Datasheet
LTC4095
11
4095fa
APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The LTC4095 provides temperature qualifi ed charging if a
grounded thermistor and a bias resistor are connected to
the NTC pin. By using a bias resistor whose value is equal
to the room temperature resistance of the thermistor (R25)
the upper and lower temperatures are pre-programmed
to approximately 40°C and 0°C, respectively (assuming
a Vishay “Curve 1” thermistor).
The upper and lower temperature thresholds can be ad-
justed by either a modifi cation of the bias resistor value
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 modifi ed 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 LTC4095’s temperature qualifi ca-
tion are internally programmed at 0.349 • IN for the hot
threshold and 0.765 • IN for the cold threshold.
Therefore, the hot trip point is set when:
R
RR
IN IN
NTC HOT
NOM NTCHOT
|
|
•.•
+
= 0 349
and the cold trip point is set when:
R
RR
IN IN
NTC COLD
NOM NTC COLD
|
|
•.•
+
= 0 765
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
Figure 3. Typical NTC Thermistor Circuit
4095 F03
R
NOM
100k
R
NTC
100k
–
+
–
+
–
+
TOO_COLD
TOO_HOT
NTC_ENABLE
0.765 • IN
(NTC
RISING)
LTC4095 NTC BLOCK
0.349 • IN
(NTC
FALLING)
6
NTC
8
IN
0.017 • IN
(NTC FALLING)
4095 F04
R
NOM
105k
R
NTC
100k
–
+
–
+
–
+
TOO_COLD
TOO_HOT
NTC_ENABLE
R1
12.7k
0.765 • IN
(NTC
RISING)
0.349 • IN
(NTC
FALLING)
6
NTC
0.017 • IN
(NTC FALLING)
8
IN