LTC4001-1_15 Datasheet, PDF (12/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001-1
APPLICATIONS INFORMATION
The battery temperature is measured by placing a negative
temperature coefficient (NTC) thermistor close to the bat-
tery pack. To use this feature, connect the NTC thermistor,
R_NTC, between the NTC pin and GNDSENS and the resistor,
R_NOM, from the NTC pin to V_INSENSE. R_NOM should be a 1%
resistor with a value equal to the value of the chosen NTC
thermistor at 25°C. The LTC4001-1 goes into hold mode
when the resistance, R_HOT, of the NTC thermistor drops to
0.41 times the value of R_NOM. For instance for R_NTC = 10k.
(The value for a Vishay NTHS0603N02N1002J thermistor
at 25°C) hold occurs at approximately 4.1k, which occurs
at 50°C. The hold mode freezes the timer and stops the
charge cycle until the thermistor indicates a return to a
valid temperature. As the temperature drops, the resistance
of the NTC thermistor rises. The LTC4001-1 is designed to
go into hold mode when the value of the NTC thermistor
increases to 2.82 times the value of R_NOM. This resistance
is R_COLD. For the Vishay 10k thermistor, this value is 28.2k,
which corresponds to approximately 0°C. The hot and cold
comparators each have approximately 3°C of hysteresis
to prevent oscillation about the trip point. Grounding the
NTC pin disables the NTC function.
Thermistors
The LTC4001-1 NTC trip points were designed to work with
thermistors whose resistance temperature characteristics
follow Vishay Dale’s “R-T Curve 2.” However, any thermis-
tor whose ratio of R_COLD to R_HOT is about 7 will also work
(Vishay Dale R-T Curve 2 shows a ratio of R_COLD to R_HOT
of 2.815/0.4086 = 6.89).
Power conscious designs may want to use thermistors
whose room temperature value is greater than 10k. Vishay
Dale has a number of values of thermistor from 10k to 100k
that follow the “R-T Curve 1.” Using these as indicated
in the NTC Thermistor section will give temperature trip
points of approximately 3°C and 47°C, a delta of 44°C. This
delta in temperature can be moved in either direction by
changing the value of R_NOM with respect to R_NTC. Increasing
R_NOM will move the trip points to higher temperatures. To
calculate R_NOM for a shift to lower temperature for example,
use the following equation:
R_NOM
=
R_COLD
2.815
• R_NTC
at
25°C
where R_COLD is the resistance ratio of R_NTC at the desired
cold temperature trip point. If you want to shift the trip points
to higher temperatures use the following equation:
R_NOM
=
R_HOT
0.4086
• R_NTC
at
25°C
where R_HOT is the resistance ratio of R_NTC at the desired
hot temperature trip point.
Here is an example using a 100k R-T Curve 1 thermistor
from Vishay Dale. The difference between trip points is
44°C, from before, and we want the cold trip point to be
0°C, which would put the hot trip point at 44°C. The R_NOM
needed is calculated as follows:
R_NOM
=
R_COLD
2.815
• R_NTC
at
25°C
=
3.266
2.815
•
100k
=
116k
The nearest 1% value for R_NOM is 115k. This is the value
used to bias the NTC thermistor to get cold and hot trip
points of approximately 0°C and 44°C respectively. To
extend the delta between the cold and hot trip points a
resistor, R1, can be added in series with R_NTC (see Figure 4).
The values of the resistors are calculated as follows:
R_NOM
=
R_COLD – R_HOT
2.815 – 0.4086
( ) ^R1=
0.4086
2.815 – 0.4086
•
R_COLD – R_HOT
– R_HOT
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