DS2780 Maxim Integrated Products, DS2780 Datasheet - Page 8

• Manufacturer Part Number: DS2780
• Description: Stand-Alone Fuel Gauge IC
• Manufacturer: Maxim Integrated Products

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Figure 5. Current Register Format
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 6. 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. Average Current Register Format
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
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.
CURRENT
MSb
“S”: sign bit(s)
IAVG
MSb
“S”: sign bit(s)
S
S
2
2
14
14
MSB—Address 0Eh
MSB—Address 08h
2
2
13
13
1.5625mV
VSNS
VSS -
2
2
12
12
2
2
11
11
78.13mA
2
2
20mW
CURRENT RESOLUTION (1 LSB)
10
10
2
2
9
9
LSb
LSb
2
2
8
8
104.2mA
8 of 27
15mW
-10
). The user must program RSGAIN cautiously to ensure
MSb
MSb
R
2
2
7
7
SNS
156.3mA
2
2
10mW
6
6
LSB—Address 0Fh
LSB—Address 09h
2
2
5
5
DS2780 Stand-Alone Fuel Gauge IC
2
2
4
4
Units:
Units:
312.5mA
5mW
2
2
3
3
2
2
2
1.5625mV/Rsns
2
1.5625mV/Rsns
Read Only
R/W
2
2
1
1
LSb
LSb
2
2
0
0