Datasheet

LTC3555/LTC3555-X
27
3555fd
Alternate NTC Thermistors and Biasing
The LTC3555 family provides temperature qualified charg-
ing if a grounded thermistor and a bias resistor are con-
nected to NTC. 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 modification 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 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-N1002F, used
in the following examples, has a nominal value of 10k
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
α
COLD
= Ratio of R
NTC|COLD
to R25
α
HOT
= Ratio of R
NTC|HOT
to R25
R
NOM
= Primary thermistor bias resistor (see Figure 5a)
R1 = Optional temperature range adjustment resistor
(see Figure 5b)
The trip points for the LTC3555 family’s temperature quali-
cation are internally programmed at 0.349 • V
BUS
for the
hot threshold and 0.765 • V
BUS
for the cold threshold.
Therefore, the hot trip point is set when:
R
NTC|HOT
R
NOM
+ R
NTC|HOT
• V
BUS
= 0.349 • V
BUS
and the cold trip point is set when:
R
NTC|COLD
R
NOM
+ R
NTC|COLD
• V
BUS
= 0.765 • V
BUS
Solving these equations for R
NTC|COLD
and R
NTC|HOT
results in the following:
R
NTC|HOT
= 0.536 • R
NOM
and
R
NTC|COLD
= 3.25 • R
NOM
APPLICATIONS INFORMATION
Figure 5. NTC Circuits
(5a)
(5b)
–
+
–
+
R
NOM
10k
R
NTC
10k
NTC
0.1V
NTC_ENABLE
3555 F05a
LTC3555/LTC3555-X
NTC BLOCK
TOO_COLD
TOO_HOT
0.765 • V
BUS
0.349 • V
BUS
–
+
3
V
BUS
V
BUS
T
–
+
–
+
R
NOM
10.5k
R
NTC
10k
R1
1.27k
NTC
V
BUS
V
BUS
0.1V
NTC_ENABLE
3555 F05b
TOO_COLD
TOO_HOT
0.765 • V
BUS
0.349 • V
BUS
–
+
3
LTC3555/LTC3555-X
NTC BLOCK
T