Datasheet

LTC4156
40
4156f
APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The LTC4156 provides temperature qualified charging if a
grounded thermistor and a bias resistor are connected to the
NTC pin. Charging is paused if the temperature rises above
an NTC_HOT_FAULT limit or falls below an NTC_TOO_
COLD limit. By using a Vishay curve 2 thermistor and a bias
resistor whose value is equal to the room temperature resis-
tance
of the thermistor (r
25
), the upper and lower tempera-
tures are preprogrammed to approximately 60°C and
0°C, respectively. The NTC_HOT_FAULT threshold also
optionally generates an interrupt.
With minor modifications to the thermistor bias network
as shown in Figure 9, it is possible to adjust either one
or both of the temperature thresholds with the constraint
that it is usually not possible to move the
temperature
thresholds closer together. Intuitively, this would require
increased temperature sensitivity from the thermistor.
α
TOO _ COLD
≡
r
TOO _ COLD
r
25
Thermistor resistance ratio at desired NTC_TOO_COLD
threshold temperature relative to its reference
temperature.
α
HOT _FAULT
≡
r
HOT _FAULT
r
25
Thermistor resistance ratio at desired NTC_HOT_FAULT
threshold temperature relative to its reference tem-
perature.
α
BIAS
≡
R
BIAS
r
25
Ratio of low drift bias resistor to r
25
.
α
TEMP _RANGE
≡
R
TEMP _RANGE
r
25
Ratio of optional low drift dilution resistor to r
25
.
Note that r
25
, r
T
, r
TOO_COLD
and r
HOT_FAULT
and are
all resistance values of the thermistor at different
temperatures, while R
BIAS
and R
TEMP_RANGE
are actual
low drift resistors.
In all of the following calculations, it will be necessary to
determine the thermistor’s
α
T
at various temperatures.
This parameter is dependent only upon the material prop-
erties of the thermistor.
α
T
for a given thermistor and
temperature may be found in one of two ways. Thermistor
manufacturers often provide a lookup table relating
α
T
to
temperature in their data sheets. For any temperature T,
α
T
may be referenced directly.
The second way to obtain
α
T
for any T requires the use
of a modeling equation and a material constant specific
to the thermistor:
α
T
= e
b
1
T






−
1
T
0














Figure 9. Alternate NTC Bias Network
R
NTC
R
BIAS
LTC4156
4156 F09
NTC
NTCBIAS
T
R
TEMP_RANGE
In the explanation below, the following notation is used.
r
25
NTC thermistor value at 25°C.
R
BIAS
Low drift bias resistor, connected between the
NTCBIAS and NTC pins.
R
TEMP_RANGE
Optional dilution resistor, connected in series with
the thermistor.
α
T
≡
r
T
r
25
Thermistor resistance ratio at any temperature T relative
to its reference temperature.