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LTC4155

4155fc

APPLICATIONS INFORMATION

substitute into the appropriate following equation to cal-

culate the value αBIAS and then RBIAS.

αBIAS = 0.34917 • αTOO_COLD

αBIAS = 1.73735 • αTOO_WARM

αBIAS = 3.35249 • αHOT_FAULT

RBIAS = αBIAS • r25

With αBIAS for the newly programmed temperature

threshold now determined, the other two dependent tem-

perature thresholds may be found by substituting αBIAS

into the remaining two equations. The following equation

may be used to convert any other NTC ADC result (NTCVAL)

back to a resistance ratio by substituting the κSPAN and

κOFFSET values found in the Electrical Characteristics table.

αT = κSPAN

• NTCVAL

+κOFFSET

−κSPAN • NTCVAL – κOFFSET

⎡

⎣

⎢

⎤

⎦

⎥αBIAS

αT may then be used to determine the temperature using

either the lookup table provided by the thermistor manu-

facturer or the curve fit model:

ln (

αT)+ β

Advanced Alternate Thermistor Bias Network

If an adjustment to RBIAS does not yield an acceptable span

between temperature thresholds, a second low drift bias

resistor may be added to the bias network between the

NTC pin and the top of the thermistor. This resistor has

the net effect of diluting the high thermal sensitivity of the

thermistor with low drift resistance. The result is reduced

thermal gain and a wider temperature span between the

preprogrammed voltage thresholds of the LTC4155. Using

this additional resistor, two of the three temperature

comparator thresholds may be adjusted. The remaining

threshold is constrained by the limited degrees of freedom

of the bias network. After determining the αT values for

the two temperature thresholds of interest, the following

equations may be used to determine αBIAS, αTEMP_RANGE,

and the third constrained temperature threshold dependent

on the choice of bias network and the thermistor.

To specify TTOO_COLD and TTOO_WARM, then calculate

THOT_FAULT:

αTEMP_RANGE = 0.25153 • αTOO_COLD – 1.25153

• αTOO_WARM

RTEMP_RANGE = αTEMP_RANGE • r25

αBIAS = 0.43699 • (αTOO_COLD – αTOO_WARM)

RBIAS = αBIAS • r25

With the bias network determined, the overconstrained

threshold may then be calculated:

αHOT_FAULT = 0.29829 • αBIAS – αTEMP_RANGE

To specify TTOO_COLD and THOT_FAULT, then calculate

TTOO_WARM:

αTEMP_RANGE = 0.11626 • αTOO_COLD – 1.11626

• αHOT_FAULT

RTEMP_RANGE = αTEMP_RANGE • r25

αBIAS = 0.38976 • (αTOO_COLD – αHOT_FAULT)

RBIAS = αBIAS • r25

With the bias network determined, the overconstrained

threshold may then be calculated:

αTOO_WARM = 0.57559 • αBIAS – αTEMP_RANGE

To specify TTOO_WARM and THOT_FAULT, then calculate

TTOO_COLD:

αTEMP_RANGE = 1.07566 • αTOO_WARM – 2.07566

• αHOT_FAULT

RTEMP_RANGE = αTEMP_RANGE • r25

αBIAS = 3.60615 • (αTOO_WARM – αHOT_FAULT)

RBIAS = αBIAS • r25

With the bias network determined, the overconstrained

threshold may then be calculated:

αTOO_COLD = 2.863946 • αBIAS – αTEMP_RANGE

It is possible to obtain a negative result for RTEMP_RANGE

which is not physically realizable using the previous equa-

tions. A negative result indicates that the two chosen tem-

perature thresholds are too close in temperature and require

more thermal sensitivity than the thermistor can provide.

LTC4099 Datasheet(PDF) 42 Page - Linear Technology

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