LTC4097EDDB View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4097EDDB

USB/Wall Adapter Standalone Li-Ion/Polymer Battery Charger

Linear Technology 

LTC4097

APPLICATIONS INFORMATION

VNTC

3

RNOM

100k

NTC

6

NTC BLOCK

0.765 • VNTC

–

+

TOO_COLD

RNTC

100k

–

0.349 • VNTC

+

TOO_HOT

+

0.1V

–

NTC_ENABLE

VNTC

3

RNOM

105k

NTC

6

R1

12.7k

RNTC

100k

NTC BLOCK

0.765 • VNTC

–

+

–

0.349 • VNTC

+

+

0.1V

–

TOO_COLD

TOO_HOT

NTC_ENABLE

4097 F04

Figure 4. Typical NTC Thermistor Circuit

4097 F05

Figure 5. NTC Thermistor Circuit with Additional Bias Resistor

and the cold trip point is set when:

RNTC|COLD • VNTC = 0.765 • VNTC

RNOM + RNTC|COLD

Solving these equations for RNTC|COLD and RNTC|HOT results

in the following:

RNTC|COLD = 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 direc-

tion. The temperature span will change somewhat due to

the non-linear 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 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 inde-

pendently programmed by using an additional bias resistor

as shown in Figure 5. 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

4097f

17

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