ltc4097 Linear Technology Corporation, ltc4097 Datasheet - Page 17

• Manufacturer Part Number: ltc4097
• Description: Usb/wall Adapter Standalone Li-ion/polymer Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
and the cold trip point is set when:
Solving these equations for R
in the following:
and
By setting R
in r
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
By using a bias resistor, R
the hot and cold trip points can be moved in either direc-
tion. The temperature span will change somewhat due to
the non-linear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R
R
R
R
R
HOT
NTC|COLD
NTC|COLD
NOM
NOM
NOM
= 0.536 and r
R
NTC COLD
=
=
+
Figure 4. Typical NTC Thermistor Circuit
R
100k
R
100k
NOM
R
r
VNTC
0 536
NTC
NOM
NTC
COLD
3 25
|
r
3
6
NTC COLD
HOT
.
= 0.536 • R
= 3.25 • R
.
|
equal to R25, the above equations result
•
0.349 • VNTC
•
0.765 • VNTC
R
R
COLD
25
25
0.1V
•
NOM
VNTC
NOM
NOM
= 3.25. Referencing these ratios
NTC|COLD
NTC BLOCK
–
+
–
+
+
–
, different in value from R25,
= 0 765
.
and R
TOO_COLD
TOO_HOT
NTC_ENABLE
•
VNTC
NTC|HOT
4097 F04
results
where r
desired hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points can
be chosen, the other is determined by the default ratios
designed in the IC. Consider an example where a 60°C
hot trip point is desired.
From the Vishay Curve 1 R-T characteristics, r
at 60°C. Using the above equation, R
to 46.4k. With this value of R
about 16°C. Notice that the span is now 44°C rather than
the previous 40°C. This is due to the decrease in “tem-
perature gain” of the thermistor as absolute temperature
increases.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 5. The following formulas can be used
to compute the values of R
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose
Figure 5. NTC Thermistor Circuit with Additional Bias Resistor
R
R
R
1 0 536
NOM
NOM
=
HOT
=
.
=
r
3 266 0 4368
COLD
R
105k
R1
12.7k
and r
R
100k
.
VNTC
NOM
NTC
NTC
3
6
•
2 714
R
.
2 714
NOM
–
.
COLD
– .
r
HOT
0.349 • VNTC
0.765 • VNTC
–
are the resistance ratios at the
r
HOT
•
0.1V
R
NOM
25
•
•
100
NOM
R R 25
NTC BLOCK
–
+
–
+
+
–
and R1:
k
, the cold trip point is
=
104 2
NOM
LTC4097
.
TOO_COLD
TOO_HOT
NTC_ENABLE
should be set
k
HOT
4097 F05
is 0.2488
17
4097f