DS2780 Maxim Integrated Products, DS2780 Datasheet - Page 9

• Manufacturer Part Number: DS2780
• Description: Stand-Alone Fuel Gauge IC
• Manufacturer: Maxim Integrated Products

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SENSE RESISTOR TEMPERATURE COMPENSATION
The DS2780 is capable of temperature compensating the current sense resistor to correct for variation in a sense
resistor’s value over temperature. The DS2780 is factory programmed with the sense resistor temperature
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/ºC in steps of 30.5ppm/ºC. The user
must program RSTC cautiously to ensure accurate current measurement.
Temperature compensation adjustments are made when the Temperature register crosses 0.5
temperature compensation is most effective with the resistor placed as close as possible to the VSS 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 DS2780 package if possible.
CURRENT ACCUMULATION
Current measurements are internally summed, or accumulated, 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 timebase. The ACR has a range of 0 to 409.6mVh with an LSb of
6.25mVh. 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, accumulation of discharge current below the minimum register value is reported at the
minimum value (8000h).
Charge currents (positive Current register values) less than 100mV are not accumulated in order to mask the effect
of accumulating small positive offset errors over long periods. This limits the minimum charge current, for coulomb-
counting purposes, to 5mA for RSNS = 0.020W and 20mA for RSNS = 0.005W.
Read and write access is allowed to the ACR. The ACR must be written MSByte first then LSByte. Whenever the
ACR is written, the fractional accumulation result bits are cleared. The write must be completed 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 accumulation begins with the second
conversion following a write to the ACR. Writing ACR clears the fractional values in ACRL. The Format of the ACR
register is shown in Figure 7, and the format of ACRL is shown in Figure 8.
In order 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 in Table 2 for specific address location and
backup frequency.
Figure 7. Accumulated Current Register Format, ACR
ACR
MSb
2
15
2
14
2
MSB—Address 10h
13
2
12
2
11
2
10
2
9
LSb
2
8
9 of 27
MSb
2
7
2
6
LSB—Address 11h
2
5
DS2780 Stand-Alone Fuel Gauge IC
2
4
Units:
2
3
2
2
6.25mVh/Rsns
o
R/W & EE
C boundaries. The
2
1
LSb
2
0