Datasheet
LTC4155
42
4155fc
APPLICATIONS INFORMATION
substitute into the appropriate following equation to cal-
culate the value
α
BIAS
and then R
BIAS
.
α
BIAS
= 0.34917 •
α
TOO_COLD
α
BIAS
= 1.73735 •
α
TOO_WARM
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 two dependent tem-
perature thresholds may be found by substituting
α
BIAS
into the remaining two equations. The following equation
may be used to convert any other NTC ADC result (NTCVAL)
back to a resistance ratio by substituting the κ
SPAN
and
κ
OFFSET
values found in the Electrical Characteristics table.
α
T
=
κ
SPAN
• NTCVAL +κ
OFFSET
1−κ
SPAN
• NTCVAL – κ
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 =
β
ln(α
T
)+
β
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 LTC4155. Using
this additional resistor, two of the three temperature
comparator thresholds may be adjusted. The remaining
threshold is constrained by the limited degrees of freedom
of the bias network. After determining the
α
T
values for
the two temperature thresholds of interest, the following
equations may be used to determine
α
BIAS
,
α
TEMP_RANGE
,
and the third constrained temperature threshold dependent
on the choice of bias network and the thermistor.
To specify T
TOO_COLD
and T
TOO_WARM
, then calculate
T
HOT_FAULT
:
α
TEMP_RANGE
= 0.25153 •
α
TOO_COLD
– 1.25153
•
α
TOO_WARM
R
TEMP_RANGE
=
α
TEMP_RANGE
• r
25
α
BIAS
= 0.43699 • (
α
TOO_COLD
–
α
TOO_WARM
)
R
BIAS
=
α
BIAS
• r
25
With the bias network determined, the overconstrained
threshold may then be calculated:
α
HOT_FAULT
= 0.29829 •
α
BIAS
–
α
TEMP_RANGE
To specify T
TOO_COLD
and T
HOT_FAULT
, then calculate
T
TOO_WARM
:
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
With the bias network determined, the overconstrained
threshold may then be calculated:
α
TOO_WARM
= 0.57559 •
α
BIAS
–
α
TEMP_RANGE
To specify T
TOO_WARM
and T
HOT_FAULT
, then calculate
T
TOO_COLD
:
α
TEMP_RANGE
= 1.07566 •
α
TOO_WARM
– 2.07566
•
α
HOT_FAULT
R
TEMP_RANGE
=
α
TEMP_RANGE
• r
25
α
BIAS
= 3.60615 • (
α
TOO_WARM
–
α
HOT_FAULT
)
R
BIAS
=
α
BIAS
• r
25
With the bias network determined, the overconstrained
threshold may then be calculated:
α
TOO_COLD
= 2.863946 •
α
BIAS
–
α
TEMP_RANGE
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.