DS2746 Low-Cost 2-Wire Battery Monitor
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AUXILARY INPUT MEASUREMENTS
The DS2746 allows for measuring two auxiliary measurement inputs, AIN0 and AIN1, with respect to V
SS
. These
inputs are designed for measuring resistor ratios, particularly useful for measuring thermistor or pack identification
resistors. At a time of t
PRE
prior to the beginning of a measurement cycle on AIN0 or AIN1, the V
OUT
pin puts out a
reference voltage in order to drive a resistive divider formed by a known resistor value, and the unknown resistance
to be measured. Making these measurements ratiometric with respect to V
OUT
removes reference tolerance from
the error calculations. Each auxiliary input measurement is updated every 660ms with the average voltage over the
220ms conversion period and placed in the AIN0 and AIN1 Registers in two’s complement form. The input
impedances of AIN0 and AIN1 are sufficiently large (>15M) to be connected to a wide range of voltage divider
resistances.
Figure 7. Auxiliary Input Registers Format
AIN0
MSB—Address 08h LSB—Address 09h
S 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 X
MSb LSb MSb LSb
“S”: sign bit, “X”: reserved Units: 1LSB = V
VOUT
* 1/2047
AIN1
MSB—Address 0Ah LSB—Address 0Bh
S 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 X
MSb LSb MSb LSb
“S”: sign bit, “X”: reserved Units: 1LSB = V
VOUT
* 1/2047
CURRENT MEASUREMENT
In the active mode of operation, the DS2746 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 V
SS
pins. The voltage sense range between SNS and V
SS
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 period. The ADC samples the input differentially and updates the current register at the completion
of each conversion. The result is updated every 878ms with the average voltage and placed in the CURRENT
register in two’s compliment form. The current measurement register format is shown in Figure 8 and specifications
for several different sense resistor options are shown in Tables 1 and 2. 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 8. Current Register Formats
MSB—Address 0Eh LSB—Address 0Fh
S 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
X X
MSb LSb MSb LSb
“S”: sign bit
Units: 2
0
= 6.25V/Rsns
