ISL6297 Intersil Corporation, ISL6297 Datasheet - Page 14

• Manufacturer Part Number: ISL6297
• Description: Li-ion/Li Polymer Battery Charger
• Manufacturer: Intersil Corporation

The ratio, K, of the TEMP pin voltage to the bias voltage is:
and
At V
Similarly, at V
Using these equations to calculate the ratio of the thermistor
hot to cold resistance results in:
and
where R
resistance values at the cold and hot temperature limits
respectively.
It is usually difficult to find an NTC thermistor that has the
exact ratio given in EQ. 12. A thermistor with a ratio larger
than 6.84, that is:
can be used in series with a regular resistor to form an
effective thermistor that has the right ratio, as shown in
Figure 12.
FIGURE 12. EFFECTIVE NTC THERMISTOR CIRCUIT
R
R
-----------------------
TMIN
R
K
R
-----------------------
K
T
K
R
R
TCOLD
THOT
TCOLD
U
=
THOT
=
=
=
TCOLD
=
,
---------- -
2.88
------------ -
1 K
ISL6297
--------------------- -
R
2.0
0.718
-------------- -
2.88
K
0.44R
–
T
R
≥
+
TMAX
T
=
=
6.84
R
×
=
and R
U
R
-----------------------
0.332R
0.6844
2.27R
TCOLD
U
0.2493
.
·
THOT
U
·
V2P8
TEMP
GND
U
=
6.84
are the NTC thermistor
14
R
R
R
T
U
S
(EQ. 10)
(EQ. 11)
(EQ. 12)
(EQ. 13)
(EQ. 14)
(EQ. 8)
(EQ. 9)
ISL6297
With the series resistor R
Once the thermistor and the temperature limits are selected,
R
and
To summarize, the NTC thermistor circuit design requires
three steps:
Example:
The charger is designed to charge the battery with the
temperature range from -5°C to +45°C. The 10kΩ NTC
thermistor NCP15XH103F03RC from Murata
(http://www.murata.com) satisfies EQ. 14. The resistance
values for this thermistor are given in Table 3. The typical
resistance at -5°C and +45°C are:
Using EQ. 16 and EQ. 17 result in:
R
Selecting R
temperature threshold of -5.1°C and a high temperature
threshold of +44.9°C.
TEMP (°C)
1. Find an NTC thermistor that satisfies EQ. 14. The
2. Calculate the series resistance according to EQ. 16.
3. Calculate the pull-up resistance according to EQ. 17.
S
S
R
R
and R
= 41.6Ω and R
temperature limits are determined by the application
requirement.
TABLE 3. RESISTANCE TABLE OF NCP15XH103F03RC
-10
TCOLD
THOT
40
45
50
55
-5
0
6
7
R
------------------------------------- -
R
R
R
S
S
U
S
+
=
+
=
U
R
= 4.9169kΩ.
R
R
------------------------------------------------------------- -
S
TCOLD
can be calculated using:
0.44
= 33.8922kΩ and
THOT
TCOLD
= 41.2Ω and R
R-LOW (kΩ)
41.4765
33.1462
26.6780
20.7560
19.9227
×
5.7443
4.8333
4.0833
3.4634
(
U
R
=
–
5.84
S
= 14.931kΩ.
6.84R
6.84
+
R
TCOLD
S
, EQ. 12 can be re-written as:
THOT
R-CENTER (kΩ)
U
= 15kΩ gives a low
)
42.5062
33.8922
27.2186
21.1230
20.2666
5.8336
4.9169
4.1609
3.5350
R-HIGH (kΩ)
43.5570
34.6515
27.7675
21.4944
20.6143
5.9238
5.0015
4.2395
3.6076
March 20, 2007
(EQ. 15)
(EQ. 16)
(EQ. 17)
FN9215.1