ISL6297 Datasheet, PDF (15/16 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6297
Hysteresis Temperature Calculation
Re-arranging EQ. 8, and including the effect of R_S, gives:
R_T = ₁-----^K_–-----K-- × R_U – R_S
(EQ. 18)
Using the K/(1-K) ratio at the hysteresis threshold, equation
18 provides the NTC thermistor resistance at the threshold.
Continuing the example above, the thermistor values are
found to be 26.2kΩ and 7.3kΩ respectively at the low and
high hysteresis temperatures. The corresponding
temperatures are found from Table 3. Cold recovery is about
1.0°C and the hot recovery is about 33.9°C. In other words,
the hysteresis temperatures for the low and high limits are
approximately 6.1°C and 11.0°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. 18, that is:
R_TMAX = ₁-----^K_–----^M-_K----_MA----^X-_A----_X-- × R_UMAX – R_SMIN
(EQ. 19)
From the Electrical Specification table, the maximum K at
cold is found to be 0.71. Assuming the resistors have 1%
accuracy, the maximum R_U is 15.15kΩ and the minimum R_S
is 40.8Ω. The resultant maximum R_T is then found to be
36.3kΩ and the corresponding temperature is about
negative 6.4°C. Hence the temperature tolerance is 1.3°C.
Similarly, at high temperature, the minimum K is .24, the
minimum R_U value is 14.85kΩ and the maximum R_S is
41.6Ω. Hence, the highest temperature is 46.0°C and the
tolerance is 1.1°C.
Charging Temperature Range
The selection of R_D follows the following equation:
R_D = R_T(@45^oC) – R_T(@45^oC + ∆T)
(EQ. 20)
where R_T(@45°C) is the thermistor resistance at 45°C and
the R_T (@45°C + ∆T) is the resistance at some desired
temperature difference above 45°C. Figure 13 shows the
temperature windows before, during, and after charging.
From the example, before and after charging, the
temperature window is -5°C to 45°C with 6°C and 4°C
hysteresis. During charging, the high temperature limit
changes to 45°C + ∆T. If this limit is exceeded, the charger is
stopped and the temperature has to come back to below
41°C for the charging to be allowed again. The low
temperature limit is also increased. However, the R_D
typically has a much lower resistance than the NTC at low
temperature, therefore, the influence on the temperature
threshold is not as much as at high temperature. Typically,
the low temperature threshold is raised by less than 2°C, as
shown in Figure 13.
45°C+∆T
44.7°C
32.0°C
0.6°C
-5.1°C
Less than
2°C
FIGURE 13. BOARD TEMPERATURE MONITORING. WHEN
NOT CHARGING, THE TEMPERATURE WINDOW
IS BETWEEN -5°C AND 45°C. ONCE THE
CHARGER STARTS, THE TEMPERATURE
WINDOW IS ~-3°C TO 45°C+ ∆T
2.9V Bias Voltage
A pre-regulator provides a regulated 2.9V on the V2P9 pin,
unless V_IN drops below 2.9V plus 250mV (typical). Then the
output voltage tracks the input voltage with a 250mV dropout
voltage. The 2.9V output turns off when V_IN drops below the
VIN (falling) threshold. A minimum 0.1µF X5R ceramic
capacitor is required for decoupling the pre-regulator.
The V2P9 output is used for biasing external circuits. The
maximum loading current on this pin is 30mA. Mainly, the
load current comes from the indication LEDs.
Board Layout Recommendations
The ISL6297 is targeted for space-limited applications. 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 pads for the 4x4
QFN package are able to have 5 vias. As much copper as
possible should be connected to the exposed pad to
minimize the thermal impedance. Refer to the ISL6297
evaluation board for layout examples.
Related Literature
• Technical Brief TB363 “Guidelines for Handling and
Processing Moisture Sensitive Surface Mount Devices
(SMDs)”
• Technical Brief TB379 “Thermal Characterization of
Packaged Semiconductor Devices”
• Technical Brief TB389 “PCB Land Pattern Design and
Surface Mount Guidelines for QFN Packages”
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FN9215.0
September 27, 2005