DS2778 Maxim Integrated Products, DS2778 Datasheet - Page 18

• Manufacturer Part Number: DS2778
• Description: 2-Cell Stand-Alone Li+ Fuel-Gauge IC
• Manufacturer: Maxim Integrated Products

2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
Protector and Optional SHA-1 Authentication
In some designs a systematic error or an application
preference requires the application of an arbitrary bias
to the current accumulation process. The Current
Accumulation Bias register (CAB) allows a user-pro-
grammed constant positive or negative polarity bias to
be included in the current accumulation process. The
value in CAB can be used to estimate battery currents
that do not flow through the sense resistor, estimate
battery self-discharge, or estimate current levels below
the current measurement resolution. The user-pro-
grammed two’s complement value, with bit weighting
the same as the current register, is added to the ACR
once per current conversion cycle. CAB is loaded on
power-up from EEPROM memory.
The cycle counter is an absolute count of the cumula-
tive discharge cycles. This register is intended to act as
a “cell odometer.” The LSb is two cycles, which allows
a maximum count of 510 discharge cycles. The register
does not loop. Once the maximum value is reached,
the register is clamped. This register is read and write
accessible while the parameter EEPROM memory block
(block 1) is unlocked. The Cycle Count register
becomes read-only once the EEPROM block is locked.
Remaining capacity estimation uses real-time mea-
sured values and stored parameters describing the cell
characteristics and application operating limits. Figure
13 describes the algorithm inputs and outputs.
Figure 11. Current Accumulation Bias Register Format
Figure 12. Cycle Counter Register Format
18
______________________________________________________________________________________
Capacity Estimation Algorithm
Accumulation Bias
Cycle Counter
“S”: SIGN BIT(S)
MSb
MSb
2
S
7
2
2
6
6
2
2
5
5
ADDRESS 61h
ADDRESS 1Eh
2
2
4
4
2
2
To achieve reasonable accuracy in estimating remain-
ing capacity, the cell performance characteristics over
temperature, load current, and charge-termination point
must be considered. Since the behavior of Li+ cells is
nonlinear, these characteristics must be included in the
capacity estimation to achieve an acceptable level of
accuracy in the capacity estimation. The FuelPack
method used in the DS2775–DS2778 is described in
general in Application Note 131: Lithium-Ion Cell Fuel
Gauging with Maxim Battery Monitor ICs. To facilitate
efficient implementation in hardware, a modified version
of the method outlined in Application Note 131 is used
to store cell characteristics in the DS2775–DS2778. Full
and empty points are retrieved in a lookup process that
retraces a piecewise linear model consisting of three
model curves named full, active empty, and standby
empty. Each model curve is constructed with five line
segments, numbered 1 through 5. Above +40°C, the
segment 5 model curves extend infinitely with zero
slope, approximating the nearly flat change in capacity
of Li+ cells at temperatures above +40°C. Segment 4 of
each model curves originates at +40°C on its upper end
and extends downward in temperature to the junction
with segment 3. Segment 3 joins with segment 2, which
in turn joins with segment 1. Segment 1 of each model
curve extends from the junction with segment 2 to infi-
nitely colder temperatures. The three junctions or break-
points that join the segments (labeled TBP12, TBP23,
and TBP34 in Figure 14) are programmable in 1°C
increments from -128°C to +40°C. The slope or deriva-
tive for segments 1, 2, 3, and 4 are also programmable
over a range of 0 to 15,555ppm in steps of 61ppm.
3
3
UNITS: 6.25μV/R
2
2
2
2
UNITS: 2 cycles
Modeling Cell Characteristics
2
2
1
1
LSb
LSb
SNS
2
2
0
0
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