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LTC4155

4155fc

thermistor current, and its associated battery drain by a

factor of 2000 from its DC value. A typical network using

a 10k thermistor causes 115nA of battery drain. A 100k

thermistor would reduce this drain to 11.5nA.

To improve measurement resolution over the temperature

range of interest, the full-scale range of the analog-to-

digital converter is restricted to the range 0.113 to 0.895

NTCBIAS. The NTC ADC result can be interpreted as follows:

αT ≡

r25

= κSPAN

•NTCVAL

+κOFFSET

−κSPAN •NTCVAL – κOFFSET

where NTCVAL is the decimal representation of the

NTCVAL[6:0] status report in the range [0-127], ADC con-

stant κSPAN = 0.006162, ADC constant κOFFSET = 0.1127,

rT is the resistance of the thermistor at temperature T,

and αT is the resistance ratio of the thermistor at the two

temperatures T and 25°C.

Thermistor manufacturer data sheets will either provide a

temperature lookup table relating αT to T, or will supply a

curve fit parameter β which can be used with the follow-

ing equations to determine the thermistor temperature:

ln (

αΤ)+

κSPAN •NTCVAL +κOFFSET

−κSPAN •NTCVAL – κOFFSET

⎛

⎝

⎜

⎞

⎠

⎟ +

where:

T = Temperature result expressed in Kelvin

T0 = Thermistor model nominal temperature, expressed

in Kelvin. Typically 298.15K (25°C + 273.15°C)

β = Thermistor model material constant, expressed in

Kelvin.

In addition to thermistor value reporting, the LTC4155

automatically pauses battery charging if the thermistor

reading falls outside of limits corresponding to the

range 0°C to 40°C for a Vishay curve 2 thermistor. The

NTC_TOO_COLD and NTC_TOO_WARM conditions

are encoded in the I2C status report NTCSTAT[1:0].

CHARGER_STATUS[2:0] will also report temperature

warnings and faults when the battery charger is enabled.

See Table 14 and Table 17. Optionally, a charger status

interrupt request may be generated when the thermistor

reading enters or exits this temperature range.

If the temperature reading is above a limit corresponding

to 60°C for a Vishay curve 2 thermistor, an optional

NTC_HOT_FAULT interrupt may be generated. In addition,

the overtemperature battery conditioner may be enabled

by I2C to minimize the time that the battery is exposed

simultaneously to high temperature and high voltage.

Refer to the Overtemperature Battery Conditioner section

for more detail.

The NTC_TOO_COLD temperature indication is triggered

when NTCVAL rises to decimal result 102. This corre-

sponds to a αCOLD,WARNING = 2.86 and 0°C for a Vishay

curve 2 thermistor. The low temperature indication is

cleared when the NTCVAL falls to decimal result 98. This

corresponds to αCOLD,RESET = 2.53 and 2°C for a Vishay

curve 2 thermistor.

The NTC_TOO_WARM temperature indication is triggered

when NTCVAL falls to decimal result 41. This corresponds

to αWARM,WARNING = 0.576 and 40°C for a Vishay curve 2

thermistor. The high temperature indication is cleared

when NTCVAL rises to decimal result 45. This corresponds

to αWARM,RESET = 0.639 and 37°C for a Vishay curve 2

thermistor.

The NTC_HOT_FAULT temperature indication is triggered

when NTCVAL falls to decimal result 19. This corre-

sponds to αCRITICAL,FAULT = 0.298 and 60°C for a Vishay

OPERATION

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