isl88731a Intersil Corporation, isl88731a Datasheet - Page 9

• Manufacturer Part Number: isl88731a
• Description: Smbus Level 2 Battery Charger
• Manufacturer: Intersil Corporation

Theory of Operation
Introduction
The ISL88731A 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 ISL88731A 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.
Figure 2 shows the ISL88731A “Typical Application Circuit”.
The ISL88731A 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 ISL88731A features a voltage regulation loop (VCOMP)
and two 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 ISL88731A 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 two 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
two voltages (0.3V) above the lower voltage. This speeds the
transition from voltage loop control to current loop control or
vice versa.
The ISL88731A 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.
9
ISL88731A
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
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 2. ACOK is an open-drain output and is active low
when ACIN is less than V
above V
57mV 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
AC-adapter. It is recommended to have an RC filter at the
ICM output for minimizing the switching noise.
VDDP Regulator
VDDP provides a 5.2V 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 the “Typical Application Circuit”
(see Figure 2) 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
GS
=
ADAPTER
, preventing cross-conduction and shoot-through.
20 I
th,rise
⋅
INPUT
. The ACIN rising threshold is 3.2V (typ) with
is the DC current drawn from the
⋅
R
S1
th,fall
, and high when ACIN is
January 7, 2009
FN6738.1
(EQ. 1)