Datasheet

LTC4155
33
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:
α
T
≡
r
T
r
25
=
κ
SPAN
•NTCVAL+κ
OFFSET
1−κ
SPAN
•NTCVAL–κ
OFFSET
where NTCVAL is the decimal representation of the
NTCVAL[6:0] status report in the range [0-127], ADC con-
stant
κ
SPAN
= 0.006162, ADC constant κ
OFFSET
= 0.1127,
r
T
is the resistance of the thermistor at temperature T,
and
α
T
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
α
T
to T, or will supply a
curve fit parameter
β which can be used with the follow-
ing equations to determine the thermistor temperature:
T =
β
ln(α
Τ
)+
β
T
0
T =
β
ln
κ
SPAN
•NTCVAL +κ
OFFSET
1−κ
SPAN
•NTCVAL – κ
OFFSET
⎛
⎝
⎜
⎞
⎠
⎟
+
β
T
0
where:
T = Temperature result expressed in Kelvin
T
0
= 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 I
2
C 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 I
2
C 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
α
COLD,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
α
COLD,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
α
WARM,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
α
WARM,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
α
CRITICAL,FAULT
= 0.298 and 60°C for a Vishay
OPERATION