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BQ29312_15 Datasheet, PDF (11/39 Pages) Texas Instruments – FOUR CELL LITHIUM-ION
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bq29312
SLUS546E – MARCH 2003 – REVISED MARCH 2005
FUNCTIONAL DESCRIPTION (continued)
Cell Voltage
The cell voltage is translated to allow a system host to measure individual series elements of the battery. The
series element voltage is translated to a GND-based voltage equal to 0.15 ±0.002 of the series element voltage.
This provides a range from 0 V to 4.5 V. The translation output is inversely proportional to the input using the
following equation.
Where, V(CELL OUT) = –K × V(CELL IN) + 0.975 (V)
Programming CELL_SEL (b1, b0) selects the individual series element. The CELL_SEL (b3, b2) selects the
voltage monitor mode, cell monitor, offset etc.
Calibration of Cell Voltage Monitor Amplifier Gain
The cell voltage monitor amplifier has an offset and to increase accuracy this can be calibrated.
There are a couple of method by calibration circumstance.
The following procedure shows how to measure and calculate the offset and gain as one of example.
• Step 1
– Set CAL1=1, CAL0=1, CELL1=0, CELL0=0, VMEN=1
– VREF is trimmed to 0.975 V within ±1%, measuring VREF eliminates its error.
– Measure internal reference voltage VREF from VCELL directly.
– VREF=measured reference voltage
• Step 2
– Set CAL1=0, CAL0=0, CELL1=0, CELL0=0, VMEN=1
– The output voltage includes the offset and represented by:
VO(4-5) = VREF + (1 + K) × VOS (V)
Where K = CELL Scaling Factor
– VOS = Offset voltage at input of the internal operational-amplifier
• Step 3
– Set CAL1=1, CAL0=0, CELL1=0, CELL0=0, VMEN=1
– Measuring scaled REF voltage through VCELL amp.
– The output voltage includes the scale factor error and offset and is represented by:
V(OUTR) = VREF + (1 + K) × VOS– K × VREF (V)
• Step 4
– Calculate (VO(4-5)– V(OUTR)/VREF
– The result is the actual scaling factor, K(ACT) and is represented by:
K(ACT) = (VO(4-5)– V(OUTR))/VREF = (VREF + (1 + K) × VOS) - (VREF + (1 + K) × VOS– K × VREF)/ VREF = K ×
VREF/VREF = K
• Step 5
– Calculate the actual offset value where:
VOS(ACT) = (VO(4-5)– VREF)/(1 + K(ACT))
• Step 6
– Calibrated cell voltage is calculated by:
VCn – VC(n+1) = {VREF + (1 + K(ACT) ) × VOS(ACT)– V(CELLOUT)}/K(ACT) – {VO(4-5)– V(CELLOUT)}/K(ACT)
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