MC33348

7

MOTOROLA ANALOG IC DEVICE DATA

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.

84

6

5

1

3

MC33348

Cell

Charge

MOSFET Q1

Discharge

MOSFET Q2

7

Figure 8. One Cell Smart Battery Pack

A functional description of the protection circuit blocks

follows. Refer to the detailed block diagram shown in

Figure 7.

Voltage Sensing

Voltage sensing is accomplished by the use of the Cell

Voltage Sample Switch in conjunction with the Over/Under

Voltage Detector and Reference block. The Sample Switch

applies the cell voltage to the top resistor of an internal

divider string. The voltage at each of the tap points is

sequentially polled and compared to an internal reference. If

a limit has been exceeded, the result is stored in the

Over/Under Data Latch and Control Logic block. The Cell

Voltage Sample Switch is gated on for a 1.0 ms period at a

one second repetition rate. This low duty cycle sampling

technique reduces the average load current that the divider

presents across the cell, thus extending the useful battery

pack capacity. The cell voltage limits are tested in the

following sequence:

Figure 9. Cell Sensing Sequence

Polling

Sequence

Time

(ms)

Tested

Limit

1

0.5

Overvoltage

2

0.5

Undervoltage

By incorporating this polling technique with a single

comparator and voltage divider, a significant reduction of

circuitry and trim elements is achieved. This results in a

smaller die size, lower cost, and reduced operating current.

Figure 10. Cell Voltage Limit Sampling

From

Cell Voltage

Sample Switch

Over/Under

Cell Voltage

Detector

&

Reference

Discharge Voltage

Threshold

Charge Voltage

Threshold

Cell Voltage

Return

Cell Voltage

+

Cell

Voltage

–

R1

R2

R3

To

Cell Negative

Terminal

The cell charge and discharge voltage limits are controlled

by the values selected for the internal resistor divider string

and the comparator input threshold. As the battery pack

reaches full charge, the Cell Voltage Detector will sense an

overvoltage fault condition when the cell exceeds the

designed overvoltage limit. The fault information is stored in

a data latch and charge MOSFET Q1 is turned off,

disconnecting the battery pack from the charging source. An

internal current source pull–up is then applied to lower tap of

the divider, creating a hysteresis voltage. As a result of an

overvoltage fault, the battery pack is available for

discharging only.

The overvoltage fault is reset by applying a load to the

battery pack. As the voltage across the cell falls below the

hysteresis level, charge MOSFET Q1 will turn on and the

current source pull–up will turn off. The battery pack will now

be available for charging or discharging.