ISL6292_06 Datasheet, PDF (15/20 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6292
Shutdown
The ISL6292 can be shutdown by pulling the EN pin to
ground. When shut down, the charger draws typically less
than 30μA current from the input power and the 2.8V output
at the V2P8 pin is also turned off. The EN pin needs be
driven with an open-drain or open-collector logic output, so
that the EN pin is floating when the charger is enabled.
TABLE 2. STATUS INDICATIONS
FAULT STATUS
INDICATION
High High
Charge completed with no fault (Inhibit) or
Standby
High Low
Charging in one of the three modes
Low High
Fault
*Both outputs are pulled up with external resistors.
Input and Output Capacitor Selection
Typically any type of capacitors can be used for the input
and the output. Use of a 0.47μF or higher value ceramic
capacitor for the input is recommended. When the battery is
attached to the charger, the output capacitor can be any
ceramic type with the value higher than 0.1μF. However, if
there is a chance the charger will be used as an LDO linear
regulator, a 10μF tantalum capacitor is recommended.
Current-Limited Adapter
Figure 24 shows the ideal current-voltage characteristics of
a current-limited adapter. V_NL is the no-load adapter output
voltage and V_FL is the full load voltage at the current limit
I_LIM. Before its output current reaches the limit I_LIM, the
adapter presents the characteristics of a voltage source. The
slope r_O represents the output resistance of the voltage
supply. For a well regulated supply, the output resistance
can be very small, but some adapters naturally have a
certain amount of output resistance.
The adapter is equivalent to a current source when running
in the constant-current region. Being a current source, its
output voltage is dependent on the load, which, in this case,
is the charger and the battery. As the battery is being
charged, the adapter output rises from a lower voltage in the
current-voltage characteristics curve, such as point A, to
higher voltage until reaching the breaking point B, as shown
in Figure 24.
The adapter is equivalent to a voltage source with output
resistance when running in the constant-voltage region;
because of this characteristic. As the charge current drops,
the adapter output moves from point B to point C, shown in
Figure 24.
The battery pack can be approximated as an ideal cell with a
lumped-sum resistance in series, also shown in Figure 24.
The ISL6292 charger sits between the adapter and the
battery.
V_NL
C
V_FL
r_O
V_NL
r_O = (V_NL - V_FL)/I_LIM
B
I_LIM
V_CELL
A
V_PACK
R_PACK
I_LIM
FIGURE 24. THE IDEAL I-V CHARACTERISTICS OF A
CURRENT LIMITED ADAPTER
Working with Current-Limited Adapter
As described earlier, the ISL6292 minimizes the thermal
dissipation when running off a current-limited AC adapter, as
shown in Figure 18. The thermal dissipation can be further
reduced when the adapter is properly designed. The
following demonstrates that the thermal dissipation can be
minimized if the adapter output reaches the full-load output
voltage (point B in Figure 24) before the battery pack voltage
reaches the final charge voltage (4.1V or 4.2V). The
assumptions for the following discussion are: the adapter
current limit = 750mA, the battery pack equivalent
resistance = 200mΩ, and the charger ON resistance is
350mΩ.
Adapter
Charger
V_ADAPTER ^RDS(ON)
I_LIM
I
V_CELL
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
(A) THE EQUIVALENT CIRCUIT IN THE
CONSTANT CURRENT REGION
(B) THE EQUIVALENT CIRCUIT IN THE
RESISTANCE-LIMIT REGION
(C) THE EQUIVALENT CIRCUIT WHEN THE
PACK VOLTAGE REACHES THE FINAL
CHARGE VOLTAGE
FIGURE 25. THE EQUIVALENT CIRCUIT OF THE CHARGING SYSTEM WORKING WITH CURRENT LIMITED ADAPTERS
15
FN9105.8
November 14, 2006