LTC3577-3 Datasheet, PDF (26/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
Alternate NTC Thermistors and Biasing
The LTC3577-3/LTC3577-4 provide temperature qualified
charging if a grounded thermistor and a bias resistor are
connected to NTC. By using a bias resistor whose value is
equal to the room temperature resistance of the thermis-
tor (R25) the upper and lower temperatures are pre-pro-
grammed 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-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)
NTCBIAS
34
R_NOM
105k
NTC
35
R1
12.7k
R_NTC
100k
NTC BLOCK
0.76 • NTCBIAS
–
+
–
0.35 • NTCBIAS
+
LTC3577-3/
LTC3577-4
TOO_COLD
TOO_HOT
0.26 • NTCBIAS
+
–
BATTERY
OVERTEMP
357734 F04
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
26
357734fa