LTC4085-3 Datasheet, PDF (19/24 Pages) Linear Technology – USB Power Manager with Ideal Diode Controller and 3.95V Li-Ion Charger

LTC4085-3/LTC4085-4
APPLICATIONS INFORMATION
V_NTC
9
R_NOM
10k
NTC
8

t
7_NTC
–
+
LTC4085-3/
LTC4085-4
TOO_COLD
R_NTC
10k
–

t
7_NTC
+
TOO_HOT
+
0.1V
–
NTC_ENABLE
V_NTC
9
R_NOM
124k
NTC
8
R1
24.3k
R_NTC
100k
408534 F04a
(4a)
Figure 4. NTC Circuits

t
7_{NTC –}
+
–

t
7_NTC
+
+
0.1V
–
(4b)
LTC4085-3/
LTC4085-4
TOO_COLD
TOO_HOT
NTC_ENABLE
408534 F04b
Alternate NTC Thermistors
The LTC4085 NTC trip points were designed to work
with thermistors whose resistance-temperature charac-
teristics follow Vishay Dale’s “R-T Curve 2.” The Vishay
NTHS0603N02N1002J is an example of such a thermis-
tor. 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 6.0 will also work (Vishay Dale R-T Curve
2 shows a ratio of 2.816/0.4839 = 5.82).
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 42°C, a delta of 39°C.
This delta in temperature can be moved in either direc-
tion 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
2.816
• R_{N T C}
at
25°C
where R_COLD is the resistance ratio of R_NTC at the desired
cold temperature trip point. To shift the trip points to higher
temperatures use the following equation:
R_NOM
=
R_HOT
0.484
• R_{N T C}
at
25°C
where R_HOT is the resistance ratio of R_NTC at the desired
hot temperature trip point.
The following example uses a 100k R-T Curve 1 Thermistor
from Vishay Dale. The difference between the trip points
is 39°C, from before—and the desired cold trip point of
0°C, would put the hot trip point at about 39°C. The R_NOM
needed is calculated as follows:
R_NOM
= ^RCOLD
2.816
• R_NTC
at
25°C =
3.266 _{• 100kΩ = 116kΩ}
2.816
408534fb
19