ISL6298_06 Datasheet, PDF (15/17 Pages) Intersil Corporation – Li-ion/Li-Polymer Battery Charger

ISL6298
0°C and 55°C are R_COLD = 27.2186kΩ and R_HOT = 3.535kΩ
respectively. Using EQ. 11 and EQ. 12 result in R_S = 360Ω
and R_U = 27.9kΩ.
Hysteresis Temperature Calculation
Using EQ. 6 is re-arranged as:
R_T
=
-----^K-------
1–K
⋅
R_U
–
R_S
(EQ. 13)
Substituting the ratio at the hysteresis threshold results in
the NTC thermistor resistance at the threshold. Continuing
the example above, the thermistor values are found to be
20.64kΩ and 4.37kΩ respectively at the low and high
hysteresis temperatures. The corresponding temperatures
are found from the Table 3 to be 7°C and 49°C respectively.
In other words, the hysteresis temperatures for the low and
high temperature limits are approximately 7°C and 6°C
respectively.
Temperature Tolerance Calculation
The temperature accuracy is affected by the accuracy of the
thresholds, R_S, R_U, and the NTC thermistor. Using the
maximum ratio K, maximum possible R_U, and minimum R_S
results in the maximum value of R_T from EQ. 13, that is:
R_{T, MAX}
=
-----^K----^M-----^A----^X-------
1 – K_MAX
⋅
R_U,
M
AX
–
R_S,
MIN
(EQ. 14)
From the Electrical Specification table, the maximum K is
found to be 52.3%. Assuming the resistors have 1%
accuracy, the maximum R_U is 28.2kΩ and the minimum R_S
is 356Ω. The resultant maximum R_T is then found to be
30.6kΩ and the corresponding temperature is -3°C. Hence
the temperature tolerance is 3°C. Similarly, the high
temperature maximum thermistor value is 3.98kΩ. Hence,
the lowest temperature is 51°C and the tolerance is 4°C.
TABLE 3. RESISTANCE TABLE OF NCP15XH103F03RC
TEMP (°C) R-Low (kΩ) R-Center (kΩ) R-High (kΩ)
-3
30.3641
31.0200
31.6869
0
26.6780
27.2186
27.7675
6
20.7560
21.1230
21.4944
7
19.9227
20.2666
20.6143
50
4.0833
4.1609
4.2395
51
4.9498
4.0262
4.1036
55
3.4634
3.5350
3.6076
Working with Current-Limited Adapter
The ISL6298 minimizes the thermal dissipation when
powered by a current-limited ac adapter. The thermal
dissipation can be further reduced when the adapter is
properly designed. For more information regarding working
with current-limited adapters, please refer to the ISL6292
datasheet available at http://www.intersil.com.
Board Layout Recommendations
The ISL6298 internal thermal foldback function limits the
charge current when the internal temperature reaches
approximately 100°C. In order to maximize the current
capability, it is very important that the exposed pad under the
package is properly soldered to the board and is connected
to other layers through thermal vias. More thermal vias and
more copper attached to the exposed pad usually result in
better thermal performance. On the other hand, the number
of vias is limited by the size of the pad. The exposed pad for
the 4x4 QFN package is able to have 5 vias. The 3x3 DFN
package allows 8 vias be placed in two rows. Since the pins
on the 3x3 DFN package are on only two sides, as much top
layer copper as possible should be connected to the
exposed pad to minimize the thermal impedance. Refer to
the ISL6298 evaluation boards for layout examples.
15
FN9173.4
March 9, 2006