DS2745 Datasheet, PDF (8/14 Pages) Maxim Integrated Products – Low-Cost I2C Battery Monitor

Figure 5. TEMPERATURE REGISTER FORMAT
DS2745 Low-Cost I²C Battery Monitor
MSB—Address 0A
LSB—Address 0B
S 2⁹ 2⁸ 2⁷ 2⁶ 2⁵ 2⁴ 2³
MSb
LSb
“S”: sign bit, “X”: reserved
2² 2¹ 2⁰ X X X X X
MSb
LSb
Units: 0.125°C
CURRENT MEASUREMENT
In the active mode of operation, the DS2745 continually measures the current flow into and out of the battery by
measuring the voltage drop across a low-value current-sense resistor, R_SNS, connected between the SNS and VSS
pins. The voltage sense range between SNS and VSS is ±51.2mV. Note that positive current values occur when
V_SNS is less than V_SS, and negative current values occur when V_SNS is greater than V_SS. Peak signal amplitudes up
to 102mV are allowed at the input as long as the continuous or average signal level does not exceed ±51.2mV over
the conversion cycle period. The ADC samples the input differentially at with an 18.6kHz sample clock and updates
the current register at the completion of each conversion cycle. Figure 6 describes the current measurement
register format and resolution. Charge currents above the maximum register value are reported at the maximum
value (7FFFh = +51.2mV). Discharge currents below the minimum register value are reported at the minimum
value (8000h = -51.2mV).
Figure 6. CURRENT REGISTER FORMATS
MSB—Address 0E
LSB—Address 0F
S 2¹⁴ 2¹³ 2¹² 2¹¹ 2¹⁰ 2⁹ 2⁸
MSb
LSb
“S”: sign bit
2⁷ 2⁶
MSb
2⁵ 2⁴ 2³ 2² 2¹ 2⁰
Units:
LSb
2⁰ = 1.5625mV/Rsns
Table 2. CURRENT RESOLUTION FOR VARIOUS RSNS VALUES
CONVERSION
TIME
3.5s
|V_SS - V_SNS|
1.5625mV
CURRENT RESOLUTION (1 LSB)
20mW
78.13mA
R_SNS
15mW
10mW
104.2mA 156.3mA
5mW
312.5mA
Table 3. CURRENT RANGE FOR VARIOUS RSNS VALUES
V_SS - V_SNS
±51.2mV
CURRENT INPUT RANGE
20mW
±2.56A
R_SNS
15mW
10mW
±3.41A
±5.12A
5mW
±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 VSS 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.
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