DS2782

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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

V/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.

CURRENT MEASUREMENT CALIBRATION

The DS2782’s current measurement gain can be adjusted through the RSGAIN register, which is factory-calibrated

to meet the data sheet specified accuracy. RSGAIN is user accessible and can be reprogrammed after module or

pack manufacture to improve the current measurement accuracy. Adjusting RSGAIN can correct for variation in an

external sense resistor’s nominal value, and allows the use of low-cost, non-precision current sense resistors.

RSGAIN is an 11-bit value stored in 2 bytes of the Parameter EEPROM Memory Block. The RSGAIN value adjusts

the gain from 0 to 1.999 in steps of 0.001 (precisely 2

accurate current measurement. When shipped from the factory, the gain calibration value is stored in two separate

locations in the Parameter EEPROM Block: RSGAIN, which is reprogrammable, and FRSGAIN, which is read only.

RSGAIN determines the gain used in the current measurement. The read-only FRSGAIN is provided to preserve

the factory value only and is not used in the current measurement.

SENSE RESISTOR TEMPERATURE COMPENSATION

The DS2782 is capable of temperature compensating the current sense resistor to correct for variation in a sense

resistor’s value over temperature. The DS2782 is factory programmed with the sense resistor temperature

coefficient, RSTC, set to zero, which turns off the temperature compensation function. RSTC is user accessible

and can be reprogrammed after module or pack manufacture to improve the current accuracy when using a high

temperature coefficient current-sense resistor. RSTC is an 8-bit value stored in the Parameter EEPROM Memory

Block. The RSTC value sets the temperature coefficient from 0 to +7782ppm/ºC in steps of 30.5ppm/ºC. The user

must program RSTC cautiously to ensure accurate current measurement.

Temperature compensation adjustments are made when the Temperature register crosses 0.5

temperature compensation is most effective with the resistor placed as close as possible to the VSS terminal to

optimize thermal coupling of the resistor to the on-chip temperature sensor. If the current shunt is constructed with

a copper PCB trace, run the trace under the DS2782 package if possible.