ISL6297_07 Datasheet, PDF (7/16 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6297
Theory of Operation
The ISL6297 is based on the Intersil Patent-pending dual-
mode charging technology. This allows the ISL6297 to
function as a traditional linear charger when powered with a
voltage-source adapter. However, when powered with a
current-limited adapter, the charger minimizes the thermal
dissipation commonly seen in traditional linear chargers.
This dual-mode technology generates very low heat, which
enables the charger to be used in space-limited applications.
The ISL6297 charges a Lithium Ion battery using the
constant current (CC) and constant voltage (CV) profile
specified by battery cell manufacturers.
As a linear charger, the constant charge current I_REF is
programmable up to 1.5A with an external resistor. The
constant charge voltage V_CH is regulated by the ISL6297 at
4.2V with a 0.7% accuracy over the entire recommended
operating range.
The ISL6297 always preconditions the battery with 10% of
the programmed current at the beginning of a charge cycle,
until the battery voltage is verified to be above the minimum
fast charge voltage, V_MIN. This low-current preconditioning
charge mode is named trickle mode.
A thermal-foldback feature removes the thermal concern
typically seen in linear chargers. The charger reduces the
charge current automatically as the IC internal temperature
rises above +100°C to prevent further temperature rise. The
thermal-foldback feature guarantees safe operation when
the printed circuit board (PCB) is space limited for thermal
dissipation.
Figure 1 shows the typical charge curves in a traditional
linear charger powered with a constant-voltage adapter. The
power dissipation P_CH is given by the following equations:
P_CH = (V_IN-V_BAT) × I_CHARGE
(EQ. 1)
where I_CHARGE is the charge current. The maximum power
dissipation occurs at the beginning of the CC mode. The
maximum power the IC is capable of dissipating is
dependent on the thermal impedance of the printed-circuit
board (PCB). Figure 1 shows, with dotted lines, two cases in
which the charge currents are limited by the maximum power
dissipation capability due to the thermal foldback.
To take advantage of the low-heat feature of the ISL6297, a
current-limited AC/DC converter is required as the power
supply to the charger. The current-limited supply has the I-V
characteristics shown in Figure 2. The supply is a DC source
before the load current reaches the limited current I_LIM.
Once the current limit is reached, the supply current cannot
increase further; instead, the supply voltage falls. The
current-limited supply is a voltage source with an equivalent
output impedance or a current source, depending on its
operating region, as shown in Figure 2.
V_NL
C
V_FL
r_O
V_NL
r_O = _{(VNL - VFL)/ILIM}
B
I _LIM
Trickle
Mode
Constant Current Constant Voltage
Mode
Mode
Inhibit
V_IN
Input Voltage
V_CH
Battery Voltage
V_MIN
I_REF
I_REF/10
P₁
P₂
P₃
Charge Current
Power Dissipation
TIMEOUT
FIGURE 1. TYPICAL CHARGE CURVES USING A
CONSTANT-VOLTAGE ADAPTER
A
I_LIM
FIGURE 2. THE I-V CHARACTERISTICS OF THE CURRENT-
LIMITED AC/DC CONVERTER
In this mode of operation, the constant current is determined
by the current limit I_LIM of the supply during the constant-
current charge mode. To ensure dual-mode operation, the
current protection level set by the ISL6297 I_REF pin should
be higher than I_LIM. In the constant-voltage charge mode,
the battery voltage is regulated at 4.2V. When the battery
voltage is below the specified V_MIN voltage, the charger
preconditions the battery using trickle charge mode.
Figure 3 shows the typical waveforms in a charge cycle of the
dual mode operation. When the battery voltage is below V_MIN,
the trickle charge mode is in effect. Since the charge current is
much less than the I_LIM, the AC/DC converter operates in the
voltage source region. Once the battery voltage exceeds
V_MIN, the charger fully turns on the internal P-channel power
MOSFET. The AC/DC converter operates in the current-
limitedregion and its voltage is pulled down to a level slightly
higher than the battery voltage.
7
FN9215.1
March 20, 2007