LTC4098EPDC-PBF(RevB) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4098EPDC-PBF
(Rev.:RevB)

USB Compatible Switching Power Managers/Li-Ion Chargers with Overvoltage Protection

Linear Technology 

LTC4098/LTC4098-1

APPLICATIONS INFORMATION

Solving these equations for RNTC|COLD and RNTC|HOT

results in the following:

RNTC|HOT = 0.536 • RNOM

and

RNTC|COLD = 3.25 • RNOM

By setting RNOM equal to R25, the previous 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 nonlinear behavior of the thermistor. The following

equations can be used to easily calculate a new value for

the bias resistor:

RNOM

=

rHOT

0.536

• R25

RNOM

=

rCOLD

3.25

•

R25

where rHOT and rCOLD are the resistance ratios at the

desired 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 previous 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 inde-

pendently programmed by using an additional bias resistor

as shown in Figure 9b. The following formulas can be used

to compute the values of RNOM and R1:

RNOM

=

rCOLD – rHOT

2.714

• R25

R1= 0.536 • RNOM – rHOT • R25

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

a Vishay Curve 1 thermistor choose:

RNOM

=

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 circuit is shown

in Figure 9b and results in an upper trip point of 45°C and

a lower trip point of 0°C.

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