ISL88731CHRTZ Intersil, ISL88731CHRTZ Datasheet - Page 11

• Manufacturer Part Number: ISL88731CHRTZ
• Description: IC BATT CHRGR SMBUS LVL2 28TQFN
• Manufacturer: Intersil

Specifications of ISL88731CHRTZ

• Function: Charge Management
• Battery Type: Lithium-Ion (Li-Ion)
• Voltage - Supply: 8 V ~ 26 V
• Operating Temperature: -10°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 28-WFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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.
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
cross-conduction and shoot-through. The same occurs for LGATE
turn on. In order for the deadtime circuit to work properly, there
must be a low resistance, low inductance path from the LGATE
11
GS
, preventing
ISL88731C
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
threshold is 3.2V (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
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. Bypass
VDDSMB to GND with a 0.1µF or greater ceramic capacitor.
The typical application connects VDDSMB to the same power source
as the SMBus master. This supply should be active and greater than
2.5V when either the adapter or the battery is present.
ISL88731C does not function when VDDSMB is below its
specified Under Voltage Lockout (UVLO) voltage. All of the SMBus
registers in ISL88731C are powered by VDDSMB and are set to
zero when it is below the UVLO threshold. Other functions are
unpredictable when VDDSMB is below the UVLO threshold.
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.
Undervoltage Detect and Battery Trickle
Charging
If the voltage at CSON falls below 2.5V ISL88731C reduces the
charge current limit to 128mA to trickle charge the battery. When
the voltage rises above 2.7V, the charge current reverts to the
programmed value in the ChargeCurrent register.
th,fall
ICM
, and high when ACIN is above V
=
adapter
20 I
⋅
INPUT
is the DC current drawn from the AC adapter. It is
⋅
R
S1
th,rise
. The ACIN rising
February 8, 2011
FN6978.2
(EQ. 1)