DS2746 Low-Cost 2-Wire Battery Monitor

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Table 1. Current Resolution for Various RSNS Values

CURRENT RESOLUTION (1 LSB)

20m

Ω

15m

Ω

10m

Ω

5m

Ω

6.25

μV

312.5µA

416.7µA

625µA

1.25mA

Table 2. Current Range for Various RSNS Values

CURRENT INPUT RANGE

20m

Ω

15m

Ω

10m

Ω

5m

Ω

±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.

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 8 describes the ACR address, format, and resolution. Table 3 shows the

ACR’s dynamic range for several different sense resistor options.

Figure 8. Accumulated Current Register Format

MSB—Address 10h

LSB—Address 11h

MSb

LSb

MSb

LSb

“S”: sign bit

Units: 6.25

μVh/Rsns