DS2780 Standalone Fuel Gauge IC
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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
Read Only
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: 1.5625V/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 DS2780’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
-10
). The user must program RSGAIN cautiously to ensure
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 DS2780 is capable of temperature compensating the current sense resistor to correct for variation in a sense
resistor’s value over temperature. The DS2780 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
o
C boundaries. The
temperature compensation is most effective with the resistor placed as close as possible to the VSS terminal. This
will optimize thermal coupling of the resistor to the on-chip temperature sensor. The current shunt trace should be
run under the DS2780 package, and it should be constructed with a copper PCB trace.
CURRENT ACCUMULATION
Current measurements are internally summed, or accumulated, at the completion of each conversion period and
the results are stored in the Accumulated Current Register (ACR). The accuracy of the ACR is dependent on the
