Rainbow Electronics DS2778 - Manual (Page 15)

DS2775/DS2776/DS2777/DS2778
2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
Protector and Optional SHA-1 Authentication
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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 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
register (ACR) is possible; however, to reduce the error,
the current measurement made just prior to the offset
conversion is retained in the Current register and is
substituted for the dropped current measurement in the
current accumulation process. Therefore the accumu-
lated current error due to offset correction is typically
much less than 1/1024.
Current Offset Bias
The current offset bias value (COB) allows a program-
mable offset value to be added to raw current measure-
ments. 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 accumula-
tion. COB can be used to correct for a static offset error
or can be used to intentionally skew the current results
and therefore the current accumulation. Read and write
access is allowed to COB. Whenever the COB is writ-
ten, the new value is applied to all subsequent current
measurements. COB can be programmed in 1.56µV
steps to any value between -199.7µV and +198.1µV.
The COBR value is stored as a two’s complement value
in volatile memory and must be initialized through the
interface on power-up. The factory default value is 00h.
Current Blanking
The current blanking feature modifies current measure-
ment result prior to being accumulated in the ACR.
Current blanking occurs conditionally when a current
measurement (raw current and COBR) falls in one of
two defined ranges. The first range prevents charge
currents less than 100µV from being accumulated. The
second range prevents discharge currents less than
25µV in magnitude from being accumulated. Charge
current blanking is always performed; however, dis-
charge current blanking must be enabled by setting the
NBEN bit in the Control register. See the
Control
Register Format description for additional information.
Current Measurement Gain
The DS2775–DS2778’s current measurement gain can
be adjusted through the RSGAIN register, which is facto-
ry 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, nonprecision 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 use caution
when programming RSGAIN 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 reprogram-
mable and FRSGAIN, which is read-only. RSGAIN deter-
mines the gain used in the current measurement. The
ADDRESS 7Bh
S 2 6 2 5 2 4 2 3 2 2 2 1 2 0
MSb LSb
“S”: SIGN BIT(S) UNITS: 1.56μV/R SNS
Figure 8. Current Offset Bias Register Format
MSB—ADDRESS 78h LSB—ADDRESS 79h
X SC0 OC1 OC0 X 2 0 2 -1 2 -2 2 -3 2 -4 2 -5 2 -6 2 -7 2 -8 2 -9 2 -10
MSb LSb MSb LSb
“S”: SIGN BIT(S) UNITS: 2–10
Figure 9. RSGAIN Register