ISL6253 datasheet(13/22 Pages) INTERSIL

13 / 22 page

Theory of Operation

Introduction

The ISL6253 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 10 amps. The

ISL6253 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 ISL6253 safely conditions over-discharged battery cells

with a percentage of full charge current until the battery

voltage exceeds 3.1V

× number of series connected cells.

When the battery voltage exceeds 3.1V

× number of series

connected cells, the ISL6253 charges the battery with

constant charge current, set by CHLIM input, until the battery

voltage rises to a programmed charge voltage set by VADJ

input; then the charger begins to operate at constant voltage

charge mode. The charger drives an adapter isolation

p-channel MOSFET to efficiently switch in the adapter

supply.

ISL6253 is a complete power source selection controller for

single battery systems and also aircraft power applications.

ISL6253 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 a single

battery system. The EN input allows shutdown of the charger

from a micro-controller. The amount of adapter current is

reported on the ICM output. Figure 16 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 17 shows the

ISL6253 typical application circuit 1 without power source

selection function. The typical application circuit 2 shown in

Figure 18 has automatic power source selection functionality

and supports aircraft power applications. 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 ISL6253 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 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 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 ISL6253 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 a

maximum duty cycle of up to 99.6%.

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.

The PWM controller is disabled when EN = GND, but the rest

of the circuitry, including the AC or DC adapter detecting

circuit and AC adapter current monitoring circuits, is still alive.

Setting the Battery Regulation Voltage

The ISL6253 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 (2.39V),

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 VCSON = 4.2V ×

number of the cells,

• Connect VADJ to VREF to set 4.41V

× number of cells,

• Connect VADJ to ground to set 3.99V

× number of cells.

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:

VCELL 0.175VVADJ

3.99V

(EQ. 1)

ISL6253

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