TC646B Datasheet, PDF (23/36 Pages) Microchip Technology – PWM Fan Speed Controllers With Auto-Shutdown, Fan Restart and FanSense™ Technology for Fault Detection

TC646B/TC648B/TC649B
The fan used in the system is a Panasonic^®, Panaflo^®-
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
- Voltage = 12V
- Current = 145 mA (data sheet number)
FIGURE 5-8:
Waveform.
FBA06T12H Fan Current
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_SENSE
and C_SENSE values later on. Also of interest for the
R_SENSE selection value is what the fan current does in
a locked-rotor condition.
FIGURE 5-9:
Fan Current.
FBA06T12H Locked-Rotor
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_SENSE) value selection, was measured at 154 mA.
This is slightly higher than the RMS value during full fan
speed operation.
Thermistor Information: Thermometrics part number:
NHQ104B425R5
- Resistance Value: 100 kΩ @ 25ºC
- Beta Value (β): 4250
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 R₁ and R₂
(see Figure 5-13), which sets the V_IN voltage.
The equation for determining the thermistor values is
shown below:
EQUATION
R_T
=
R_TO exp
β_----(_---T_{---O-----–----}T_----)_-
T • T_O
R_T0 is the thermistor value at 25ºC. T₀ is 298.15 and T
is the temperature of interest. All temperatures are in
degrees kelvin.
Using this equation, the values for the thermistor are
found to be:
- R_T (20ºC) = 127,462Ω
- R_T (40ºC) = 50,520Ω
 2003 Microchip Technology Inc.
DS21755B-page 23