LTC4055 Linear Technology, LTC4055 Datasheet - Page 17

• Manufacturer Part Number: LTC4055
• Description: USB Power Controller and Li-Ion Linear Charger
• Manufacturer: Linear Technology

OPERATIO
THERMISTORS
The LTC4055 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
is about 7.0 will also work (Vishay Dale R-T Curve 2 shows
a ratio of 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
Increasing R
temperatures. Likewise a decrease in R
R
NTC
will move the trip points to higher temperatures. To
R
100k
R
100k
NOM
V
NTC
NTC
NTC
15
16
COLD
NOM
to R
0.29 • V
U
0.74 • V
will move both trip points to lower
HOT
0.1V
NTC
NTC
of 2.815/0.4086 = 6.89).
(3a)
–
+
–
+
+
–
NOM
NTC BLOCK
LTC4055
with respect to R
NOM
with respect to
TOO_COLD
TOO_HOT
NTC_ENABLE
COLD
Figure 3. NTC Circuits
to R
4055 F03a
NTC
HOT
.
calculate R
ample, 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 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
needed is calculated as follows:
R
R
R
NOM
NOM
NOM
R
121k
R1
13.3k
R
100k
NOM
V
NTC
NTC
NTC
15
16
COLD
HOT
R
3 266
2 815
2 815
NOM
R
0 4086
2 815
.
.
COLD
.
is the resistance ratio of R
R
.
COLD
is the resistance ratio of R
.
HOT
for a shift to lower temperature for ex-
0.29 • V
•
0.74 • V
•
100
R
•
•
NTC
R
0.1V
R
NTC
k
NTC
NTC
NTC
at
116
at
25
(3b)
at
–
+
–
+
+
–
25
k
NTC BLOCK
25
C
LTC4055
C
C
NTC
NTC
LTC4055
TOO_COLD
TOO_HOT
NTC_ENABLE
at the desired
at the desired
4055 F03b
17
NOM
4055p