ISL6292_06 Datasheet, PDF (16/20 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6292
When charging in the constant-current region, the pass
element in the charger is fully turned on. The charger is
equivalent to the on-resistance of the internal P-channel
MOSFET. The entire charging system is equivalent to the
circuit shown in Figure 25(A). The charge current is the
constant current limit I_LIM, and the adapter output voltage
can be easily found out as,
V_Adapter= I_LIM ⋅ r_DS(ON) + V_PACK
(EQ. 13)
where V_PACK is the battery pack voltage. The power
dissipation in the charger is given in Equation 2, where
I_CHARGE = I_LIM.
A critical condition of the adapter design is that the adapter
output reaches point B in Figure 24 at the same time as the
battery pack voltage reaches the final charge voltage (4.1V
or 4.2V), that is:
V_Critical= I_LIM ⋅ r_DS(ON) + V_CH
(EQ. 14)
For example, if the final charge voltage is 4.2V, the r_DS(ON)
is 350mΩ, and the current limit I_LIM is 750mA, the critical
adapter full-load voltage is 4.4625V.
When the above condition is true, the charger enters the
constant-voltage mode simultaneously as the adapter exits
the current-limit mode. The equivalent charging system is
shown in Figure 25(C). Since the charge current drops at a
higher rate in the constant-voltage mode than the increase
rate of the adapter voltage, the power dissipation decreases
as the charge current decreases. Therefore, the worst case
thermal dissipation occurs in the constant-current charge
mode. Figure 26 (A) shows the I-V curves of the adapter
output, the battery pack voltage and the cell voltage during
the charge. The 5.9V no-load voltage is just an example
value higher than the full-load voltage. The cell voltage
4.05V uses the assumption that the pack resistance is
200mΩ. Figure 27 (A) 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 4.2V (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 27 (C). 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 25(B). 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 25(C) becomes the
equivalent circuit until charging ends. In this case, the worst-
case thermal dissipation also occurs in the constant-current
charge mode. Figure 26(B) 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
27(B) 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 above 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. Figure 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
(A)
V_ADAPTER
V_PACK
V_CELL
4.4625V
4.2V
4.05V
V_NL
4.2V
(B)
0.75A
V_ADAPTER
V_PACK
V_CELL
4.2V
4.0V
3.775V
3.625V
0.55A
0.75A
FIGURE 26. THE I-V CHARACTERISTICS OF THE CHARGER
WITH DIFFERENT CURRENT LIMITED
ADAPTERS
16
FN9105.8
November 14, 2006