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LTC4160/LTC4160-1

41601f

Therefore, the hot trip point is set when:

NTCBIAS

NTCHOT

NOM

NTCHOT

•.

•

0 349

And the cold trip point is set when:

NTCBIAS

NTC COLD

NOM

NTC COLD

•.

•

0 765

Solving these equations for RNTC|COLDandRNTC|HOTresults

in the following:

RNTC|HOT = 0.536 • RNOM

and

RNTC|COLD = 3.25 • RNOM

By setting RNOM equal to R25, the above equations result

in rHOT = 0.536 and rCOLD = 3.25. Referencing these ratios

to the Vishay Resistance-Temperature Curve 1 chart gives

a hot trip point of about 40°C and a cold trip point of about

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

is approximately 40°C.

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

R25, the hot and cold trip points can be moved in either

direction. The temperature span will change somewhat

due to the non-linear behavior of the thermistor. The fol-

lowing equations can be used to calculate a new value for

the bias resistor:

NOM

HOT

NOM

COLD

0 536

325

•

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 60°C

hot trip point is desired.

From the Vishay Curve 1 R-T characteristics, rHOT is

0.2488 at 60°C. Using the above equation, RNOM should

be set to 46.4k. With this value of RNOM, rCOLD is 1.436

and the cold trip point is about 16°C. Notice that the span

is now 44°C rather than the previous 40°C. This is due to

the decrease in “temperature gain” of the thermistor as

absolute temperature increases.

The upper and lower temperature trip points can be inde-

pendently programmed by using an additional bias resistor,

R1, as shown in Figure 8. The following formulas can be

used to compute the values of RNOM and R1:

NOM

COLD

HOT

NOM

HOT

–

•

.•

–

•

2 714

1 0 536

R25

For example, to set the trip points to 0°C and 45°C with

a Vishay Curve 1 thermistor choose:

NOM ==

3 266 0 4368

2 714

100

104 2

.– .

•.

the nearest 1% value is 105k:

R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k

the nearest 1% value is 12.7k. The ﬁnal solution is shown

in Figure 8 and results in an upper trip point of 45°C and

a lower trip point of 0°C.

APPLICATIONS INFORMATION

Figure 7. Standard NTC Conﬁguration

–

RNOM

100k

RNTC

100k

NTC

NTCBIAS

0.1V

NTC_ENABLE

41601 F07

LTC4160/LTC4160-1

NTC BLOCK

TOO_COLD

TOO_HOT

0.765 • NTCBIAS

0.349 • NTCBIAS

–

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