Figure 3. current register format, Figure 4. average current register format, Current offset correction – Rainbow Electronics DS2756 User Manual
Page 7: Current accumulation
DS2756: High-Accuracy Battery Fuel Gauge with Programmable Suspend Mode
7 of 26
error is introduced into both the Average Current and Accumulated Current values by the current sample timing
discontinuity introduced with each trigger of the Snapshot mode, use of Snapshot once every 5s does not produce
a significant error.
The following register formats specify the update interval and units for the Current and Average Current registers.
Values are posted in two’s complement format. Positive values represent charge currents (V
IS1
> V
IS2
) and negative
values represent discharge currents (V
IS2
> V
IS1
). Positive currents above the maximum register value are reported
at the maximum value, 0x7FFF. Negative currents below the minimum register value are reported at the minimum
value, 0x8000.
Figure 3. Current Register Format
12-bit + sign resolution (13-bit), 88ms update interval
MSB-Address
0Eh
LSB-Address
0Fh
S 2
11
2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
X X X
MSb
LSb
MSb LSb
“S”: sign bit(s)
Units:2
0
=
15.625
mV/Rsns
Figure 4. Average Current Register Format
15-bit + sign resolution (16-bit), 2.8s update interval
MSB-Address
1Ah
LSB-Address
1Bh
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
“S”: sign bit(s)
Units:2
0
= 1.953
mV/Rsns
CURRENT OFFSET CORRECTION
Continuous offset cancellation is performed automatically to correct for offsets in the current measurement system.
Individual values reported by the Current register have a maximum offset of ±0.5 LSb’s (±7.8125
mV). Individual
values reported in the Average Current register have a maximum offset of ±2 LSb’s (±7.8125
mV).
CURRENT ACCUMULATION
The DS2756 measures current for coulomb counting purposes, with an accuracy of ±2% (±3.9
mV) over a range of
+/-64mV. Using a 20m
W sense resistor, current accumulation is performed over a range of ±3.2A while measuring
standby currents with an accuracy of ±195
mA. Current measurements are internally summed, or accumulated, with
the results displayed in the Accumulated Current Register (ACR). The accuracy of the ACR is dependent on both
the current measurement and the accumulation timebase. The 16-bit ACR has a range of ±204.8mVh with an LSb
of 6.25
mVh. Accumulation of charge current above the maximum register value is reported at the maximum value;
conversely, accumulation of discharge current below the minimum register value is reported at the minimum value.
Read and write access is allowed to the ACR. The ACR must be written MSB first then LSB. Whenever the ACR is
written, internal fractional accumulation result bits are cleared. In order to preserve the ACR value in case of power
loss, the ACR MSB and LSB are automatically backed up to EEPROM after incrementing or decrementing by
100
mVh (5.0mAh for Rsns = 20mW). The ACR value is recovered from EEPROM on power-up or by a Recall Data