ISL6253 Datasheet, PDF (13/22 Pages) Intersil Corporation – Highly Integrated Battery Charger for Notebook Computers

ISL6253
Theory of Operation
Introduction
The ISL6253 includes all of the functions necessary to
charge 2 to 4 cell Li-Ion and Li-polymer batteries. A high
efficiency synchronous buck converter is used to control the
charging voltage and charging current up to 10 amps. The
ISL6253 has input current limiting and analog inputs for
setting the charge current and charge voltage; CHLIM inputs
are used to control charge current and VADJ inputs are used
to control charge voltage.
The ISL6253 safely conditions over-discharged battery cells
with a percentage of full charge current until the battery
voltage exceeds 3.1V × number of series connected cells.
When the battery voltage exceeds 3.1V × number of series
connected cells, the ISL6253 charges the battery with
constant charge current, set by CHLIM input, until the battery
voltage rises to a programmed charge voltage set by VADJ
input; then the charger begins to operate at constant voltage
charge mode. The charger drives an adapter isolation
p-channel MOSFET to efficiently switch in the adapter
supply.
ISL6253 is a complete power source selection controller for
single battery systems and also aircraft power applications.
ISL6253 drives a battery selector p-channel MOSFET to
efficiently select between a single battery and the adapter. It
controls the battery discharging MOSFET and switches to
the battery when the AC adapter is removed, or, switches to
the AC adapter when the AC adapter is inserted for a single
battery system. The EN input allows shutdown of the charger
from a micro-controller. The amount of adapter current is
reported on the ICM output. Figure 16 shows the IC
functional block diagram.
The synchronous buck converter uses external N-channel
MOSFETs to convert the input voltage to the required
charging current and charging voltage. Figure 17 shows the
ISL6253 typical application circuit 1 without power source
selection function. The typical application circuit 2 shown in
Figure 18 has automatic power source selection functionality
and supports aircraft power applications. The voltage at
CHLIM and the value of R1 sets the charging current. The
DC-DC converter generates the control signals to drive two
external N-channel MOSFETs to regulate the voltage and
current set by the ACLIM, CHLIM, VADJ and CELLS inputs.
The ISL6253 features a voltage regulation loop (VCOMP)
and two current regulation loops (ICOMP). The VCOMP
voltage regulation loop monitors CSON to ensure that its
voltage never exceeds the voltage set by VADJ. The ICOMP
current regulation loops regulate the battery charging current
delivered to the battery to ensure that it never exceeds the
charging current limit set by CHLIM; and the ICOMP current
regulation loops regulate the input current drawn from the
AC adapter to ensure that it never exceeds the input current
limit set by ACLIM, and to prevent a system crash and AC
adapter overload.
PWM Control
The ISL6253 employs a fixed frequency PWM current mode
control architecture with a feed forward function. The feed-
forward function maintains a constant modulator gain of 11
to achieve fast line regulation as the buck input voltage
changes. When the battery charge voltage approaches the
input voltage, the DC-DC converter operates in dropout
mode, where there is a timer to prevent the frequency from
dropping into the audible frequency range. It can achieve a
maximum duty cycle of up to 99.6%.
An adaptive gate drive scheme is used to control the dead
time between two switches. The dead time control circuit
monitors the LGATE output and prevents the upper side
MOSFET from turning on until LGATE is fully off, preventing
cross-conduction and shoot-through. In order for the dead
time circuit to work properly, there must be a low resistance,
low inductance path from the LGATE driver to MOSFET
gate, and from the source of MOSFET to PGND. The
external Schottky diode is between the VDDP pin and BOOT
pin to keep the bootstrap capacitor charged.
The PWM controller is disabled when EN = GND, but the rest
of the circuitry, including the AC or DC adapter detecting
circuit and AC adapter current monitoring circuits, is still alive.
Setting the Battery Regulation Voltage
The ISL6253 uses a high-accuracy trimmed band-gap
voltage reference to regulate the battery charging voltage.
The VADJ input adjusts the charger output voltage, and the
VADJ control voltage can vary from 0 to VREF (2.39V),
providing a 10% adjustment range (from 4.2V -5% to
4.2V +5%) on CSON regulation voltage. An overall voltage
accuracy of better than 0.5% is achieved.
The per-cell battery termination voltage is a function of the
battery chemistry. Consult the battery manufacturers to
determine this voltage.
Float VADJ to set the battery voltage V_CSON = 4.2V ×
number of the cells,
• Connect VADJ to VREF to set 4.41V × number of cells,
• Connect VADJ to ground to set 3.99V × number of cells.
Note that other battery charge voltages can be set by
connecting a resistor divider from VREF to ground. The
resistor divider should be sized to draw no more than 100µA
from VREF; or connect a low impedance voltage source like
the D/A converter in the micro-controller. The programmed
battery voltage per cell can be determined by the following
equation:
V_CELL= 0.175V_VADJ + 3.99V
(EQ. 1)
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