DS2778 Maxim Integrated Products, DS2778 Datasheet - Page 17

• Manufacturer Part Number: DS2778
• Description: 2-Cell Stand-Alone Li+ Fuel-Gauge IC
• Manufacturer: Maxim Integrated Products

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.
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
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 measurements are internally summed, or accu-
mulated, at the completion of each conversion period
Figure 10. Accumulated Current Register Format
Table 2. Resolution and Range vs. Sense Resistor
Current Resolution
Current Range
ACR Resolution
ACR Range
TYPE OF RESOLUTION/RANGE
“S”: SIGN BIT(S)
MSb
2
15
Protector and Optional SHA-1 Authentication
Sense-Resistor Temperature
2
2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
14
______________________________________________________________________________________
2
13
MSB—ADDRESS 10h
Current Accumulation
2
12
2
11
Compensation
2
±409.6mVh
10
V
1.5625μV
±51.2mV
SS
6.25μVh
- V
SS
2
SNS
9
terminal
LSb
2
8
312.5μAh
±20.48Ah
78.13μA
±2.56A
20m
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
= 0.020Ω and 20mA for R
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.
UNITS: 6.25μV/R
MSb
2
7
2
±27.30Ah
6
416.7μAh
104.2μA
±3.41A
15m
2
SNS
LSB—ADDRESS 11h
5
R
SNS
2
4
SNS
±40.96Ah
156.3μA
625μAh
±5.12A
10m
2
= 0.005Ω (see Table 2 for
3
2
2
www.DataSheet4U.com
2
1
1.250mAh
±81.92Ah
312.5μA
±10.2A
5m
LSb
2
0
SNS
17