DS2786B Maxim Integrated Products, DS2786B Datasheet - Page 11

• Manufacturer Part Number: DS2786B
• Description: Stand-Alone OCV-Based Fuel Gauge
• Manufacturer: Maxim Integrated Products

The Current Offset Bias Register (COBR) allows a pro-
grammable offset value to be added to raw current
measurements. The result of the raw current measure-
ment plus the COBR value is displayed as the current
measurement result in the Current Register, and is used
for current accumulation and detection of an OCV con-
dition. The COBR value can be used to correct for a sta-
tic offset error, or can be used to intentionally skew the
current results and therefore the current accumulation.
Read and write access is allowed to COBR. Whenever
the COBR is written, the new value is applied to all sub-
sequent current measurements. COBR can be pro-
grammed in 25µV steps to any value between
+3.175mV and -3.2mV. The COBR value is stored as a
two’s-complement value in nonvolatile (NV) memory.
The COBR factory default value is 00h. Figure 9 shows
the Current Offset Bias Register format.
An Internal Accumulated Current Register (IACR)
serves as an up/down counter holding a running count
of charge since the last OCV condition. Current mea-
surement results, plus a programmable bias value are
internally summed, or accumulated, at the completion
of each current measurement-conversion period. The
IACR has a range of ±204.8mVh. The IACR uses the
Initial or Learned Cell Capacity Registers to increment
or decrement the Relative Capacity Register as current
flows into or out of the battery. In this way, the fuel
gauge is accurate even when an OCV condition does
not occur for an extended time period. See Table 2 for
the accumulated current range for various R
Figure 9. Current Offset Bias Register Format
“S”: SIGN BIT
MSB
S
2
6
2
5
______________________________________________________________________________________
Current Accumulation
ADDRESS 60h
2
4
Current Offset Bias
Stand-Alone OCV-Based Fuel Gauge
2
3
2
UNITS: 25μV/R
2
2
SNS
1
values.
LSB
2
SNS
0
The DS2786B uses a hybrid OCV measurement and
coulomb-counting algorithm to estimate remaining cell
capacity. During periods of charging or discharging the
cell, the DS2786B counts charge flow into and out of
the cell. When the application becomes inactive, the
DS2786B waits for the cell voltage to relax and then
adjusts the coulomb count based on an open-circuit
voltage cell model stored in device EEPROM. The
resulting calculation is reported to the system as a per-
centage value between 0 and 100%. As the cell ages, a
learn feature adjusts for changes in capacity.
The Relative Capacity Register reports remaining cell
charge as a percentage of full. Relative capacity is
reported with a resolution of 0.5% and is limited to a
value between 0% and 100%. The Relative Capacity
Register is updated each time the IC performs a current
measurement or open-circuit cell-voltage measurement.
See Figure 10.
Figure 10. Relative Capacity Register Format
Table 2. Accumulated Current Range for
Various R
±204.8mVh
V
SS
MSB
2
7
- V
SNS
2
6
±10.24Ah
SNS
20m
Cell-Capacity Estimation
2
5
Values
ADDRESS 02h
IACR RANGE
2
±13.65Ah
4
15m
2
3
R
SNS
±20.48Ah
10m
2
2
www.DataSheet4U.com
UNITS: 0.5%
2
1
±40.96Ah
5m
LSB
2
0
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