Datasheet

DS2746 Low-Cost 2-Wire Battery Monitor
10 of 17
Table 1. Current Resolution for Various RSNS Values
CURRENT RESOLUTION (1 LSB)
R
SNS
V
SS
- V
SNS
20m 15m 10m 5m
6.25V
312.5µA 416.7µA 625µA 1.25mA
Table 2. Current Range for Various RSNS Values
CURRENT INPUT RANGE
R
SNS
V
SS
- V
SNS
20m 15m 10m 5m
±51.2mV ±2.56A ±3.41A ±5.12A ±10.24A
Every 1024th conversion, the ADC measures its input offset to facilitate offset correction. Offset correction occurs
approximately once per hour. The resulting correction factor is applied to the subsequent 1023 measurements.
During the offset correction conversion, the ADC does not measure the SNS to V
SS
signal. A maximum error of
1/1024 in the accumulated current register (ACR) is possible, however, to reduce the error, the current
measurement just prior to the offset conversion is displayed in the current register and is substituted for the
dropped current measurement in the current accumulation process. The error due to offset correction is typically
much less than 1/1024 of the expected reading.
CURRENT ACCUMULATION
The Accumulated Current register (ACR) serves as an up/down counter holding a running count of charge stored in
the battery. Current measurement results, plus a programmable bias value are internally summed, or accumulated,
at the completion of each current measurement conversion period with the results displayed in the ACR. The ACR
has a range of 0mVh to +409.6mVh with an LSb of 6.25μVhAdditional registers hold fractional results of each
accumulation, however, these bits are not user accessible. The ACR count clamps at FFFFh when accumulating
charge values and at 0000h when accumulating discharge values.
Read and write access is allowed to the ACR. Whenever the ACR is written, fractional accumulation results are
cleared. A write to the ACR also forces the ADC to measure its offset and update the offset correction factor.
Current measurement and accumulation resume (using the new offset correction) with the second conversion
following the write to the ACR. Figure 9 describes the ACR address, format, and resolution. Table 3 shows the
ACR’s dynamic range for several different sense resistor options.
Figure 9. Accumulated Current Register Format
MSB—Address 10h LSB—Address 11h
2
15
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
Units: 6.25Vh/Rsns