DS2438 データシートの表示（PDF） - Maxim Integrated

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DS2438

Smart Battery Monitor

Maxim Integrated 

DS2438

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 RSENS involves a tradeoff. On the one hand, the impedance of RSENS must be minimized

to avoid excessive voltage drop during peak current demands. On the other hand, the impedance of RSENS

should be maximized to achieve the finest resolution for current measurement and accumulation. Table 9

below lists several example RSENS values, the LSb of the current calculation ( 1/(4096 * RSENS) ) and the

LSb of the remaining capacity calculation ( 1/(2048 * RSENS) ). The user should carefully consider

voltage drop at maximum current and required current measurement/accumulation resolution when

selecting RSENS.

SENSE RESISTOR TRADEOFFS Table 9

SENSE RESISTOR

VALUE (RSENS)

CURRENT lsb

25 mW

9.76 mA

50 mW

4.88 mA

100 mW

2.44 mA

200 mW

1.22 mA

REMAINING

CAPACITY lsb

19.53 mAHr

9.76 mAHr

4.88 mAHr

2.44 mAHr

MAX REMAINING

CAPACITY VALUE

5000 mAhr

2500 mAhr

1250 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.

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