Datasheet

LTC3677-3
25
36773f
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
–
+
–
+
R
NOM
105k
R
NTC
100k
R1
12.7k
NTC
NTCBIAS
34
0.26 • NTCBIAS
BATTERY
OVERTEMP
36773 F04
NTC BLOCK
LTC3677-3
TOO_COLD
TOO_HOT
0.76 • NTCBIAS
0.35 • NTCBIAS
–
+
35
operaTion
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modified but not both. The
other trip point will be determined by the characteristics
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tempera-
ture trip points can be independently programmed with
the constraint that the difference between the upper and
lower temperature thresholds cannot decrease. Examples
of each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N011-N1003F, used
in the following examples, has a nominal value of 100k
and follows the Vishay curve 1 resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the thermistor at 25°C
R
NTC|COLD
= Value of thermistor at the cold trip point
R
NTC|HOT
= Value of the thermistor at the hot trip point
r
COLD
= Ratio of R
NTC|COLD
to R25
r
HOT
= Ratio of R
NTC|HOT
to R25
R
NOM
= Primary thermistor bias resistor (see Figure 3)
R1 = Optional temperature range adjustment resistor
(see Figure 4)
The trip points for the LTC3677-3’s temperature qualifica-
tion are internally programmed at 0.35 • V
NTC
for the hot
threshold and 0.76 • V
NTC
for the cold threshold.
Therefore, the hot trip point is set when:
R
R R
NTCBIAS NTCBIAS
NOM
NTC|HOT
NTC|HOT
+
=• . •0 35
and the cold trip point is set when:
R
R R
NTCBIAS NTCBIA
NOM
NTC|COLD
NTC|COLD
+
=• . •0 76 SS
Solving these equations for R
NTC|COLD
and R
NTC|HOT
results
in the following:
R
NTC|HOT
= 0.538 • R
NOM
and
R
NTC|COLD
= 3.17 • R
NOM
By setting R
NOM
equal to R25, the above equations result
in r
HOT
= 0.538 and r
COLD
= 3.17. Referencing these ratios
to the Vishay resistance-temperature curve 1 chart gives a
hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
By using a bias resistor, R
NOM
, different in value from
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R
r
R
R
r
R
NOM
HOT
NOM
COLD
=
=
0 538
25
3 17
25
.
•
.
•
where r
HOT
and r
COLD
are the resistance ratios at the
desired hot and cold trip points. Note that these equations