Datasheet

LTC4156
33
4156f
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
=
k
SPAN
• NTCVAL
+ k
OFFSET
1− k
SPAN
• NTCVAL – k
OFFSET
where NTCVAL is the decimal representation of the
NTCVAL[6:0] status report in the range [0-127], ADC con-
stant
k
SPAN
= 0.006162, ADC constant k
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
b which can be used with the follow-
ing equations to determine the thermistor temperature:
T =
b
ln (α
Τ
) +
b
T
0
T =
b
ln
k
SPAN
• NTCVAL + k
OFFSET
1− k
SPAN
• NTCVAL – k
OFFSET






+
b
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)
b = Thermistor model material constant, expressed in
Kelvin.
In addition to thermistor value reporting, the LTC4156
automatically pauses battery charging if the thermistor
reading falls outside of limits corresponding to the
range 0°C to 60°C for a Vishay curve 2 thermistor.
The NTC_TOO_COLD and NTC_HOT_
FAULT 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 13 and Table 16. 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 also be generated.
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_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
curve 2 thermistor. The critically hot temperature indica-
tion is cleared when NTCVAL rises to decimal result 23.
This corresponds to
α
CRITICAL,RESET
= 0.341 and 55.5°C
for a Vishay curve 2 thermistor.
It is possible to modify the thermistor bias network to adjust
either one or both of the above temperature thresholds. See
Alternate NTC Thermistors and Biasing in the Applications
Information section for implementation details.
OPERATION