DS2782: Stand-Alone Fuel Gauge IC

9 of 25

CURRENT MEASUREMENT

In the ACTIVE mode of operation, the DS2782 continually measures the current flow into and out of the battery by

SNS and VSS is ±51.2mV. The input linearly converts peak signal amplitudes up to 102.4mV as long as the

continuous signal level (average over the conversion cycle period) does not exceed ±51.2mV. The ADC samples

the input differentially at 18.6kHz and updates the Current register at the completion of each conversion cycle.

The Current register is updated every 3.515s with the current conversion result in two’s complement form. Charge

currents above the maximum register value are reported at the maximum value (7FFFh = +51.2mV). Discharge

currents below the minimum register value are reported at the minimum value (8000h = -51.2mV).

Figure 6. Current Register Format

CURRENT

Read Only

MSB—Address 0Eh

LSB—Address 0Fh

S

2

14

2

13

2

12

2

11

2

10

2

9

2

8

2

7

2

6

2

5

2

4

2

3

2

2

2

1

2

0

MSb

LSb

MSb

LSb

“S”: sign bit(s)

Units:

1.5625

mV/Rsns

CURRENT RESOLUTION (1 LSB)

VSS -

VSNS

20m

W

15m

W

10m

W

5m

W

1.5625

mV

78.13

mA

104.2

mA

156.3

mA

312.5

mA

AVERAGE CURRENT MEASUREMENT

The Average Current register reports an average current level over the preceding 28 seconds. The register value is

updated every 28s in two’s complement form, and is the average of the 8 preceding Current register updates. The

format of the Average Current register is shown in Figure 7. Charge currents above the maximum register value

are reported at the maximum value (7FFFh = +51.2mV). Discharge currents below the minimum register value are

reported at the minimum value (8000h = -51.2mV).

Figure 7. Average Current Register Format

IAVG

R/W

MSB—Address 08h

LSB—Address 09h

S

2

14

2

13

2

12

2

11

2

10

2

9

2

8

2

7

2

6

2

5

2

4

2

3

2

2

2

1

2

0

MSb

LSb

MSb

LSb

“S”: sign bit(s)

Units:

1.5625

mV/Rsns

CURRENT OFFSET CORRECTION

Every 1024th conversion, the ADC measures its input offset to facilitate offset correction. Offset correction occurs

approximately once per hour. The resulting correction factor is applied to the subsequent 1023 measurements.

During the offset correction conversion, the ADC does not measure the sense resistor signal. A maximum error of

1/1024 in the accumulated current register (ACR) is possible; however, to reduce the error, the current

measurement made just prior to the offset conversion is displayed in the current register and is substituted for the

dropped current measurement in the current accumulation process. This results in an accumulated current error

due to offset correction of less than 1/1024.