Rainbow Electronics DS2788 - Manual (Page 10)

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8 current-sense resistors. RSGAIN is an 11-bit value
stored in 2 bytes of the parameter EEPROM memory
block. The RSGAIN value adjusts the gain from 0 to
1.999 in steps of 0.001 (precisely 2 -10 ). The user must
program RSGAIN cautiously to ensure accurate current
measurement. When shipped from the factory, the gain
calibration value is stored in two separate locations in
the parameter EEPROM block: RSGAIN, which is repro-
grammable, and FRSGAIN, which is read only. RSGAIN
determines the gain used in the current measurement.
The read-only FRSGAIN (address B0h and B1h) is pro-
vided to preserve the factory value only and is not used
in the current measurement.
Sense Resistor Temperature
Compensation
The DS2788 is capable of temperature compensating
the current-sense resistor to correct for variation in a
sense resistor’s value over temperature. The DS2788 is
factory programmed with the sense resistor temperature
coefficient, RSTC, set to zero, which turns off the tem-
perature compensation function. RSTC is user accessi-
ble and can be reprogrammed after module or pack
manufacture to improve the current accuracy when
using a high temperature coefficient current-sense
resistor. RSTC is an 8-bit value stored in the parameter
EEPROM memory block. The RSTC value sets the tem-
perature coefficient from 0 to +7782ppm/°C in steps of
30.5ppm/°C. The user must program RSTC cautiously to
ensure accurate current measurement.
Temperature compensation adjustments are made
when the Temperature register crosses 0.5 ° C bound-
aries. The temperature compensation is most effective
with the resistor placed as close as possible to the V SS
terminal to optimize thermal coupling of the resistor to
the on-chip temperature sensor. If the current shunt is
constructed with a copper PCB trace, run the trace
under the DS2788 package if possible.
Current Accumulation
Current measurements are internally summed, or accu-
mulated, at the completion of each conversion period
with the results displayed in the ACR. The accuracy of
the ACR is dependent on both the current measure-
ment and the conversion time base. The ACR has a
range of 0 to 409.6mVh with an LSb (least significant
bit) of 6.25µVh. Additional read-only registers (ACRL)
hold fractional results of each accumulation to avoid
truncation errors. Accumulation of charge current
above the maximum register value is reported at the
maximum register value (7FFFh); conversely, accumu-
lation of discharge current below the minimum register
value is reported at the minimum value (8000h).
Read and write access is allowed to the ACR. The ACR
must be written MSB (most significant byte) first, then
LSB (least significant byte). The write must be complet-
ed within 3.515s (one ACR register update period). A
write to the ACR forces the ADC to perform an offset
correction conversion and update the internal offset
correction factor. Current measurement and accumula-
tion begins with the second conversion following a write
to the ACR. Writing the ACR clears the fractional values
in ACRL. ACR’s format is shown in Figure 8, and
ACRL’s format is shown in Figure 9.
To preserve the ACR value in case of power loss, the
ACR value is backed up to EEPROM. The ACR value is
recovered from EEPROM on power-up. See the memo-
ry map in Table 3 for specific address location and
backup frequency.
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
10 ______________________________________________________________________________________
ACR R/W AND EE
MSB—ADDRESS 10h LSB—ADDRESS 11h
2 15 2 14 2 13 2 12 2 11 2 10 2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0
MSb LSb MSb LSb
UNITS: 6.25µVh/R SNS
Figure 8. Accumulated Current Register (ACR) Format