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

LTC4001
APPLICATIO S I FOR ATIO
The battery temperature is measured by placing a negative
temperature coefficient (NTC) thermistor close to the
battery pack. To use this feature, connect the NTC ther-
mistor, 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 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 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 oscilla-
tion about the trip point. Grounding the NTC pin disables
the NTC function.
Thermistors
The LTC4001 NTC trip points were designed to work with
thermistors whose resistance temperature characteristics
follow Vishay Dale’s “R-T Curve 2.” However, any ther-
mistor 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 indi-
cated 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 tem-
peratures. To calculate R_NOM for a shift to lower tempera-
ture 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 • 100k = 116k
2.815
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
4001f
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