ISL6251A Intersil Corporation, ISL6251A Datasheet - Page 12

• Manufacturer Part Number: ISL6251A
• Description: Low Cost Multi-chemistry Battery Charger Controller
• Manufacturer: Intersil Corporation

Theory of Operation
Introduction
The ISL6251, ISL6251A 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 ISL6251, ISL6251A 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 ISL6251, ISL6251A 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 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 11 shows the IC functional block
diagram.
The synchronous buck converter uses external N-channel
MOSFETs to convert the input voltage to the required
charging current and charging voltage. Figure 12 shows the
ISL6251, ISL6251A typical application circuit with charging
current and charging voltage fixed at specific values. The
typical application circuit shown in Figure 13 shows the
ISL6251, ISL6251A typical application circuit which uses a
micro-controller to adjust the charging current set by CHLIM
input. The voltage at CHLIM and the value of R1 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 ISL6251, ISL6251A 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 ISL6251, ISL6251A 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
12
ISL6251, ISL6251A
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.
Setting the Battery Regulation Voltage
The ISL6251, ISL6251A uses a high-accuracy trimmed
band-gap voltage reference to regulate the battery charging
voltage. The VADJ input adjusts the charger output voltage,
and the VADJ control voltage can vary from 0 to VREF,
providing a 10% adjustment range (from 4.2V-5% to
4.2V+5%) on CSON regulation voltage. An overall voltage
accuracy of better than 0.5% is achieved.
The per-cell battery termination voltage is a function of the
battery chemistry. Consult the battery manufacturers to
determine this voltage.
• Float VADJ to set the battery voltage V
• Connect VADJ to VREF to set 4.41V × number of cells,
• Connect VADJ to ground to set 3.99V × number of the
So, the maximum battery voltage of 17.6V can be achieved.
Note that other battery charge voltages can be set by
connecting a resistor divider from VREF to ground. The
resistor divider should be sized to draw no more than 100µA
from VREF; or connect a low impedance voltage source like
the D/A converter in the micro-controller. The programmed
battery voltage per cell can be determined by the following
equation:
An external resistor divider from VREF sets the voltage at
VADJ according to:
V
V
CELL
VADJ
number of the cells,
cells.
=
=
. 0
VREF
175
V
VADJ
×
------------------------------------------------------------------------------------------------ -
R
top_VADJ 514k
+
3.99
R
bot_VADJ
V
||
+
R bot_VADJ 514k
||
514k
CSON
||
= 4.2V ×
May 10, 2006
FN9202.2