DS2786B Maxim Integrated Products, DS2786B Datasheet - Page 14

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

Stand-Alone OCV-Based Fuel Gauge
Figure 17. Voltage Breakpoint Register Format
The DS2786B calculates relative capacity immediately
upon power-up. During initialization, the DS2786B
makes a voltage measurement and uses the OCV cell
model data to determine a starting point for the Relative
Capacity Register. This estimation occurs regardless of
the load on the cell. Any error induced from cell loading
is removed at the next OCV adjustment. The initial volt-
age measurement used in determining the starting
point is stored in the Initial Voltage Register until the IC
is power cycled. See Figure 18.
As the cell ages, the Initial Capacity Scaling Factor
Register value might no longer accurately reflect the
true capacity of the cell, causing error in relative capaci-
ty calculation while in coulomb-counting mode of opera-
tion. The DS2786B has a learn feature that allows the IC
to remain accurate as the cell changes. The DS2786B
Figure 18. Initial Voltage Register Format
Figure 19. Last OCV Register Format
14
MSB
“S”: SIGN BIT(S), “X”: RESERVED
MSB
“X”: RESERVED
2
S
11
______________________________________________________________________________________
2
2
10
11
MSB—EVEN ADDRESSES 68h–78h
Initial Capacity Estimation
2
2
10
New Capacity Learning
9
MSB—ADDRESS 14h
2
2
9
8
2
2
8
7
MSB
2
7
2
2
6
7
2
6
2
2
5
6
2
5
LSB
LSB
2
2
5
4
ADDRESS 16h
2
4
compares the percent relative capacity difference
between the last two OCV updates to the change in the
coulomb count to learn the new cell capacity. The Last
OCV Register maintains the relative capacity percent-
age at the previous OCV adjustment point used for
learning the new cell capacity. The last OCV is updated
with a new value at each OCV adjustment. Figure 19
shows the Last OCV Register format.
Example: Assuming a 15mΩ sense resistor, the
DS2786B adjusts the relative capacity of a 1000mAh
cell to 10% based on an OCV measurement during an
idle period of the application. The cell is then charged
by 500mAh (to 60% expected) based on the internal
coulomb count multiplied by the learned capacity scal-
ing factor value of 55h. The next OCV adjustment deter-
mines the relative capacity should actually be at 65%,
not 60%. The DS2786B then adjusts the learned capac-
ity scaling factor value upward to (65% - 10%)/(500mAh
x 0.015Ω) = 5Eh, lowering the expected cell capacity
by approximately 10%.
2
3
MSB
MSB
2
2
3
4
2
2
2
2
2
3
UNITS: 0.5%
2
LSB—ODD ADDRESS 69h–79h
1
2
2
1
2
LSB—ADDRESS 15h
LSB
2
0
2
2
0
1
2
X
0
X
X
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UNITS: 1.22mV
UNITS: 1.22mV
X
X
LSB
LSB
X
X