MAX6616AEG+T【PDF 17/20 页】IC TEMP MON FAN CNTRL 24QSOP

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Applications Information

Thermistor Considerations

NTC thermistors are resistive temperature sensors

whose resistance decreases with increasing tempera-

ture. They are available in a wide variety of packages

that are useful in difficult applications such as measure-

ment of air or liquid temperature. Some can operate

over temperature ranges beyond that of most ICs. The

relationship between temperature and resistance in an

NTC thermistor is very nonlinear and can be described

by the following approximation:

where T is absolute temperature in Kelvin, R is the ther-

mistors resistance, and A, B, and C are coefficients that

vary with manufacturer and material characteristics.

The highly nonlinear relationship between temperature

and resistance in an NTC thermistor makes it somewhat

more difficult to use than a digital-output temperature-

sensor IC. However, by connecting the thermistor in

series with a properly chosen resistor and using the

MAX6615/MAX6616 to measure the voltage across the

resistor, a reasonably linear transfer function can be

obtained over a limited temperature range. Accuracy

increases over smaller temperature ranges.

Figures 9 and 10 show a good relationship between

temperature and data. This data was taken using a

popular thermistor model, the Betatherm 10K3A1, with

EXT

= 1.6k&. Using these values produces data with

good conformance to real temperature over a range of

about +30癈 to +100癈. Different combinations of ther-

mistors and R

EXT

result in different curves.

ADC Noise Filtering

The integrating ADC has inherently good noise rejec-

tion, especially at low-frequency signals such as

60Hz/120Hz power-supply hum. Lay out the PCB care-

fully with proper external noise filtering for high-accura-

cy thermistor measurements in electrically noisy

environments.

Filter high-frequency electromagnetic interference

(EMI) at TH_ and REF with an external 100pF capacitor

connected between the two inputs. This capacitor can

be increased to about 2000pF (max), including cable

capacitance. A capacitance higher than 2000pF intro-

duces errors due to the rise time of the switched cur-

rent source.

Chip Information

PROCESS: BiCMOS

A BIn R CIn R

= +

( ) [ ( )]

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

______________________________________________________________________________________ 17

MAX6615/MAX6616 ERROR

-12

-10

-6

-8

-2

-4

40 60

80 100 120 140

TEMPERATURE (癈)

OPTIMIZED FOR +30癈 TO +100癈

Figure 9. Data Error vs. Temperature Using a Betatherm

10K3A1 Thermistor

100

120

-50

100

150

MEASUREMENT vs. TEMPERATURE

TEMPERATURE (癈)

Figure 10. Measured Temperature vs. Actual Temperature

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