LTC2941I(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC2941I
(Rev.:RevA)

Battery Gas Gauge with I2C Interface

Linear Technology 

LTC2941

Applications Information

The value of the external sense resistor is determined by

the maximum input range of VSENSE and the maximum

current of the application:

RSENSE

≤

50mV

IMAX

The choice of the external sense resistor value influences

the gain of the coulomb counter. A larger sense resistor

gives a larger differential voltage between SENSE+ and

SENSE– for the same current which results in more precise

coulomb counting. Thus the amount of charge represented

by the least significant bit (qLSB) of the accumulated charge

(registers C, D) is given by:

qLSB

= 0.085mAh •

50mΩ

RSENSE

•

M

128

or

qLSB

=

0.085mAh

•

50mΩ

RSENSE

when the prescaler is set to its default value of M=128.

Note that 1mAh = 3.6A • s = 3.6C (coulombs).

Choosing RSENSE = 50mV/IMAX is not sufficient in

applications where:

A. the battery capacity (QBAT) is very large compared to

the maximum current (IMAX):

QBAT > IMAX • 5.5 hours

B. the battery capacity (QBAT) is very small compared to

the maximum current (IMAX):

QBAT < IMAX • 0.1 hours

For case A: In low current applications using a large bat-

tery, choosing RSENSE according to RSENSE = 50mV/IMAX

can lead to a qLSB smaller than QBAT/216 and the 16-bit

accumulated charge may underflow before the battery is

exhausted or overflow during charge. Choose in this case

a maximum RSENSE of:

RSENSE

≤

0.085mAh • 216

QBAT

• 50mΩ

In an example application where the maximum current is

IMAX = 100mA, calculating RSENSE = 50mV/IMAX would

lead to a sense resistor of 500mΩ. This gives a qLSB of

8.5µAh and the accumulated charge register can represent

a maximum battery capacity of QBAT = 8.5µAh • 65535 =

557mAh. If the battery is larger, RSENSE must be lowered.

For example, RSENSE must be reduced to 150mΩ if a bat-

tery with a capacity of 1800mAh is used.

For case B: In applications using a small battery but having

a high maximum current, qLSB can get quite large with

respect to the battery capacity. For example, if the battery

capacity is 100mAh and the maximum current is 1A, the

standard equation leads to choose a sense resistor value

of 50mΩ, resulting in:

qLSB = 0.085mAh = 306mC

The battery capacity then corresponds to only 1176 qLSBs

and less than 2% of the accumulated charge register is

utilized.

To preserve digital resolution in this case, the LTC2941

includes a programmable prescaler. Lowering the prescaler

factor M allows reducing qLSB to better match the accu-

mulated charge registers C and D to the capacity of the

battery. The prescaling factor M can be chosen between 1

and its default value 128. The charge LSB then becomes:

qLSB

= 0.085mAh • 50mΩ

RSENSE

•M

128

To use as much of the range of the accumulated charge

registers C and D as possible the prescaler factor M should

2941fa

9

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