DS2438 Dallas Semiconducotr, DS2438 Datasheet - Page 9

• Manufacturer Part Number: DS2438
• Description: Smart Battery Monitor
• Manufacturer: Dallas Semiconducotr

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
to avoid excessive voltage drop during peak current demands. On the other hand, the impedance of R
should be maximized to achieve the finest resolution for current measurement and accumulation. Table 9
below lists several example R
the current calculation ( 1/(4096 * R
R
measurement/accumulation resolution when selecting R
SENSE RESISTOR TRADEOFFS Table 9
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.
SENSE RESISTOR
SENS
VALUE (R
) ). The user should carefully consider voltage drop at maximum current and required current
100 m
200 m
25 m
50 m
SENS
)
SENS
involves a tradeoff. On the one hand, the impedance of R
V
SENS
100 mV
200 mV
400 mV
50 mV
SENS
AT I=2A
values, the voltage drop across R
SENS
) ) and the LSb of the remaining capacity calculation ( 1/(2048 *
CURRENT lsb
9.76 mA
4.88 mA
2.44 mA
1.22 mA
9 of 29
SENS
.
CAPACITY lsb
REMAINING
19.53 mAHr
SENS
9.76 mAHr
4.88 mAHr
2.44 mAHr
at 2A (as an example), the LSb of
SENS
CAPACITY VALUE
MAX REMAINING
must be minimized
5000 mAhr
2500 mAhr
1250 mAhr
625 mAhr
DS2438
SENS