ISL88731HRZ View Datasheet(PDF) - Intersil

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ISL88731HRZ

SMBus Level 2 Battery Charger

Intersil 

ISL88731

Theory of Operation

Introduction

The ISL88731 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 ISL88731 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

ISL88731 “Typical Application Circuit” is shown on Figure 2.

The ISL88731 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 ISL88731 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 ISL88731 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 ISL88731 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 VGS, preventing cross-conduction and shoot-through.

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 Vth,fall, and high when ACIN is

above Vth,rise. The ACIN rising threshold is 3.2V (typ) with

57mV hysteresis.

Current Measurement

Use ICM to monitor the adapter current being sensed across

CSSP and CSSN. The output voltage range is 0 to 2.5V. The

voltage of ICM is proportional to the voltage drop across

CSSP and CSSN, and is given by Equation 1:

ICM = 20 ⋅ IINPUT ⋅ RS1

(EQ. 1)

where Iadapter is the DC current drawn from the 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” 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.

9

FN9258.0

November 20, 2006

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