LTC4055-1_15 Datasheet, PDF (17/24 Pages) Linear Technology – USB Power Controller and Li-Ion Charger

OPERATION
V_NTC
15
R_NOM
100k
NTC
16
LTC4055/LTC4055-1
NTC BLOCK
0.74 • V_NTC
–
+
TOO_COLD
R_NTC
100k
–
0.29 • V_NTC
+
TOO_HOT
+
0.1V
–
NTC_ENABLE
V_NTC
15
R_NOM
121k
NTC
16
R1
13.3k
R_NTC
100k
(3a)
4055 F03a
Figure 3. NTC Circuits
LTC4055/LTC4055-1
LTC4055/LTC4055-1
NTC BLOCK
0.74 • V_NTC
–
+
TOO_COLD
–
0.29 • V_NTC
+
TOO_HOT
+
0.1V
–
(3b)
NTC_ENABLE
4055 F03b
THERMISTORS
The LTC4055/LTC4055-1 NTC trip points were designed
to work with thermistors whose resistance-temperature
characteristics follow Vishay Dale’s “R-T Curve 2.” The
Vishay NTHS0603N02N1002J is an example of such a
thermistor. However, Vishay Dale has many thermistor
products that follow the “R-T Curve 2” characteristic in
a variety of sizes. Furthermore, any thermistor whose
ratio of R_COLD to R_HOT is about 7.0 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 both trip points to lower temperatures.
Likewise a decrease in R_NOM with respect to R_NTC 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 the 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
4055fb
17