TC646B datasheet(23/36 Pages) MICROCHIP

23 / 36 page

 2003 Microchip Technology Inc.

DS21755B-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

Ω @ 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 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) = 127,462Ω

-RT (40ºC) = 50,520Ω

RTO

β TO T

–

()

TTO

•

------------------------

exp

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