DS2788 MAXIM [Maxim Integrated Products], DS2788 Datasheet - Page 13

• Manufacturer Part Number: DS2788
• Description: Stand-Alone Fuel-Gauge IC with LED Display Drivers
• Manufacturer: MAXIM [Maxim Integrated Products]

ing of three curves named full, active empty, and
standby empty. Each model curve is constructed with
five line segments, numbered 1 through 5. Above
+50°C, the segment 5 model curves extend infinitely
with zero slope, approximating the nearly flat change in
capacity of Li+ cells at temperatures above +50°C.
Segment 4 of each model curves originates at +50°C
on its upper end and extends downward in temperature
to +25°C. 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. Segment slopes are stored
as µVh ppm change per °C. The two junctions or break-
points that join the segments (labeled TBP12 and
TBP23 in Figure 12) are programmable in 1°C incre-
ments from -128°C to +25°C. They are stored in two’s
complement format, TBP23 at 7Ch, and TBP12 at 7Dh.
The slope or derivative for segments 1, 2, 3, and 4 are
also programmable.
Figure 12. Cell Model Example Diagram
100%
______________________________________________________________________________________
SEGMENT 1
DERIVATIVE
(ppm/°C)
Stand-Alone Fuel-Gauge IC with
TBP12
SEGMENT 2
ACTIVE
EMPTY
TBP23
Full: The full curve defines how the full point of a given
cell stack depends on temperature for a given charge
termination. The charge termination method used in the
application is used to determine the table values. The
DS2788 reconstructs the full line from cell characteristic
table values to determine the full capacity of the battery
at each temperature. Reconstruction occurs in one-
degree temperature increments. Full values are stored
as ppm change per °C. For example, if a cell had a
nominal capacity of 1214mAh at +50°C, a full value of
1199mAh at +25°C, and 1182mAh at 0°C (TBP23), the
slope for segment 3 would be:
1 LSB of the slope registers equals 61ppm so the full
segment 3 slope register (location 0x6Dh) would be
programmed with a value of 0x09h. Each Slope register
has a dynamic range 0ppm to 15555ppm.
STANDBY
FULL
SEGMENT 3
EMPTY
((1199mAh - 1182mAh) / (1214mAh / 1M)) /
LED Display Drivers
+25°C
(25°C - 0°C) = 560ppm/°C
SEGMENT 4
CHARACTERIZATION
CHARACTERIZATION
CELL
CELL
+50°C
SEGMENT 5
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