ISL78692 Datasheet, PDF (14/18 Pages) Intersil Corporation – Li-ion/Li-Polymer Battery Charger

ISL78692
Input and Output Capacitor Selection
The use of a 10µF Tantalum type TCA106M016R0200 or
Ceramic type C3216X7RC1106KT000N or equivalent is
recommended for the input. When used as a charger, the output
capacitor should be 2x10µF Tantalum type AVX
TCJA106M016R0200 or equivalent. The device partially relies on
the ESR (equivalent series resistance) of the output capacitor for
the loop stability. If there is a need to use ceramic capacitors for
device output, it is recommended to use a 220mΩ, 0.25W
resistor, in series with the VBAT pin followed by 2x10µF, 16V, X7R
ceramic capacitor C3216X7RC1106KT000N or equivalent for an
I_BAT = 0.5A (see Figure 25).
TO INPUT
ISL78692
VIN
VBAT
C1
10µF
Ceramic
GND
220m, 0.25W _{TO BATTERY}
R1
C₂
LARGE
CERAMIC
CAPACITOR
FIGURE 25. INSERTING R₁ TO IMPROVE THE STABILITY OF
APPLICATIONS WITH LARGE CERAMIC CAPACITOR
USED AT THE OUTPUT
Current-Limited Adapter
Figure 26 shows the ideal current voltage characteristics of a
current-limited adapter. The 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 26.
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 26.
The battery pack can be approximated as an ideal cell with a
lumped-sum resistance in series, also shown in Figure 26. The
ISL78692 charger sits between the adapter and the battery.
VNL
VFL
C
RO
VNL
RO = (VNL - VFL )/ILIM
B
ILIM
VCELL
VPACK
RPACK
A
ILIM
FIGURE 26. THE IDEAL I-V CHARACTERISTICS OF A CURRENT
LIMITED POWER SUPPLY
Working with Current-Limited Power Supply
As described earlier, the ISL78692 minimizes the thermal
dissipation when running off a current-limited AC adapter, as
shown in Figure 19. 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 26) before the battery pack voltage reaches the final
charge voltage (4.1V). The assumptions for the following
discussion are: the adapter current limit = 500mA, the battery
pack equivalent resistance = 200mΩ, and the charger
ON-resistance is 350mΩ.
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 27A.
The charge current is the constant current limit I_LIM, and the
adapter output voltage can be easily found out as calculated in
Equation 12:
V_Adapter= I_LIM  r_{DSON}V_PACK
(EQ. 12)
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 26 at the same time as the
battery pack voltage reaches the final charge voltage (4.1V), that
is given by Equation 13:
V_Critical= I_LIM  r_DSON + V_CH
(EQ. 13)
For example, if the final charge voltage is 4.1V, the r_DS(ON) is
350mΩ, and the current limit I_LIM is 500mA, the critical adapter
full-load voltage is 4.275V.
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 27C. 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 27A 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
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FN8692.0
September 10, 2014