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MC33348D-2

Lithium Battery Protection Circuit for One Cell Battery Packs

ON Semiconductor 

MC33348

INTRODUCTION

The insatiable demand for smaller lightweight portable electronic

equipment has dramatically increased the requirements of battery

performance. Batteries are expected to have higher energy densities,

superior cycle life, be safe in operation and environmentally friendly.

To address these high expectations, battery manufacturers have

invested heavily in developing rechargeable lithium−based cells.

Today’s most attractive chemistries include lithium−polymer,

lithium−ion, and lithium−metal. Each of these chemistries require

electronic protection in order to constrain cell operation to within the

manufacturers limits.

Rechargeable lithium−based cells require precise charge and

discharge termination limits for both voltage and current in order to

maximize cell capacity, cycle life, and to protect the end user from a

catastrophic event. The termination limits are not as well defined as

with older non−lithium chemistries. These limits are dependent upon a

manufacturer’s particular lithium chemistry, construction technique,

and intended application. Battery pack assemblers may also choose to

enhance cell capacity at the expense of cycle life. In order to address

these requirements, six versions of the MC33348 protection circuit

were developed. These devices feature charge overvoltage protection,

discharge current limit protection with delayed shutdown, low

operating current, a virtually zero current sleepmode state, and

requires few external components to implement a complete one cell

smart battery pack.

Operating Description

The MC33348 is specifically designed to be placed in the battery

pack where it is continuously powered from a single lithium cell. In

order to maintain cell operation within specified limits, the protection

circuit senses both cell voltage and discharge current, and

correspondingly controls the state of two N−channel MOSFET

switches. These switches, Q1 and Q2, are placed in series with the

negative terminal of the Cell and the negative terminal of the battery

pack. This configuration allows the protection circuit to interrupt the

appropriate charge or discharge path FET in the event that either a

voltage threshold or the discharge current limit for the cell has been

exceeded.

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