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參數(shù)資料
| 型號： | TC3405A |
| 廠商： | Linear Technology Corporation |
| 元件分類： | 熱敏電阻 |
| 英文描述： | Standalone Linear Li-lon Battery Charger with Thermistor Input |
| 中文描述： | 獨(dú)立線性鋰離子電池充電器熱敏電阻輸入 |
| 文件頁數(shù)： | 16/20頁 |
| 文件大小： | 665K |
| 代理商： | TC3405A |
LTC4061
16
4061fa
APPLICATIOU
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.
W
U
U
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 10k
Ω
.
Vishay Dale has a number of values of thermistor from
10k
Ω
to 100k
Ω
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
R
3 266
.
R
at
C
NOM
COLD
NTC
=
°
25
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
R
0 5325
.
R
at
C
NOM
HOT
NTC
=
°
25
where R
HOT
is the resistance ratio of R
NTC
at the desired
hot temperature trip point.
Here is an example using 10k
Ω
R-T Curve 2 thermistor
from Vishay Dale. The difference between the trip points
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
R
3.
2 816
3 266
.
R
at
C
k
k
NOM
COLD
.
NTC
=
°
=
=
25
10
8 62
.
The nearest 1% value for R
NOM
is 8.66k
Ω
. 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
R
3 266 0 5325
0 5325
3 266 0 5325
R
R
R
R
R
NOM
COLD
HOT
COLD
HOT
HOT
=
=
–
.
– .
.
.
.
(
–
)–
1
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
R
NOM
=
=
.
– .
.
,8 87
8 8
R
k
k
k is the nearest
value
COLD
3 266 0 5325
HOT
=
–
2 816 0 4086
.
3 266 0 5325
1
– .
)
– .
%
.
10
The final solution is R
NOM
= 8.87k
Ω
, R1 = 604
Ω
and
R
NTC
= 10k
Ω
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 100k
Ω
NTC thermistor has
±10% tolerance. By looking up the temperature coef-
ficient of the thermistor at 40°C, the tolerance error can
R
k
is the nearest
value
1
10
0 5325
.
– .
– .
3 266 0 5325
604
,
1
%
604
=
=
.
2 816 0 4086
)–0 4086
.
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