ISL6292_07 Datasheet, PDF (16/20 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

Adapter
Charger
V_ADAPTER ^RDS(ON)
I_LIM
I
V_CELL
ISL6292
V_PACK
R_PACK
Battery
Pack
Adapter
r_O
Charger
V_ADAPTER ^RDS(ON)
V_NL
I
V_CELL
V_PACK
R_PACK
Battery
Pack
Adapter
r_O
V_NL
Charger
V_{ADAPTER 4.2V DC}
Output
I
V_CELL
V_PACK
R_PACK
Battery
Pack
FIGURE 25A. THE EQUIVALENT CIRCUIT IN FIGURE 25B. THE EQUIVALENT CIRCUIT IN FIGURE 25C. THE EQUIVALENT CIRCUIT WHEN
THE CONSTANT CURRENT
THE RESISTANCE-LIMIT
THE PACK VOLTAGE REACHES
REGION
REGION
THE FINAL CHARGE VOLTAGE
FIGURE 25. THE EQUIVALENT CIRCUIT OF THE CHARGING SYSTEM WORKING WITH CURRENT LIMITED ADAPTERS
If the battery pack voltage reaches 4.2V (or 4.1V) before the
adapter reaches point B in Figure 24, 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 18 as well as Figure 27C. 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 4.2V (or 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 25B. Eventually, the battery pack
voltage will reach 4.2V (or 4.1V) because the adapter no-load
voltage is higher than 4.2V (or 4.1V), then Figure 25C becomes
the equivalent circuit until charging ends. In this case, the
worst-case thermal dissipation also occurs in the constant-
current charge mode. Figure 26B shows the I-V curves of the
adapter output, the battery pack voltage and the cell voltage for
the case V_FL = 4V. In the case, the full-load voltage is lower
than the final charge voltage (4.2V), but the charger is still able
to fully charge the battery as long as the no-load voltage is
above 4.2V. Figure 26B 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 14. Even if that is not true, the
power dissipation is still much less than the power
dissipation in the traditional linear charger. Figures 28 and
29 are scope-captured waveforms to demonstrate the
operation with a current-limited adapter.
The waveforms in Figure 28 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 28 also shows the
resistance-limit mode before entering the CV mode.
5.9V
4.2V
V_ADAPTER
V_PACK
V_CELL
4.4625V
4.2V
4.05V
V_NL
4.2V
FIGURE 26A.
0.75A
V_ADAPTER
V_PACK
V_CELL
4.2V
4.0V
3.775V
3.625V
0.55A
0.75A
FIGURE 26B.
FIGURE 26. THE I-V CHARACTERISTICS OF THE CHARGER
WITH DIFFERENT CURRENT LIMITED ADAPTERS
Figure 29 shows the actual captured waveforms depicted in
Figure 27C. The constant charge current is 750mA. A step in
the adapter voltage during the transition from CC mode to
CV mode is demonstrated.
16
FN9105.9
December 17, 2007