Current register format figure 5, Current accumulators, Accumulated current register format figure 6 – Rainbow Electronics DS2770 User Manual
Page 10: Current offset compensation
DS2770
10 of 27
CURRENT REGISTER FORMAT Figure 5
MSB—Address 0E
LSB—Address 0F
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
Units: 62.5
mAh for internal sense resistor
1.56
mVh for external sense resistor
CURRENT ACCUMULATORS
The current accumulator facilitates remaining capacity estimation by tracking the net current flow into
and out of the battery. Current flow into the battery increments the current accumulator while current
flow out of the battery decrements it. Data is maintained in the current accumulator in two’s complement
format and updated every 3.52s. Figure 6 the format of the current accumulator.
When the internal sense resistor is used, the DS2770 maintains the current accumulator in units of amp-
hours, with a resolution of 250
mA and full-scale range of ±8.19Ah. When using an external sense resistor,
the DS2770 maintains the current accumulator in units of volt-hours, with a resolution of 6.25
mVh and a
full-scale range of
±205mVh.
ACCUMULATED CURRENT REGISTER FORMAT Figure 6
MSB—Address 10
LSB—Address 11
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
Units: 0.25mAh for internal sense resistor
6.25
mVh for external sense resistor
CURRENT OFFSET COMPENSATION
Current measurement and consequently the current accumulation are internally compensated for offset on
a continual basis to minimize errors from variations in device temperature and supply voltage. Offset
compensation is corrected to ±1LSB at least once per hour. Additionally, the Current Offset Bias Register
provides a user-programmable constant bias value that may be used to correct for errors due to circuit
layout or topology that result in current measurement or current accumulation errors. It can also be used
to arbitrarily apply a constant bias to force the current measurement to be pessimistic, or correct for room
temperature self-discharge. The current offset bias value is stored in EEPROM addresses 32h and 33h in
two’s complement format. The stored value is subtracted from each current measurement; therefore,
positive values (0001h to 7fffh) bias the current measurement and current accumulation in the discharge
direction. Figure 7 shows the format of the Current Offset Bias Register.