LTC3559-1_15 Datasheet, PDF (15/24 Pages) Linear Technology – Linear USB Battery Charger with Dual Buck Regulators

LTC3559/LTC3559-1
APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The battery charger provides temperature qualified
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 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-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-
fication 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_NTC|HOT
R_NOM + R_NTC|HOT
• V_CC
= 0.349 • V_CC
and the cold trip point is set when:
R_NTC|COLD
R_NOM + R_NTC|COLD
•
V_CC
=
0.765
•
V_CC
V_CC
16
R_NOM
100k
NTC BLOCK
0.765 • V_CC
(NTC RISING) _–
NTC
+
13
R_NTC
100k
–
0.349 • V_CC
(NTC FALLING) ₊
TOO_COLD
TOO_HOT
V_CC
16
R_NOM
105k
0.765 • V_CC
(NTC RISING) _–
NTC
+
13
R1
12.7k
R_NTC
100k
–
0.349 • V_CC
(NTC FALLING) ₊
TOO_COLD
TOO_HOT
+
0.017 • V_CC
–
(NTC FALLING)
NTC_ENABLE
3559 F03
Figure 3. Typical NTC Thermistor Circuit
+
0.017 • V_CC
–
(NTC FALLING)
NTC_ENABLE
3559 F04
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
3559fb
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