LTC2942I(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LTC2942I
(Rev.:RevA)

Battery Gas Gauge with Temperature, Voltage Measurement

Linear Technology 

LTC2942

Applications Information

Alert/Charge Complete Configuration B[2:1]

The AL/CC pin is a dual function pin configured by the

control register. By setting bits B[2:1] to [10] (default)

the AL/CC pin is configured as an alert pin following the

SMBus protocol. In this configuration the AL/CC pin is a

digital output and is pulled low if one of the three mea-

sured quantities (charge, voltage, temperature) exceeds

its high or low threshold or if the value of the accumulated

charge register overflows or underflows. An alert response

procedure started by the master resets the alert at the

AL/CC pin. For further information see the Alert Response

Protocol section.

Setting the control bits B[2:1] to [01] configures the AL/CC

pin as a digital input. In this mode, a high input on the

AL/CC pin communicates to the LTC2942 that the battery

is full and the accumulated charge register is set to its

maximum value FFFFh. The AL/CC pin would typically

be connected to the “charge complete” output from the

battery charger circuitry.

If neither the alert nor the charge complete functionality

is desired, bits B[2:1] should be set to [00]. The AL/CC

pin is then disabled and should be tied to GND.

Avoid setting B[2:1] to [11] as it enables the alert and the

charge complete modes simultaneously.

Choosing RSENSE

To achieve the specified precision of the coulomb counter,

the differential voltage between SENSE+ and SENSE– must

stay within ±50mV. For differential input signals up to

±300mV the LTC2942 will remain functional but the preci-

sion of the coulomb counter is not guaranteed.

The required value of the external sense resistor, RSENSE,

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 equal to:

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 = 3.6C (coulomb).

Choosing RSENSE = 50mV/IMAX is not sufficient in ap-

plications where the battery capacity (QBAT) is very large

compared to the maximum current (IMAX):

QBAT > IMAX • 5.5 Hours

For such low current applications with a large battery,

choosing RSENSE according to RSENSE = 50mV/IMAX can

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

mulated charge register 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 capacity is larger, RSENSE must be

lowered. For example, RSENSE must be reduced to 150mΩ

if a battery with a capacity of 1800mAh is used.

2942fa

10

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