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LTC4091

4091fa

For more information www.linear.com/LTC4091

The trip points for the LTC4091’s temperature qualifica-

tion are internally programmed at 0.29 • VNTC for the hot

threshold and 0.74 • VNTC for the cold threshold.

Therefore, the hot trip point is set when:

RNTC|HOT

RNOM+RNTC|HOT

• VNTC = 0.29• VNTC

and the cold trip point is set when:

RNTC|COLD

RNOM+RNTC|COLD

• VNTC = 0.74• VNTC

SolvingtheseequationsforRNTC|COLDandRNTC|HOTresults

in the following:

RNTC|HOT = 0.409 • RNOM

and

RNTC|COLD = 2.815 • RNOM

By setting RNOM equal to R25C,theaboveequationsresult

inrHOT=0.409andrCOLD=2.815.Referencingtheseratios

to the Vishay Resistance-Temperature Curve 2 chart gives

a hot trip point of about 50°C and a cold trip point of about

0°C. The difference between the hot and cold trip points

is approximately 50°C.

By using a bias resistor, RNOM, different in value from

R25C, the hot and cold trip points can be moved in either

direction.Thetemperaturespanwillchangesomewhatdue

to the nonlinear behavior of the thermistor. The following

equations can be used to easily calculate a new value for

the bias resistor:

RNOM=

rHOT

0.409

•R25C

RNOM=

rCOLD

2.815

•R25C

where rHOT and rCOLD are the resistance ratios at the de-

sired hot and cold trip points. Note that these equations

are linked. Therefore, only one of the two trip points can

be chosen, the other is determined by the default ratios

designed in the IC. Consider an example where a 40°C

hot trip point is desired.

FromtheVishayCurve2R-Tcharacteristics,rHOTis0.5758

at 40°C. Using the above equation, RNOM should be set to

14k. With this value of RNOM, the cold trip point is about

–7°C. Notice that the span is now 47°C rather than the

previous 50°C. This is due to the increase in temperature

gain of the thermistor as absolute temperature decreases.

The upper and lower temperature trip points can be in-

dependently programmed by using an additional dilution

resistor as shown in Figure 9. The following formulas can

be used to compute the values of RNOM and R1:

RNOM=

rCOLD–rHOT

2.815

•R25C

R1= 0.409•RNOM–rHOT •R25C

For example, to set the trip points to –5°C and 55°C with

a Vishay Curve 2 thermistor choose

RNOM=

3.532– 0.3467

2.815– 0.409

•10k = 13.2k

APPLICATIONS INFORMATION

–

RNOM

13.2k

RNTC

10k

1.97k

NTC

VNTC

0.1V

NTC_ENABLE

4091 F10

TOO_COLD

TOO_HOT

0.738 • VNTC

0.29 • VNTC

–

NTC BLOCK

Figure 9. NTC Thermistor Circuit with Additional Dilution

Resistor

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