DS2770

10 of 27

CURRENT REGISTER FORMAT Figure 5

MSB—Address 0E

LSB—Address 0F

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

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.