LTC3559 Linear Technology, LTC3559 Datasheet - Page 15

• Manufacturer Part Number: LTC3559
• Description: Linear USB Battery Charger
• Manufacturer: Linear Technology

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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.
R
100k
NOM
R
100k
NTC
16
13
Figure 3. Typical NTC Thermistor Circuit
V
NTC
CC
(NTC FALLING)
(NTC FALLING)
0.349 • V
(NTC RISING)
0.017 • V
0.765 • V
CC
CC
CC
NTC BLOCK
–
+
–
+
+
–
TOO_HOT
NTC_ENABLE
TOO_COLD
3559 F03
In the explanation below, the following notation is used.
R25 = Value of the thermistor at 25°C
R
R
r
r
R
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
hot threshold and 0.765 • V
Therefore, the hot trip point is set when:
and the cold trip point is set when:
COLD
HOT
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
NTC|COLD
NTC|HOT
NOM
R
R
NOM
NOM
= Ratio of R
= Ratio of R
= Primary thermistor bias resistor (see Figure 3)
R
105k
R1
12.7k
R
R
NOM
R
100k
NTC
NTCHOT
NTC COLD
+
+
= Value of the thermistor at the hot trip point
16
13
= Value of thermistor at the cold trip point
R
R
|
|
NTCHOT
NTC COLD
V
NTC
CC
|
|
NTC|HOT
(NTC FALLING)
(NTC FALLING)
NTC|COLD
0.349 • V
(NTC RISING)
0.017 • V
0.765 • V
•
•
V
V
CC
CC
to R25
CC
CC
CC
= 0 349
to R25
CC
= 0 765
–
+
–
+
+
–
for the cold threshold.
.
.
•
•
V
V
CC
LTC3559
CC
TOO_HOT
NTC_ENABLE
TOO_COLD
CC
15
3559 F04
for the
3559f