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Maxim > Design Support > Technical Documents > Application Notes > 1-Wire® Devices > APP 4114
Keywords: RSGAIN, Standalone, Stand Alone, Fuel Gauge,
APPLICATION NOTE 4114
Calibrating RSGAIN for the DS278x Family of Fuel
Gauges
Sep 11, 2007
Abstract: The DS278x family of stand-alone fuel gauges provides an accurate estimation of the
remaining capacity available in rechargeable lithium-ion or lithium-polymer batteries. These fuel gauges
are factory calibrated to provide the accuracy specified in the data sheet. However, users can reprogram
the current-measurement gain factor (RSGAIN) to improve current-measurement accuracy after module
or pack manufacture. Adjusting RSGAIN to correct for variation in the external sense resistor's nominal
value allows the use of low-cost, nonprecision current-sense resistors. This application note details a
procedure for calibrating the RSGAIN of a DS278x stand-alone fuel gauge.
Introduction
The DS278x family of stand-alone fuel gauges—which includes the DS2780, DS2781, DS2782, DS2784,
and DS2788—provides an accurate estimation of the remaining capacity that is available in rechargeable
lithium-ion or lithium-polymer batteries. The accuracy of the fuel gauge is determined by the cell
characteristics and application parameters that are stored in EEPROM, as well as the accuracy of the
current readings.
Each device is factory calibrated to meet the data-sheet-specified current accuracy. However, the
current-measurement gain factor (RSGAIN) is also user accessible and can be reprogrammed to improve
current-measurement accuracy after module or pack manufacture. Users can adjust RSGAIN to correct
for variation in an external sense resistor's nominal value, thus allowing the use of low-cost, nonprecision
current-sense resistors.
Description
RSGAIN is a scaling factor that is used by the DS278x family of devices to accurately scale currents that
are measured to flow across the sense resistor. Each device measures the current and then multiplies
that value by the RSGAIN scaling factor to provide an accurate current measurement, which is reported
in the current register and accumulated in the Accumulated Current Register (ACR).
Reported Current (mA) = Measured Current (mA) × RSGAIN
(Eq. 1)
Figure 1 illustrates the RSGAIN Register format. RSGAIN is an 11-bit value stored in 2 bytes of the
Parameter EEPROM Memory Block. The RSGAIN Register value can be adjusted from 0 to 1.999 in
steps of 0.001 (precisely 2
-10
). The MSb has a value weight of 1 and the LSb has a value weight of
1/1024 (or 2
-10
).
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