Current accumulation – Rainbow Electronics DS2784 User Manual
Page 15
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CURRENT MEASUREMENT CALIBRATION
The DS2784’s current measurement gain can be adjusted through the RSGAIN register, which is factory calibrated
to meet the data sheet-specified accuracy. RSGAIN is user accessible and can be reprogrammed after module or
pack manufacture to improve the current measurement accuracy. Adjusting RSGAIN can correct for variation in an
external sense resistor’s nominal value, and allows the use of low-cost, nonprecision, 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 reprogrammable, and FRSGAIN, which is read only.
RSGAIN determines the gain used in the current measurement. The FRSGAIN value is provided to preserve the
factory calibration value only and is not used to calibrate the current measurement. The 16-bit FRSGAIN value is
readable from addresses B0h and B1h.
CURRENT MEASUREMENT GAIN REGISTER FORMAT
MSB—ADDRESS
78h
LSB—ADDRESS
79h
X X X X X 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: 2
-10
SENSE RESISTOR TEMPERATURE COMPENSATION
The DS2784 can of temperature compensate the current-sense resistor to correct for variation in a sense resistor’s
value overtemperature. The DS2784 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
o
C boundaries. The
temperature compensation is most effective with the resistor placed as close as possible to the V
SS
terminal. This
optimizes thermal coupling of the resistor to the on-chip temperature sensor.
SENSE RESISTOR TEMPERATURE COMPENSATION REGISTER FORMAT
ADDRESS
7Ah
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb
LSb
Units: 30.5ppm/ºC
CURRENT ACCUMULATION
Current measurements are internally summed, or accumulated, at the completion of each conversion period and
the results are stored in the 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 6.25
μVh. Additional registers hold
fractional 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; conversely,
accumulation of discharge current below the minimum register value is reported at the minimum value.
Charge currents (positive current register values) less than 100
μV 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 R
SNS
= 0.020
Ω and 20mA for R
SNS
= 0.005
Ω.