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LTC3554

3554f

OPERATION

Therefore, the hot trip point is set when:

NTC|HOT

NOM + RNTC|HOT

•V

BUS = 0.35 • VBUS

and the cold trip point is set when:

NTC|COLD

NOM + RNTC|COLD

•V

BUS = 0.76 • V BUS

Solving these equations for RNTC|COLD and RNTC|HOT

results in the following:

RNTC|HOT = 0.538 • RNOM

and

RNTC|COLD = 3.17 • RNOM

By setting RNOM equal to R25, the above equations result

in rHOT = 0.538 and rCOLD = 3.17. 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 nonlinear behavior of the thermistor. The following

equations can be used to easily calculate a new value for

the bias resistor:

NOM =

HOT

0.538

•R25

NOM =

COLD

3.17

•R25

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 independently set, the other is determined by the de-

fault 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, 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 tem-

perature gain of the thermistor as absolute temperature

increases.

The upper and lower temperature trip points can be in-

dependently programmed by using an additional bias

resistor as shown in Figure 2. The following formulas can

be used to compute the values of RNOM and R1:

NOM =

COLD –r HOT

2.714

•R25

1 = 0.536 • RNOM –r HOT •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

• 100k =104.2k

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 2 and results in an upper trip point of 45°C and

a lower trip point of 0°C.

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