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TC649B Datasheet(Fiches technique) 23 Page - Microchip Technology

Numéro de pièce TC649B
Description  PWM Fan Speed Controllers With Auto-Shutdown, Fan Restart and FanSense™ Technology for Fault Detection
Télécharger  36 Pages
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Fabricant  MICROCHIP [Microchip Technology]
Site Internet  http://www.microchip.com
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 23 page
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 2002-2013 Microchip Technology Inc.
DS21755C-page 23
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:
FBA06T12H Fan Current
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 RSENSE
and CSENSE values later on. Also of interest for the
RSENSE selection value is what the fan current does in
a locked-rotor condition.
FIGURE 5-9:
FBA06T12H Locked-Rotor
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
(RSENSE) 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 k
 @ 25ºC
- Beta Value (
): 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 VIN voltage.
The equation for determining the thermistor values is
shown below:
EQUATION
RT0 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:
-RT (20ºC) = 127,462
-RT (40ºC) = 50,520
R
T
R
TO
 T
O
T

TT
O
------------------------
exp
=




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