DS2438
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OPERATION - CURRENT ACCUMULATORS
The DS2438 tracks the remaining capacity of a battery using the Integrated Current Accumulator (ICA).
The ICA maintains a net accumulated total of current flowing into and out of the battery; therefore, the
value stored in this register is an indication of the remaining capacity in a battery and may be used in
performing fuel gauge functions. In addition, the DS2438 has another register that accumulates only
charging (positive) current (CCA) and one that accumulates only discharging (negative) current (DCA).
The CCA and DCA give the host system the information needed to determine the end of life of a
rechargeable battery, based on total charge/discharge current over its lifetime.
The current measurement described above yields the voltage across sense resistor R
SENS
measured every
27.46 ms. This value is then used to increment or decrement the ICA register, increment the CCA (if
current is positive), or increment the DCA (if current is negative). The ICA is a scaled 8-bit volatile
binary counter that integrates the voltage across R
SENS
over time. The ICA is only
incremented/decremented if the IAD bit is set to 1 in the Status/Configuration Register. Table 5 illustrates
the contents of the ICA. See Memory Map section for the address location of the ICA.
ICA REGISTER FORMAT Table 5
(This register accumulates the voltage measured across current sense resistor R
SENS
. This
value can be used to calculate remaining battery capacity using the equation below.)
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb (unit = 0.4882 mVhr) LSb
Remaining battery capacity is calculated from the ICA value using the equation:
Remaining Capacity
= ICA / (2048 * R
SENS
)
(where R
SENS
is in W)
For example, if a battery pack has 0.625 Ahr of remaining capacity, and the pack uses a 0.025W sense
resistor, the ICA will contain the value 32. From this value, remaining capacity can be calculated to be:
Remaining Capacity = 32 / ( 2048 * 0.025) = 0.625 Ahr
Since the accuracy of the current ADC is +2 LSb, measurements of very small currents can be inaccurate
by a high percentage. Because these inaccuracies can turn into large ICA errors when accumulated over a
long period of time, the DS2438 provides a method for filtering out potentially erroneous small signals so
that they are not accumulated. The DS2438’s Threshold Register specifies a current measurement
magnitude (after offset cancellation) above which the measurement is accumulated in the ICA, CCA and
DCA and below which it is not accumulated. The format of the Threshold Register is shown in Table 6.
The power-on default Threshold Register value is 00h (no threshold).
NOTE:
When writing to the Threshold Register, current measurement must be disabled (IAD bit set to “0”).
