for an over-temperature
condition as this is being reported elsewhere.
Step 1: Gathering Information.
The first step in the design process is to gather the
needed data on the fan and thermistor. For the fan, it is
also a good idea to look at the fan current waveform, as
indicated earlier in the data sheet.
Fan Information: Panasonic number: FBA06T12H
FIGURE 5-8:
Waveform.
From the waveform in Figure 5-8, the fan current has
an average value of 120 mA, with peaks up to 150 mA.
This information will help in the selection of the R
and C
R
a locked-rotor condition.
SENSE
2003 Microchip Technology Inc.
- Voltage = 12V
- Current = 145 mA (data sheet number)
SENSE
selection value is what the fan current does in
values later on. Also of interest for the
FBA06T12H Fan Current
®
, Panaflo
SENSE
®
TC646B/TC648B/TC649B
-
FIGURE 5-9:
Fan Current.
From Figure 5-9, it is seen that in a locked-rotor fault
condition, the fan goes into a pulsed current mode of
operation. During this mode, when the fan is conduct-
ing current, the peak current value is 360 mA for peri-
ods of 200 msec. This is significantly higher than the
average full fan speed current shown in Figure 5-8.
However, because of the pulse mode, the average fan
current in a locked-rotor condition is lower and was
measured at 68 mA. The RMS current during this
mode, which is necessary for current sense resistor
(R
This is slightly higher than the RMS value during full fan
speed operation.
Thermistor Information: Thermometrics part number:
NHQ104B425R5
From this information, the thermistor values at 20ºC
and 40ºC must be found. This information is needed in
order to select the proper resistor values for R
(see Figure 5-13), which sets the V
The equation for determining the thermistor values is
shown below:
EQUATION
R
is the temperature of interest. All temperatures are in
degrees kelvin.
Using this equation, the values for the thermistor are
found to be:
T0
- Resistance Value: 100 k @ 25ºC
- Beta Value ( ): 4250
- R
- R
SENSE
is the thermistor value at 25ºC. T
T
T
(20ºC) = 127,462
(40ºC) = 50,520
) value selection, was measured at 154 mA.
R
T
=
R
TO
FBA06T12H Locked-Rotor
exp
------------------------
T T
T
O
–
IN
O
T
DS21755B-page 23
0
voltage.
is 298.15 and T
1
and R
2