DS2438Z データシートの表示（PDF） - Dallas Semiconductor -> Maxim Integrated

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DS2438Z

Smart Battery Monitor

Dallas Semiconductor -> Maxim Integrated 

CURRENT REGISTER FORMAT Table 3

(This register actually stores the voltage measured across current sense resistor RSENS.

This value can be used to calculate battery pack current using the equation below.)

DS2438

27

26 25 24

23

22

21

20

LSB

MSb

(unit = 0.2441mV) LSb

S

214 213 212 211 210

29

28 MSB

The battery pack current is calculated from the Current Register value using the equation:

I = Current Register / (4096 * RSENS)

(where RSENS is in Ω)

For example, if 1.25A is flowing into the pack, and the pack uses a 0.025Ω sense resistor, the DS2438

will write the value 12810 to the Current Register. From this value, battery pack current can be calculated

to be:

I = 128 / ( 4096 * 0.025) = 1.25A

Because small current ADC offset errors can have a large cumulative effect when current is integrated

over time, the DS2438 provides a method for canceling offset errors in the current ADC. After each

current measurement is completed, the measured value is added to the contents of the Offset Register and

the result is then stored in the Current Register. The Offset Register is a two-byte nonvolatile read/write

register formatted in two’s-complement format. The four MSb’s of the register contain the sign of the

offset, as shown in Table 4.

OFFSET REGISTER FORMAT Table 4

24 23

MSb

XX

22 21 20

0

(unit = 0.2441 mV)

X S 28 27

0 0 LSB

LSb

26 25 MSB

The following process can be used to calibrate the current ADC:

1. Write all zeroes to the Offset Register

2. Force zero current through RSENS

3. Read the Current Register value

4. Disable the current ADC by setting the IAD bit in the Status/Configuration Register to “0”

5. Change the sign of the previously-read Current Register value by performing the two’s complement

and write the result to the Offset Register

6. Enable the current ADC by setting the IAD bit in the Status/Configuration Register to “1”

NOTE:

When writing to the Offset Register, current measurement MUST be disabled (IAD bit set to “0”).

The current ADC calibration process is done for each DS2438 device prior to shipment. However, for

best results, battery pack manufacturers should calibrate the current ADC during initial battery pack

testing, and the host system should calibrate whenever possible (during battery charging, for example).

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