Rainbow Electronics DS2778 - Manual (Page 16)

FRSGAIN value is provided to preserve the factory cali-
bration value only and is not used to calibrate the current
measurement. The 16-bit FRSGAIN value is readable
from addresses B0h and B1h.
Sense-Resistor Temperature
Compensation
The DS2775–DS2778 can temperature compensate the
current-sense resistor to correct for variation in a sense
resistor’s value over temperature. The DS2775–DS2778
are factory programmed with the sense-resistor temper-
ature coefficient, RSTC, set to zero, which turns off the
temperature compensation function. RSTC is user
accessible 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 temperature 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 boundaries. 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
DS2775–DS2778 package whenever possible.
Current Accumulation
Current measurements are internally summed, or accu-
mulated, at the completion of each conversion period
with the results displayed in the Accumulated Current
register (ACR). The accuracy of the ACR is dependent
on both the current measurement and the conversion
time base. The ACR has a range of 0 to +409.6mVh
with an LSb of 6.25µVh. Additional registers hold frac-
tional results of each accumulation to avoid truncation
errors. The fractional result bits are not user accessible.
Accumulation of charge current above the maximum
register value is reported at the maximum value; con-
versely, accumulation of discharge current below the
minimum register value is reported at the minimum
value.
Charge currents (positive Current register values) less
than 100µV are not accumulated in order to mask the
effect of accumulating small positive offset errors over
long periods. This effect limits the minimum charge cur-
rent, for coulomb counting purposes, to 5mA for R
SNS
= 0.020 Ω and 20mA for R
SNS = 0.005 Ω (see Table 2 for
more details).
Read and write access is allowed to the ACR. The
ACR must be written most significant byte (MSB) first,
then LSB. Whenever the ACR is written, the fractional
accumulation result bits are cleared. The write must
be completed in 3.5s. A write to the ACR forces the
ADC to perform an offset correction conversion and
update the internal offset correction factor. The cur-
rent measurement and accumulation begin with the
second conversion following a write to the ACR. 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
Memory Map for specific address location and back-
up frequency.
DS2775/DS2776/DS2777/DS2778
2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
Protector and Optional SHA-1 Authentication
16 ______________________________________________________________________________________
R
SNS
TYPE OF RESOLUTION/RANGE V SS - V SNS
20m 15m 10m 5m
Current Resolution 1.5625μV 78.13μA 104.2μA 156.3μA 312.5μA
Current Range ±51.2mV ±2.56A ±3.41A ±5.12A ±10.2A
ACR Resolution 6.25μVh 312.5μAh 416.7μAh 625μAh 1.250mAh
ACR Range ±409.6mVh ±20.48Ah ±27.30Ah ±40.96Ah ±81.92Ah
Table 2. Resolution and Range vs. Sense Resistor
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
“S”: SIGN BIT(S) UNITS: 6.25μV/R SNS
Figure 10. Accumulated Current Register Format