MAX1645 Datasheet, PDF(29/32 Page) Maxim Integrated Products – Advanced Chemistry-Independent, Level 2 Battery Chargers with Input Current Limiting

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MAX1645 Datasheet, PDF (29/32 Pages) Maxim Integrated Products – Advanced Chemistry-Independent, Level 2 Battery Chargers with Input Current Limiting

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Advanced Chemistry-Independent, Level 2

Battery Chargers with Input Current Limiting

(Table 8). Level 3 smart battery chargers are supersets

of Level 2 chargers and, as such, support all Level 2

charger commands.

Level 2 Smart Battery Charger

The Level 2 or smart battery-controlled smart battery

charger interprets the smart batteryâs critical warning

messages and operates as an SMBus slave device to

respond to the smart batteryâs ChargingVoltage() and

ChargingCurrent() messages. The charger is obliged to

adjust its output characteristics in direct response to

the ChargingVoltage() and ChargingCurrent() mes-

sages it receives from the battery. In Level 2 charging,

the smart battery is completely responsible for initiating

the communication and providing the charging algo-

rithm to the charger.

The smart battery is in the best position to tell the smart

battery charger how it needs to be charged. The charg-

ing algorithm in the battery may request a static charge

condition or may choose to periodically adjust the

smart battery chargerâs output to meet its present

needs. A Level 2 smart battery charger is truly chem-

istry independent and, since it is defined as an SMBus

slave device only, the smart battery charger is relatively

inexpensive and easy to implement.

Selecting External Components

Table 10 lists the recommended components and

refers to the circuit of Figure 1; Table 9 lists the suppli-

ersâ contacts. The following sections describe how to

select these components.

MOSFETs and Schottky Diodes

Schottky diode D1 provides power to the load when the

AC adapter is inserted. Choose a 3A Schottky diode or

higher. This diode may not be necessary if P1 is used.

The P-channel MOSFET P1 turns on when VCVS >

VBATT. This eliminates the voltage drop and power con-

sumption of the Schottky diode. To minimize power loss,

select a MOSFET with an RDS(ON) of 50mâ¦ or less. This

MOSFET must be able to deliver the maximum current

as set by R1. D1 and P1 provide protection from

reversed voltage at the adapter input.

The N-channel MOSFETs N1 and N2 are the switching

devices for the buck controller. High-side switch N1

should have a current rating of at least 6A and have an

RDS(ON) of 50mâ¦ or less. The driver for N1 is powered

by BST; its current should be less than 10mA. Select a

MOSFET with a low total gate charge and determine

the required drive current by IGATE = QGATE Â· f (where f

is the DC-DC converter maximum switching frequency

of 400kHz).

The low-side switch N2 should also have a current rat-

ing of at least 3A, have an RDS(ON) of 100mâ¦ or less,

and a total gate charge less than 10nC. N2 is used to

provide the starting charge to the BST capacitor C14.

During normal operation, the current is carried by

Schottky diode D2. Choose a 3A or higher Schottky

diode.

Table 9. Component Suppliers

COMPONENT

Inductor

MOSFET

Sense Resistor

MANUFACTURER

Sumida

Coilcraft

Coiltronics

Internal Rectifier

Fairchild

Vishay-Siliconix

Dale

IRC

PART

CDRH127 series

D03316P series

UP2 series

IRF7309

FDS series

Si4435/6

WSL series

LR2010-01 series

D3 is a signal-level diode, such as the 1N4148. This

diode provides the supply current to the high-side

MOSFET driver.

The P-channel MOSFET P2 delivers the current to the

load when the AC adapter is removed. Select a MOS-

FET with an RDS(ON) of 50mâ¦ or less to minimize power

loss and voltage drop.

Inductor Selection

Inductor L1 provides power to the battery while it is

being charged. It must have a saturation current of at

least 3A plus 1/2 of the current ripple (âIL).

ISAT = 3A + 1/2 âIL

Capacitor

Diode

AVX

TPS series,

TAJ series

The controller determines the constant off-time period,

which is dependent on BATT voltage. This makes the

Sprague

Motorola

595D series

1N5817â1N5822

ripple current independent of input and battery voltage

and should be kept to less than 1A. Calculate the âIL

with the following equation:

Nihon

Central

Semiconductor

NSQ03A04

CMSH series

âIL = 21VÂµs / L

Higher inductor values decrease the ripple current.

Smaller inductor values require higher saturation cur-

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