Datasheet
DS2438
9 of 29
Figure 2 illustrates the activity of the ICA, CCA, and DCA over a sample charge/discharge cycle of a
battery pack, assuming the DS2438 is configured for the ICA to function and the CCA/DCA to function
and shadow data to EEPROM. To simplify the illustration of the accumulators, they are treated as analog
values, although they are digital counters in the DS2438. Note that when the battery becomes fully
discharged, i.e., the ICA value reaches 0, the CCA and DCA register values are maintained.
CURRENT ACCUMULATOR ACTIVITY Figure 2
SENSE RESISTOR SELECTION
The selection of R
SENS
involves a tradeoff. On the one hand, the impedance of R
SENS
must be minimized
to avoid excessive voltage drop during peak current demands. On the other hand, the impedance of R
SENS
should be maximized to achieve the finest resolution for current measurement and accumulation. Table 9
below lists several example R
SENS
values, the LSb of the current calculation ( 1/(4096 * R
SENS
) ) and the
LSb of the remaining capacity calculation ( 1/(2048 * R
SENS
) ). The user should carefully consider
voltage drop at maximum current and required current measurement/accumulation resolution when
selecting R
SENS
.
SENSE RESISTOR TRADEOFFS Table 9
SENSE RESISTOR
VALUE (R
SENS
)
CURRENT lsb
REMAINING
CAPACITY lsb
MAX REMAINING
CAPACITY VALUE
25 mW
9.76 mA 19.53 mAHr 5000 mAhr
50 mW
4.88 mA 9.76 mAHr 2500 mAhr
100 mW
2.44 mA 4.88 mAHr 1250 mAhr
200 mW
1.22 mA 2.44 mAHr 625 mAhr
OPERATION - ELAPSED TIME METER
An internal oscillator is used as the timebase for the timekeeping functions. The elapsed time functions
are double buffered, allowing the master to read elapsed time without the data changing while it is being
read. To accomplish this, a snapshot of the counter data is transferred to holding registers which the user
accesses. This occurs after the 8th bit of the Recall Memory command.
The elapsed time meter (ETM) is a 4-byte binary counter with 1-second resolution. The ETM can
accumulate 136 years of seconds before rolling over. Time/date is represented by the number of seconds
since a reference point, which is determined by the user. For example, 12:00 A.M., January 1, 1970 could
be used as a reference point.