isl88731c Intersil Corporation, isl88731c Datasheet - Page 11

• Manufacturer Part Number: isl88731c
• Description: Smbus Level 2 Battery Charger
• Manufacturer: Intersil Corporation

Theory of Operation
Introduction
The ISL88731C includes all of the functions necessary to
charge 1 to 4 cell Li-Ion and Li-polymer batteries. A high
efficiency synchronous buck converter is used to control
the charging voltage up to 19.2V and charging current up
to 8A. The ISL88731C also has input current limiting up
to 11A. The Input current limit, charge current limit and
charge voltage limit are set by internal registers written
with SMBus. The ISL88731C “Typical Application Circuit”
is shown in Figure 4.
The ISL88731C charges the battery with constant
charge current, set by the ChargeCurrent register, until
the battery voltage rises to a voltage set by the
ChargeVoltage 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, battery charge current is reduced
so the charger does not reduce the adapter current
available to the system.
The ISL88731C features a voltage regulation loop
(VCOMP) and 2 current regulation loops (ICOMP). The
VCOMP voltage regulation loop monitors VFB to limit the
battery charge voltage. The ICOMP 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 ICOMP current
regulation loop also limits the input current drawn from
the AC-adapter to ensure that it never exceeds the limit
set by the InputCurrent register, and to prevent a system
crash and AC-adapter overload.
PWM Control
The ISL88731C employs a fixed frequency PWM 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 input
voltage changes.
The duty cycle of the buck regulator is controlled by the
lower of the voltages on ICOMP and VCOMP. The voltage
on ICOMP and VCOMP are inputs to a Lower Voltage
Buffer (LVB) who’s output is the lower of the 2 inputs.
The output of the LVB is compared to an internal
400kHz ramp to produce the Pulse Width Modulated
signal that controls the UGATE and LGATE drivers. An
internal clamp holds the higher of the 2 voltages (0.3V)
above the lower voltage. This speeds the transition from
voltage loop control to current loop control or vice
versa.
The ISL88731C can operate up to 99.6% duty cycle if
the input voltage drops close to or below the battery
charge voltage (drop out mode). The DC/DC converter
has a timer to prevent the frequency from dropping into
the audible frequency range.
To prevent boosting of the system bus voltage, the
battery charger drives the lower FET in a way that
prevents negative inductor current.
11
ISL88731C
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 20ns after
LGATE falls below 1V V
and shoot-through. The same occurs for LGATE turn on.
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. An internal Schottky diode between
the VDDP pin and BOOT pin keeps the bootstrap
capacitor charged.
AC-Adapter Detection
Connect the AC-adapter voltage through a resistor
divider to ACIN to detect when AC power is available, as
shown in Figure 4. ACOK is an open-drain output and is
active low when ACIN is less than V
ACIN is above V
(typ) with 60mV hysteresis.
Current Measurement
Use ICM to monitor the adapter current being sensed
across CSSP and CSSN. The output voltage range is 0V
to 2.5V. The voltage of ICM is proportional to the voltage
drop across CSSP and CSSN, and is given by Equation 1:
where I
adapter. It is recommended to have an RC filter at the
ICM output for minimizing the switching noise.
VDDP Regulator
VDDP provides a 5.1V supply voltage from the internal
LDO regulator from DCIN and can deliver up to 30mA of
continuous current. The MOSFET drivers are powered by
VDDP. VDDP also supplies power to VCC through a low
pass filter as shown in “TYPICAL APPLICATION CIRCUIT”
on page 2. Bypass VDDP and VCC with a 1µF capacitor.
VDDSMB Supply
The VDDSMB input provides power to the SMBus
interface. Connect VDDSMB to VCC, or apply an external
supply to VDDSMB to keep the SMBus interface active
while the supply to DCIN is removed. When VDDSMB is
biased the internal registers are maintained. Bypass
VDDSMB to GND with a 0.1µF or greater ceramic
capacitor.
Short Circuit Protection and 0V Battery
Charging
Since the battery charger will regulate the charge current
to the limit set by the ChargeCurrent register, it
automatically has short circuit protection and is able to
provide the charge current to wake up an extremely
discharged battery. Undervoltage trickle charge folds
back current if there is a short circuit on the output.
ICM
=
20 I
adapter
⋅
INPUT
is the DC current drawn from the AC
th,rise
⋅
R
S1
. The ACIN rising threshold is 3.2V
GS
, preventing cross-conduction
th,fall
, and high when
September 9, 2010
(EQ. 1)
FN6978.1