slyt334.pdf

Power Management

Texas Instruments Incorporated

Selecting the right charge-

management solution

By Masoud Beheshti

Director/Product Line Manager

Introduction

Today’s designers of portable devices have choices of

many types of battery chemistries, charger topologies,

and charge-management solutions. Selecting the right

solution should be simple, but in most cases it is a bit

complicated. The designer needs to strike a balance

between performance, cost, form factor, and other key

requirements. This article provides an overview of several

portable-power solutions.

The three C’s of charge management

Charge management is a critical function in any portable

design utilizing rechargeable batteries. Sound design tech-

niques ensure that requirements for the following three

considerations are met (see Table 1):

1. Cell safety— This is not limited to a simple requirement

like, for example, meeting the voltage-regulation toler-

ance of ±1% during the final phase of charge for a Li-Ion

battery. Safety functions also include safety timers, cell-

temperature monitoring, and a preconditioning mode to

safely handle deeply discharged cells.

2. Cell capacity— Any charge-management solution needs

to ensure that the batteries are charged to full capacity

in every cycle. Early charge termination results in

reduced run time and is not desirable in today’s power-

hungry portable devices.

3. Cell cycle life— Adhering to the recommended charge

algorithm is an important step towards ensuring that

the end user gets the maximum number of charge

cycles from each pack. Qualifying each charge with the

cell temperature and voltage, preconditioning deeply

discharged cells, and avoiding late or improper charge

termination are some of the steps necessary for maxi-

mizing cycle life.

Managing battery-chemistry requirements

System designers today have the option to select from a

variety of battery chemistries. The selection is typically

based on a number of criteria, including energy density;

size and form factor; cost; and usage pattern and cycle life.

Although there has been a strong trend towards Li-Ion and

Li-Pol chemistries in recent years, the NiCd and NiMH

chemistries are still viable options for a variety of con-

sumer applications.

Table 1. The three C’s of charge management

CELL

SAFETY

CELL

CAPACITY

CELL

CYCLE LIFE

CHARGE FEATURE

Accurate voltage and/or

current regulation

Charge qualification (voltage

and temperature)

Temperature monitoring

Preconditioning

End-of-charge termination

Charge timer

Charge-status reporting

Detection of battery insertion

and removal

Minimal battery drainage

Short-circuit current limit

Automatic recharge

Regardless of the choice of chemistry, it is critical to

adhere to the appropriate charge-management techniques

for each chemistry. These techniques ensure that batteries

are charged to their maximum capacities in every cycle

without compromising safety or cycle life.

NiCd/NiMH

Before a fast-charge cycle starts, NiCd and NiMH batteries

must be qualified and possibly conditioned. Fast charge is

prohibited if the battery voltage or temperature is outside

the allowed limits. For safety, any charging of a “hot” bat-

tery (typically above 45°C) is suspended until the battery

cools to the normal operating-temperature range. To con-

dition a “cold” battery (typically below 10°C) or an over-

discharged battery (typically below 1 V per cell), a gentle

trickle current is applied.

Fast charge begins when the battery temperature and

voltage are valid. NiMH batteries are typically charged with

a constant current of 1C or less. Certain NiCd batteries

can be charged at rates of up to 4C. Proper charge

termination is required to prevent harmful overcharge.

For nickel-based rechargeable batteries, fast-charge

termination can be based on either voltage or temperature.

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2Q 2009

Analog Applications Journal

Texas Instruments Incorporated

Power Management

As shown in Figure 1, a typical voltage-

termination method is peak-voltage detec-

tion (PVD), where fast charging is termi-

nated within a range of 0 to –4 mV per cell

of the peak cell voltage. The temperature

method monitors the rate of battery tem-

perature rise, DT/Dt, to detect full charge.

The typical DT/Dt rate is 1°C/minute.

Li-Ion/Li-Pol

Similar to NiCd and NiMH batteries, Li-Ion

and Li-Pol batteries must be qualified and

possibly conditioned before fast charge. A

qualification and conditioning method simi-

lar to the one described earlier is used.

As shown in Figure 2, following qualifica-

tion and preconditioning, a lithium-based

battery is first charged with a current of 1C

or less until it reaches its charge-voltage

limit. This stage of charge typically replen-

ishes up to 70% of the capacity. The battery

is then charged with a constant voltage of

typically 4.2 V. To maximize safety and the

available capacity, the charge voltage must

be regulated to at least ±1%. During this

stage of charge, the charging current drawn

by the battery tapers down. The charge is

typically terminated once the current level

falls below 10 to 15% of the initial charging

current at a 1C charging rate.

Linear versus switch-mode

charging topology

Linear and switch-mode topologies are

commonly used for controlling the charging

current and voltage in applications using

rechargeable batteries. Each topology pro-

vides unique advantages for its intended

applications.

The linear topology is well suited for low

cell counts and charging currents. It offers

the designer several advantages: low imple-

mentation cost, design simplicity, and

“quiet” operation due to the absence of

high-frequency switching. The linear topol-

ogy also introduces some power dissipation

into the system, in this case mostly during

the current-regulation phase of the charge

cycle. This is a drawback if the designer

has no means to manage the thermal issues

in the design.

The switch-mode topology is well suited

for higher cell counts and charging currents. Its main

advantage is increased efficiency. Unlike linear regulators,

the power switch or switches are operated in the satura-

tion region, which substantially reduces the overall losses.

The main sources of power loss in a buck converter

Figure 1. Charge profile for NiCd/NiMH batteries

Precharge

Phase

Current-Regulation

Phase

Charge

Termination

Regulation Current

PVD

– 

Charge Current

Battery Voltage

Minimum Charge Voltage



T/ t

Battery Temperature

Preconditioning and Taper Detect

Time

Figure 2. Charge profile for Li-Ion/Li-Pol batteries

Charge

Termination

Precharge

Phase

Current-Regulation

Phase

Voltage-Regulation

Phase

Regulation Voltage

Regulation Current

Charge Current

Charge Voltage

Minimum Charge Voltage

Preconditioning and Taper Detect

Time

include the switching losses (in the power switches) and

the DC losses in the filter inductor. Depending on the

design parameters, it is not uncommon to see efficiencies

of well over 95% in these applications.

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Power Management

Texas Instruments Incorporated

Inductive charging

Inductive (wireless) power has been around for a

long time and has found applications in many areas.

In the industrial area, for instance, induction heating

has provided a practical and efficient way to melt

large amounts of metals in a manufacturing environ-

ment. In the consumer area, inductive power has

been used successfully to charge toothbrushes and

other small personal-care products. However, when it

comes to charging the new generation of portable

appliances such as cellular phones, portable media

players, and Bluetooth ® headsets, the use of wireless

power is in its infancy.

The wireless chargers commonly used in the con-

sumer market for devices such as toothbrushes are

not optimized for efficiency or speed. These chargers

“trickle charge” at a low rate, and the form factor is

customized to accept only the intended end equip-

ment. However, the demands for portable power are

changing; and most consumers now own a multitude

of portable devices, each with its own power cable

and, in many cases, proprietary connectors. Consum-

ers are beginning to look for the same convenience

in charging their portable devices as is offered by

wireless data transfer. This concept, although simple,

presents a number of barriers for design solution and

acceptance:

•Unlikethebatteryforatoothbrush,batteriesfor

the new portable devices need to be charged at a

standard fast-charge rate, reaching 70% of capacity

in about an hour. The solution must therefore be

very power-efficient.

•Thebatteryforeachportabledeviceisadifferent

size and has a different charge rate (i.e., power

rating), so the concept of “one size fits all” does

not apply. The wireless charger needs to have the

intelligence to recognize these variations and

adjust itself accordingly.

•Consumersafetyisveryimportant,sothewireless

charger needs not only to differentiate between a

coin and a cell phone but also to make certain that

no hazardous situations are created under any

operating condition.

•Ultimately,whatconsumerswillpayforisconvenience,

so the wireless charger needs to be substantially easier

to use than the easiest corded charger available.

There are a variety of solutions being developed to

address these concerns. A great example is eCoupled™

technology developed by Fulton Innovation. This technol-

ogy includes an inductively coupled power-supply circuit

that dynamically seeks resonance, adapting its operation to

match the needs of each device it supplies (see Figure 3).

By communicating with each device individually in real

time, eCoupled technology not only determines power

needs but also takes into account the age of a battery or

device and its charging life cycles. This supplies the optimal

Figure 3. Concept of inductively coupled

power supply

Receiving Coil Built into Device

Power-Supply Coil Built

into Surface

Electromagnetic Field

Figure 4. TI Battery Chargers Quick Search tool

amount of power to the device and keeps it operating at

peak efficiency.

Selecting the charger

Texas Instruments offers a variety of tools to make the

process of selecting the right charger easier for designers.

Figure 4 shows the “Battery Chargers Quick Search” tool

available at power.ti.com (Scroll down to “Analog eLab™

Design Support” to view links under “Design, Simulation,

and Selection Tools.”)

Related Web site

power.ti.com

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