LTC4124 datasheet(20/24 Pages) AD

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LTC4126-ADJ

Rev. 0

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Figure 15. NTC Thermistor Connection with

Desensitizing Resistor RD

LTC4126-ADJ

NTC

BAT

NTC RESISTOR

THERMALLY COUPLED

WITH BATTERY

VCC

4126-ADJ F15

RBIAS

RNTC

Li-Ion

APPLICATIONS INFORMATION

Similarly, since the Too Cold comparator threshold in

the LTC4126-ADJ is internally set to 76.5% of VCC, the

resistance of the thermistor at the cold threshold, RCOLD,

can be computed using the following equation:

RCOLD

RCOLD +RBIAS

= 0.765

This can be simplified as:

RCOLD

RBIAS

= 3.25

Again, if RBIAS is chosen to have a value equal to the

value of the chosen NTC thermistor at 25°C (R25), then

RCOLD/R25 = 3.25. For the same Vishay thermistor with

β25/85 = 3950k, the ratio RT/R25 = 3.25 corresponds to

approximately 0°C.

The hot/cold temperature thresholds can be increased or

decreased by choosing an RBIAS value which is not the

same as R25. For example, if a hot temperature threshold

of 50°C is desired, consult the resistance/temperature

conversion table of the thermistor to find the ratio R50/

R25. For the same Vishay thermistor used above, this

ratio is 0.3631. Since RHOT/RBIAS = 0.536, RBIAS can be

calculated as follows:

RBIAS =

RHOT

0.536

0.3631•R25

0.536

= 0.677 •R25

This means: choose an RBIAS value which is 67.7% of the

value of the thermistor at 25°C to set the hot temperature

threshold to 50°C. However, this will automatically shift

the cold temperature threshold upward too. The cold

temperature threshold can be recalculated by computing

the RCOLD/R25 ratio as follows:

RCOLD

R25

RCOLD

RBIAS

•

RBIAS

R25

= 3.25 • 0.677 = 2.202

From the conversion table, this ratio corresponds to

about 8°C. Note that changing the value of RBIAS to be

smaller than R25 moves both the hot and cold thresholds

higher. Similarly, RBIAS with a value greater than R25 will

move both the hot and cold thresholds lower. Also note

that with only one degree of freedom (i.e. adjusting the

value of RBIAS), the user can only set either the cold or

hot threshold but not both.

It is possible to adjust the hot and cold threshold indepen-

dently by introducing another resistor as a second degree

of freedom (Figure 15). The resistor RD in effect reduces

the sensitivity of the resistance between the NTC pin and

ground.Therefore,intuitivelythisresistorwillmovethehot

threshold to a hotter temperature and the cold threshold

to a colder temperature. The value of RBIAS and RD can

now be set according to the following formula:

RBIAS =

RCOLD –RHOT

(

)

2.714

RD = 0.197 •RCOLD – 1.197 •RHOT

Note that this method can only be used to push the hot and

cold temperature thresholds apart from each other. When

using the formulas above, if the user finds that a negative

value is needed for RD, the two temperature thresholds

selected are too close to each other and a higher sensitiv-

ity thermistor is needed. For example, this method can be

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