DS2788 MAXIM [Maxim Integrated Products], DS2788 Datasheet - Page 9

• Manufacturer Part Number: DS2788
• Description: Stand-Alone Fuel-Gauge IC with LED Display Drivers
• Manufacturer: MAXIM [Maxim Integrated Products]

The Average Current (IAVG) register reports an aver-
age current level over the preceding 28 seconds. The
register value is updated every 28s in two’s comple-
ment form, and is the average of the eight preceding
Current register updates. Figure 6 shows the format of
the Average Current register. Charge currents above
the maximum register value are reported at the maxi-
mum value (7FFFh = +51.2mV). Discharge currents
below the minimum register value are reported at the
minimum value (8000h = -51.2mV).
Every 1024th conversion the ADC measures its input
offset to facilitate offset correction. Offset correction
occurs approximately once per hour. The resulting cor-
rection factor is applied to the subsequent 1023 mea-
surements. During the offset correction conversion, the
ADC does not measure the sense resistor signal. A
maximum error of 1/1024 in the Accumulated Current
(ACR) register 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 measure-
ment in the current accumulation process. This results
in an accumulated current error due to offset correction
of less than 1/1024.
Figure 6. Average Current Register Format
Figure 7. Current Offset Bias Register Format
COB
IAVG
MSb
“S”: SIGN BIT(S)
S
Average Current Measurement
2
14
Current Offset Correction
MSB—ADDRESS 08h
2
_______________________________________________________________________________________
13
2
12
“S”: SIGN BIT(S)
MSb
2
11
S
2
10
2
Stand-Alone Fuel-Gauge IC with
6
2
9
2
5
LSb
ADDRESS 7Bh
2
8
2
4
2
The Current Offset Bias (COB) register allows a pro-
grammable offset value to be added to raw current mea-
surements. The result of the raw current measurement
plus COB is displayed as the current measurement
result in the Current register, and is used for current
accumulation. COB can be used to correct for a static
offset error, or can be used to intentionally skew the cur-
rent results and therefore the current accumulation.
COB allows read and write access. Whenever the COB
is written, the new value is applied to all subsequent
current measurements. COB can be programmed in
1.56µV steps to any value between +198.1µV and -
199.7µV. The COB value is stored as a two’s comple-
ment value in nonvolatile memory.
The DS2788’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 cur-
rent measurement accuracy. Adjusting RSGAIN can
correct for variation in an external sense resistor’s nom-
inal value, and allows the use of low-cost, nonprecision
3
MSb
2
7
UNITS: 1.56μV/R
2
2
LED Display Drivers
2
6
2
1
LSB—ADDRESS 09h
2
5
LSb
2
Current Measurement
SNS
0
2
4
Current Offset Bias
2
3
UNITS: 1.5625μV/R
2
2
Calibration
READ ONLY
R/W AND EE
2
1
LSb
SNS
2
0
9