Datasheet

LTC4155
41
4155fc
APPLICATIONS INFORMATION
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.
α
TOO_COLD
≡
r
TOO_COLD
r
25
Thermistor resistance ratio at desired NTC_TOO_COLD
threshold temperature relative to its reference
temperature.
α
TOO_ WARM
≡
r
TOO_ WARM
r
25
Thermistor resistance ratio at desired NTC_TOO_WARM
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
, r
TOO_WARM
, 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
β
1
T
⎛
⎝
⎜
⎞
⎠
⎟
−
1
T
0
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
where:
e = Natural logarithm base, approximately 2.71828
T = Temperature of interest, expressed in Kelvin
T
0
= Thermistor model nominal temperature, expressed
in Kelvin. Typically 298.15K (25°C + 273.15°C)
β = Model material constant, expressed in Kelvin. This
model is a curve fit at T
0
and a second temperature. β is
close to 4000K for most thermistors. Higher
β results in
increased temperature sensitivity at the expense of reduced
linearity over a wide temperature range
Simple Alternate Thermistor Bias Network
By simply adjusting the bias resistor, R
BIAS
, and omitting
the optional R
TEMP_RANGE
, one of the three temperature
thresholds may be adjusted. The other two temperature
comparator thresholds will be determined by the choice
of the first temperature threshold and the NTCVAL
thresholds fixed in the LTC4155. Increasing R
BIAS
above
r
25
shifts all three temperature thresholds colder while
simultaneously slightly compressing the temperature span
between thresholds. Likewise, decreasing R
BIAS
below
r
25
shifts all three temperature thresholds warmer while
simultaneously slightly expanding the temperature span
between thresholds. To program a single temperature
threshold, obtain the value of either
α
TOO_COLD
,
α
TOO_WARM
or α
HOT_FAULT
for the chosen temperature
threshold using one of the aforementioned methods and