DS2751
8 of 19
CURRENT MEASUREMENT
In the active mode of operation, the DS2751 continually measures the current flow into and out of the
battery by measuring the voltage drop across a current-sense resistor. The DS2751 is available in two
configurations: 1) internal 25mW current-sense resistor, and 2) external user-selectable sense resistor. In
either configuration, the DS2751 considers the voltage difference between pins IS1 and IS2 (V
IS
= V
IS1
-
V
IS2
) to be the filtered voltage drop across the sense resistor. A positive V
IS
value indicates current is
flowing into the battery (charging), while a negative V
IS
value indicates current is flowing out of the
battery (discharging).
V
IS
is measured with a signed resolution of 12 bits. The Current Register is updated in two’s-complement
format every 88ms with an average of 128 measurements. Currents outside the range of the register are
reported at the limit of the range. The format of the Current Register is shown in Figure 3.
For the internal sense resistor configuration, the DS2751 maintains the Current Register in units of amps,
with a resolution of 0.625mA and full-scale range of no less than ±1.9A (see Note 7 on I
FS
spec for more
details). The DS2751 automatically compensates for internal sense-resistor process variations and
temperature effects when reporting current.
For the external sense resistor configuration, the DS2751 updates the measured V
IS
voltage to the Current
Register in units of volts, with a resolution of 15.625mV and a ±64mV full-scale range.
Figure 3. CURRENT REGISTER FORMAT
MSB—Address 0E LSB—Address 0F
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
Units: 0.625mA for internal sense resisto
15.625mV for external sense resisto
CURRENT ACCUMULATOR
The Current Accumulator facilitates remaining capacity estimation by tracking the net current flow into
and out of the battery. Current flow into the battery increments the Current Accumulator, while current
flow out of the battery decrements it. Data is maintained in the Current Accumulator in two’s-
complement format. The format of the Current Accumulator is shown in Figure 4.
When the internal sense resistor is used, the DS2751 maintains the Current Accumulator in units of amp-
hours, with a resolution of 0.25mAhrs and a ±8.2Ahrs full-scale range. When using an external sense
resistor, the DS2751 maintains the Current Accumulator in units of volt-hours, with a resolution of
6.25mVhrs and a ±205mVhrs full-scale range.
The Current Accumulator is a read/write register that can be altered by the host system as needed.
