ISL78692 Datasheet, PDF (15/18 Pages) Intersil Corporation – Li-ion/Li-Polymer Battery Charger

ISL78692
4.05V uses the assumption that the pack resistance is 200mΩ.
Figure 28A illustrates the adapter voltage, battery pack voltage,
the charge current and the power dissipation in the charger
respectively in the time domain.
If the battery pack voltage reaches 4.1V before the adapter
reaches point B in Figure 26, a voltage step is expected at the
adapter output when the pack voltage reaches the final charge
voltage. As a result, the charger power dissipation is also
expected to have a step rise. This case is shown in Figure 19 as
well as Figure 29C. Under this condition, the worst case thermal
dissipation in the charger happens when the charger enters the
constant voltage mode.
If the adapter voltage reaches the full-load voltage before the
pack voltage reaches 4.1V, the charger will experience the
resistance-limit situation. In this situation, the ON-resistance of
the charger is in series with the adapter output resistance. The
equivalent circuit for the resistance-limit region is shown in
Figure 27B. Eventually, the battery pack voltage will reach 4.1V
because the adapter no-load voltage is higher than 4.1V, then
Figure 27C becomes the equivalent circuit until charging ends. In
this case, the worst-case thermal dissipation also occurs in the
constant current charge mode. Figure 28B shows the I-V curves of
the adapter output, the battery pack voltage and the cell voltage
for the case VFL = 4V. In the case, the full-load voltage is lower
than the final charge voltage (4.1V), but the charger is still able to
fully charge the battery as long as the no-load voltage is above
4.1V. Figure 28B illustrates the adapter voltage, battery pack
voltage, the charge current and the power dissipation in the
charger respectively in the time domain.
Based on the previous discussion, the worst-case power
dissipation occurs during the constant current charge mode if the
adapter full-load voltage is lower than the critical voltage given in
Equation 13. Even if that is not true, the power dissipation is still
much less than the power dissipation in the traditional linear
charger. Figures 26 and 27 are scope-captured waveforms to
demonstrate the operation with a current-limited adapter.
The waveforms in Figure 26 are the adapter output voltage
(1V/div), the battery voltage (1V/div), and the charge current
(200mA/div) respectively. The time scale is 1ks/div. The adapter
current is limited to 600mA and the charge current is
programmed to 1A. Note that the voltage difference is only
approximately 200mV and the adapter voltage tracks the battery
voltage in the CC mode. Figure 26 also shows the resistance limit
mode before entering the CV mode.
Figure 27 shows the actual captured waveforms depicted in
Figure 29C. The constant charge current is 750mA. A step in the
adapter voltage during the transition from CC mode to CV mode
is demonstrated.
ADAPTER
CHARGER
VADAPTER ^rDS(ON)
ILIM
I
VCELL
VPACK
RPACK
BATTERY
PACK
ADAPTER
RO
CHARGER
VADAPTER ^rDS(ON)
VNL
I
VCELL
VPACK
RPACK
BATTERY
PACK
ADAPTER
RO
VNL
CHARGER
VADAPTER
4.1V DC
OUTPUT
I
VCELL
VPACK
RPACK
BATTERY
PACK
FIGURE 27A. THE EQUIVALENT CIRCUIT IN THE FIGURE 27B. THE EQUIVALENT CIRCUIT IN THE FIGURE 27C. THE EQUIVALENT CIRCUIT WHEN
CONSTANT CURRENT REGION
RESISTANCE-LIMIT REGION
THE PACK VOLTAGE REACHES
THE FINAL CHARGE VOLTAGE
FIGURE 27. THE EQUIVALENT CIRCUIT OF THE CHARGING SYSTEM WORKING WITH CURRENT LIMITED ADAPTERS
5.9V
4.1V
V_ADAPTER
V_PACK
VCELL
4.275V
4.1V
3.9V
VNL
4.1V
V_ADAPTER
V_CELL
VPACK
4.1V
4.0V
3.775V
3.625V
500mA
500mA
FIGURE 28A.
FIGURE 28B.
FIGURE 28. THE I-V CHARACTERISTICS OF THE CHARGER WITH DIFFERENT CURRENT LIMITED POWER SUPPLIES
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FN8692.0
September 10, 2014