Datasheet

LTC4156
41
4156f
APPLICATIONS INFORMATION
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)
b = Model material constant, expressed in Kelvin. This
model is a curve fit at T
0
and a second temperature. b is
close to 4000K for most thermistors. Higher
b 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 temperature thresholds
may be adjusted. The other temperature comparator
threshold will be determined by the choice of the first
temperature threshold and the NTCVAL thresholds fixed
in the LTC4156. Increasing R
BIAS
above r
25
shifts both
temperature thresholds colder while
simultaneously
slightly compressing the temperature span between
thresholds. Likewise, decreasing R
BIAS
below r
25
shifts
both 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
or α
HOT_FAULT
for
the chosen temperature threshold using one of the afore-
mentioned methods and substitute into the appropriate
following equation to calculate the value
α
BIAS
and then
R
BIAS
.
α
BIAS
= 0.34917 • α
TOO_COLD
α
BIAS
= 3.35249 • α
HOT_FAULT
R
BIAS
= α
BIAS
• r
25
With α
BIAS
for the newly programmed temperature
threshold now determined, the other dependent tempera-
ture threshold may be found by substituting
α
BIAS
into
the remaining equation.
The following equation may be used to convert any other
NTC ADC result (NTCVAL) by substituting the
k
SPAN
and
k
OFFSET
values found in the Electrical Characteristics table.
α
T
=
k
SPAN
• NTCVAL + k
OFFSET
1− k
SPAN
• NTCVAL – k
OFFSET






α
BIAS
α
T
may then be used to determine the temperature using
either the lookup table provided by the thermistor manu-
facturer or the curve fit model:
T =
b
ln(α
T
)+
b
T
0
Advanced Alternate Thermistor Bias Network
If an adjustment to R
BIAS
does not yield an acceptable span
between temperature thresholds, a second low drift bias
resistor may be added to the bias network between the
NTC pin and the top of the thermistor. This resistor has
the net effect of diluting the high thermal sensitivity of the
thermistor with low drift resistance. The result is reduced
thermal gain
and a wider temperature span between the
preprogrammed voltage thresholds of the LTC4156. Using
this additional resistor, both of the temperature comparator
thresholds may be adjusted. After determining the
α
T
values for the two temperature thresholds of interest, the
following equations may be used to determine
α
BIAS
and
α
TEMP_RANGE
.
α
TEMP_RANGE
= 0.11626 • α
TOO_COLD
– 1.11626
• α
HOT_FAULT
R
TEMP_RANGE
= α
TEMP_RANGE
• r
25
α
BIAS
= 0.38976 • ( α
TOO_COLD
– α
HOT_FAULT
)
R
BIAS
= α
BIAS
• r
25
It is possible to obtain a negative result for R
TEMP_RANGE
which is not physically realizable using the previous equa-
tions. A negative result indicates that the two chosen tem-
perature thresholds are too close in temperature and require
more thermal sensitivity than the thermistor can provide.