LTC4061 Datasheet, PDF (16/20 Pages) Linear Technology – Standalone Linear Li-lon Battery Charger with Thermistor Input

LTC4061
APPLICATIO S I FOR ATIO
It is important to remember that LTC4061 applications do
not need to be designed for worst-case thermal conditions,
since the IC will automatically reduce power dissipation if
the junction temperature reaches approximately 105°C.
Thermistors
The LTC4061 NTC comparator trip points were designed
to work with thermistors whose resistance-temperature
characteristics follow Vishay Dale’s “R-T Curve 1.” The
Vishay NTHS0603N01N1003J is an example of such a
thermistor. However, Vishay Dale has many thermistor
products that follow the “R-T Curve 1” characteristic in a
variety of sizes. Furthermore, any thermistor whose ratio
of R_COLD to R_HOT is about 6 also works (Vishay Dale R-T
Curve 1 shows a ratio of R_COLD to R_HOT of 3.266/0.5325
= 6.13).
Power conscious designers may want to use thermistors
whose room temperature value is greater than 10kΩ.
Vishay Dale has a number of values of thermistor from
10kΩ to 100kΩ that follow the “R-T Curve 1.” Using dif-
ferent R-T curves, such as Vishay Dale “R-T Curve 2,” is
also possible. This curve, combined with LTC4061 internal
thresholds, gives temperature trip points of approximately
0°C (falling) and 40°C (rising), a delta of 40°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
moves both trip points to lower temperatures. Likewise
a decrease in R_NOM with respect to R_NTC moves the trip
points to higher temperatures. To calculate R_NOM for a shift
to lower temperatures, use the following equation:
R_NOM
=
R_COLD
3.266
• 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 equations:
R_NOM
=
R_HOT
0.5325
• 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 10kΩ R-T Curve 2 thermistor
from Vishay Dale. The difference between the trip points
16
is 40°C, from before, and we want the cold trip point to
be 0°C, which would put the hot trip point at 40°C. The
R_NOM needed is calculated as follows:
R_NOM
=
R_COLD
3.266
• R_NTC
at
25°C
= ^2.816 •10kΩ = 8.62kΩ
3.266
The nearest 1% value for R_NOM is 8.66kΩ. This is the
value used to bias the NTC thermistor to get cold and hot
trip points of approximately 0°C and 40°C respectively.
To extend the delta between the cold and hot trip points, a
resistor, R1, can be added in series with R_NTC. The values
of the resistors are calculated as follows:
R_NOM
=
R_COLD – R_HOT
3.266 – 0.5325
R₁ = ^{⎛⎝⎜} _{3.2606.5−}^{302.55325⎞⎠⎟} • (R_COLD – R_HOT ) – R_HOT
where R_NOM is the value of the bias resistor, R_HOT and
R_COLD are the values of R_NTC at the desired temperature
trip points. Continuing the example from before with a
desired hot trip point of 50°C:
R_NOM
=
R_COLD – R_HOT
3.266 – 0.5325
=
10k • (2.816 – 0.4086)
3.266 – 0.5325
= 8.8kΩ, 8.87k is the nearest 1%value.
R₁ = 10k • ^{⎛⎝⎜} _{3.2606.5–302.55325}^{⎞⎠⎟}
• (2.816 – 0.4086) – 0.4086
= 604Ω, 604 is the nearest 1%value.
The final solution is R_NOM = 8.87kΩ, R1 = 604Ω and
R_NTC = 10kΩ at 25°C.
NTC Trip Point Error
When a 1% resistor is used for R_HOT, the major error
in the 40°C trip point is determined by the tolerance of
the NTC thermistor. A typical 100kΩ NTC thermistor has
±10% tolerance. By looking up the temperature coef-
ficient of the thermistor at 40°C, the tolerance error can
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