Datasheet

LTC3559/LTC3559-1
15
3559fb
APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The battery charger 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, respec-
tively (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 battery charger’s temperature quali-
fi cation are internally programmed at 0.349 • V
CC
for the
hot threshold and 0.765 • V
CC
for the cold threshold.
Therefore, the hot trip point is set when:
R
RR
VV
NTCHOT
NOM NTCHOT
CC CC
|
|
•.•
+
= 0 349
and the cold trip point is set when:
R
RR
VV
NTCCOLD
NOM NTCCOLD
CC CC
|
|
•.•
+
= 0 765
3559 F03
R
NOM
100k
R
NTC
100k
–
+
–
+
–
+
TOO_COLD
TOO_HOT
NTC_ENABLE
0.765 • V
CC
(NTC RISING)
NTC BLOCK
0.349 • V
CC
(NTC FALLING)
13
NTC
16
V
CC
0.017 • V
CC
(NTC FALLING)
3559 F04
R
NOM
105k
R
NTC
100k
–
+
–
+
–
+
TOO_COLD
TOO_HOT
NTC_ENABLE
R1
12.7k
0.765 • V
CC
(NTC RISING)
0.349 • V
CC
(NTC FALLING)
13
NTC
0.017 • V
CC
(NTC FALLING)
16
V
CC
Figure 3. Typical NTC Thermistor Circuit Figure 4. NTC Thermistor Circuit with Additional Bias Resistor