Current measurement, Figure 4. current register format, Current accumulator – Rainbow Electronics DS2761 User Manual
Page 8
DS2761
8 of 24
Summary. All of the protection conditions described above are OR'ed together to affect the
CC
and
DC
outputs.
DC
= (Undervoltage) or (Overcurrent, Either Direction) or (Short Circuit) or
(Protection Register Bit DE = 0) or (Sleep Mode)
CC
= (Overvoltage) or (Undervoltage) or (Overcurrent, Charge Direction) or (Protection Register
bit CE = 0) or (Sleep Mode)
CURRENT MEASUREMENT
In the active mode of operation, the DS2761 continually measures the current flow into and out of the
battery by measuring the voltage drop across a current-sense resistor. The DS2761 is available in two
configurations: 1) internal 25m
W current-sense resistor, and 2) external user-selectable sense resistor. In
either configuration, the DS2761 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 readings. 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 4.
For the internal sense resistor configuration, the DS2761 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 DS2761 automatically compensates for internal sense resistor process variations and
temperature effects when reporting current.
For the external sense resistor configuration, the DS2761 writes the measured V
IS
voltage to the current
register, with a resolution of 15.625
mV and a full-scale range of ±64mV.
Figure 4. 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 Resistor
15.625
mV for External Sense Resistor
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 5.