ISL6252 INTERSIL [Intersil Corporation], ISL6252 Datasheet - Page 12

• Manufacturer Part Number: ISL6252
• Description: Highly Integrated Battery Charger Controller for Notebook Computers
• Manufacturer: INTERSIL [Intersil Corporation]

VADJ
VADJ adjusts battery regulation voltage. VADJ = VREF for
4.2V + 5%/cell; VADJ = Floating for 4.2V/cell; VADJ = GND
for 4.2V - 5%/cell. Connect to a resistor divider to program
the desired battery cell voltage between 4.2V - 5% and
4.2V + 5%.
CHLIM
CHLIM is the battery charge current limit set pin. CHLIM
input voltage range is 0.1V to 3.6V. When CHLIM = 3.3V, the
set point for CSOP to CSON is 165mV. The charger shuts
down if CHLIM is forced below 88mV.
ACLIM
ACLIM is the adapter current limit set pin. ACLIM = VREF for
100mV, ACLIM = Floating for 75mV, and ACLIM = GND for
50mV. Connect a resistor divider to program the adapter
current limit threshold between 50mV and 100mV.
VREF
VREF is a 2.39V reference output pin. It is internally
compensated. Do not connect a decoupling capacitor.
Theory of Operation
Introduction
Unless otherwise noted, all descriptions of ISL6252 refer to
both ISL6252 and ISL6252A. The ISL6252 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 10A. The ISL6252 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 ISL6252 charges the battery with constant charge
current, set by CHLIM input, until the battery voltage rises up
to a programmed charge voltage set by VADJ input; then the
charger begins to operate at a constant voltage charge mode.
The charger also drives an adapter isolation P-Channel
MOSFET to efficiently switch in the adapter supply.
ISL6252 is a complete power source selection controller for
single battery systems and also aircraft power applications.
It 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 single battery
system.
The EN input allows shutdown of the charger through a
command from a micro-controller. It also uses EN to safely
shutdown the charger when the battery is in extremely hot
conditions. The amount of adapter current is reported on the
ICM output. Figure 1 shows the “IC Functional Block Diagram”
on page 3.
12
ISL6252, ISL6252A
The synchronous buck converter uses external N-Channel
MOSFETs to convert the input voltage to the required
charging current and charging voltage. Figure 2 shows the
ISL6252 typical application circuit with charging current and
charging voltage fixed at specific values. The typical
application circuit shown in Figure 3 shows the ISL6252
typical application circuit, which uses a micro-controller to
adjust the charging current set by CHLIM input for aircraft
power applications. The voltage at CHLIM and the value of R
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 ISL6252 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 and regulates the battery
charge 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
also 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 ISL6252 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
duty cycle of up to 99.6%.
To prevent boosting of the system bus voltage, the battery
charger operates in standard-buck mode when
CSOP-CSON drops below 4.25mV. Once in standard-buck
mode, hysteresis does not allow synchronous operation of
the DC/DC converter until CSOP-CSON rises above
12.5mV.
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.
August 25, 2010
FN6498.3
1