ltc4001 Linear Technology Corporation, ltc4001 Datasheet - Page 12

• Manufacturer Part Number: ltc4001
• Description: 2a Synchronous Buck Li-ion Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC4001
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
resistor, R
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
thermistor drops to 0.41 times the value of R
instance for R
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
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
work (Vishay Dale R-T Curve 2 shows a ratio of R
R
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
12
HOT
of 2.815/0.4086 = 6.89).
NTC
NOM
NOM
, between the NTC pin and GNDSENS and the
. This resistance is R
, from the NTC pin to V
NTC
U
= 10k. (The value for a Vishay
COLD
U
NOM
to R
COLD
HOT
with respect to R
W
is about 7 will also
. For the Vishay 10k
HOT
INSENSE
, of the NTC
U
NOM
. R
COLD
. For
NOM
NTC
to
.
Increasing R
peratures. To calculate R
ture for example, use the following equation:
where R
cold temperature trip point. If you want to shift the trip
points to higher temperatures use the following equation:
where R
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
needed is calculated as follows:
The nearest 1% value for R
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
Figure 4). The values of the resistors are calculated as
follows:
R
R
R
R
R
NOM
NOM
NOM
NOM
1
=
COLD
HOT
2 815 0 4086
=
=
=
=
=
.
R
R
0 4086
3 266
2 815
2 815 0 4086
2 815
2 815
R
is the resistance ratio of R
0 4086
NOM
R
.
.
.
.
COLD
is the resistance ratio of R
COLD
.
.
.
COLD
HOT
– .
will move the trip points to higher tem-
– .
•
•
•
–
100
•
R
R
R
R
NTC
NTC
NTC
HOT
k
NOM
•
=
(
at
at
R
NOM
at
116
COLD
for a shift to lower tempera-
25
25
25
k
°
°
is 115k. This is the value
°
C
C
C
–
R
HOT
NTC
NTC
)
–
at the desired
at the desired
R
HOT
NTC
(see
NOM
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