11
FN6775.0
December 8, 2008
Theory of Operation
Introduction
The ISL9518 differs from the ISL9518A only in the default
states of the internal registers at power-up. ISL9518 defaults
are for systems with an 8.4V (2-cell) battery and ISL9518A
defaults are for systems with a 12.6V battery (3-cell). Unless
otherwise noted, all specifications and descriptions of
ISL9518 refer to both the ISL9518 and ISL9518A.
A high efficiency synchronous buck converter is used to
control the system voltage up to 19.2V and charging current
up to 8A. The ISL9518 also has input current limiting up to
8.064A (or higher with lower values of sense resistor). The
Input current limit, charge current limit, minimum and
maximum system voltage are set by internal registers written
with SMBus. The ISL9518 “Typical Application Circuit” is
shown in Figure 2.
The ISL9518 charges the battery with constant charge
current, set by the ChargeCurrent register, until the battery
voltage rises to a voltage set by the MaxSystemVoltage
register. The charger will then operate at a constant voltage.
The adapter current is monitored and if the adapter current
rises to the limit set by the InputCurrent register, system
voltage and battery charge current are reduced to limit
adapter current. If battery voltage is below the min system
voltage, the trickle charge system is activated.
The ISL9518 features two voltage regulation loops and two
current regulation loops. The max system voltage loop
controls the voltage at CSON with a precision voltage divider
to the voltage error amplifier GM2. The min system voltage
prevents the system voltage from dropping below a minimum
value even if a deeply discharged battery is inserted that is
below the minimum. The Charge Current regulation loop
limits the battery charging current delivered to the battery to
ensure that it never exceeds the current set by the
ChargeCurrent register. The Input Current regulation loop
limits the current drawn from the AC-adapter to ensure that it
never exceeds the limit set by the InputCurrent register to
prevent adapter overload.
PWM Control
The ISL9518 employs a fixed frequency pulse width
modulator (PWM) with feed forward. The switching
frequency can be reduced with an SMBus command for
improved light load efficiency
AC-adapter Detection
AC-adapter voltage is connected through a resistor divider to
ADET to detect when AC power is available, as shown in
Figure 2. ADPR is an open-drain output and is active low
when ADET is less than V
th,fall
, and high Z when ADET is
above V
th,rise
. The ADET rising threshold is 3.2V (typ) with
57mV hysteresis. ADET must be above the threshold to
Enable the output voltage.
Current Measurement
ACMON is an output voltage that is proportional to the
adapter current being sensed across CSIP and CSIN. The
output voltage range is 0.1V to 3.2V. The voltage of ACMON
is given by Equation 1:
where I
INPUT
is the DC current drawn from the AC-adapter.
A capacitor is required at the ACMON output to stabilize the
ACMON amplifier and to minimize switching noise.
VDD Regulator
VDD provides a 5.1V supply voltage from the internal LDO
regulator from DCIN and can deliver up to 30mA of
continuous current. VDD also supplies power to VDDP
through a low pass filter as shown in the “Typical Application
Circuit” in Figure 2. The MOSFET drivers are powered by
VDDP. Bypass VDDP and VDD with a 1µF capacitor.
VSMB Supply
The VSMB input provides power to the SMBus interface.
Connect an external supply to VSMB to keep the SMBus
interface active while the supply to DCIN is removed. When
VSMB is biased, the internal registers are maintained. Bypass
VSMB to AGND with a 0.1µF or greater ceramic capacitor.
SGATE Function
If ADET > 3.2V and VDD > 4.5V and ISOLATE_ADAPTER bit
is 0 (default state) then SGATE will be ON (meaning SGATE
will be driven to ground turning on the inrush limit and the
adapter isolation FETs ON). In all other cases, SGATE is OFF
(meaning the chip will not pull-down SGATE and the off chip
resistor will pull the gates of the in-rush limit and adapter
isolation FETs to their sources, turning them OFF).
BGATE Function
The BGATE pin drives the gate of an external PFET to
control the minimum system voltage. If a battery is
connected that is discharged below the value set in the
MinSystemVoltage register, BGATE controls the system
voltage at the value set in the MinSystemVoltage register.
Trickle Charging
If a battery that is discharged below the value set in the
MinSystemVoltage register is connected to the system, the
trickle charge system is activated. In trickle charge mode,
the charge current is reduced to 256mA. The value in the
ChargeCurrent register is not changed. The BGATE FET is
controlled in a linear mode to regulate the system voltage at
min system voltage and to drop voltage between the min
system voltage and the battery. This state is communicated
to the host system by the trickle bit in the control register and
a low state on the TRKLN pin.
When the battery is charged to the min system voltage, the
BGATE FET becomes fully enhanced and BGATE is pulled
more than 5V below the system voltage. This changes the
ACMON 20 I
INPUT
R
S1
⋅⋅=
(EQ. 1)
ISL9518, ISL9518A
