BQ29312A Datasheet, PDF (11/35 Pages) Texas Instruments – TWO-CELL, THREE-CELL, AND FOUR-CELL LITHIUM-ION OR LITHIUM-POLYMER BATTERY PROTECTION AFE

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bq29312A
SLUS629A – JANUARY 2005 – REVISED AUGUST 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 methods by calibration circumstance.
The following procedure shows how to measure and calculate the offset and gain as an example.
• Step 1
– Set CAL1=1, CAL0=1, CELL1=0, CELL0=0, VMEN=1
– V_REF is trimmed to 0.975 V within ±1%, measuring V_REF eliminates its error.
– Measure internal reference voltage V_REF from VCELL directly.
– VREF=measured reference voltage
• Step 2
– Set CAL1=0, CAL0=0, CELL1=0, CELL0=1, VMEN=1
– The output voltage includes the offset and represented by:
V_O(4-5) = V_REF + (1 + K) × V_OS (V)
Where K = CELL Scaling Factor
– V_OS = 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 amplifier.
– The output voltage includes the scale factor error and offset and is represented by:
V_(OUTR) = V_REF + (1 + K) × V_OS– K × V_REF (V)
• Step 4
– Calculate (V_O(4-5)– V_(OUTR)/V_REF
– The result is the actual scaling factor, K_(ACT), and is represented by:
K_(ACT) = (V_O(4-5)– V_(OUTR))/V_REF = (V_REF + (1 + K) × V_OS) - (V_REF + (1 + K) × V_OS– K × V_REF)/ V_REF = K ×
V_REF/V_REF = K
• Step 5
– Calculate the actual offset value where:
V_OS(ACT) = (V_O(4-5)– V_REF)/(1 + K_(ACT))
• Step 6
– Calibrated cell voltage is calculated by:
VCn – VC(n+1) = {V_REF + (1 + K_(ACT) ) × V_OS(ACT) – V_(CELLOUT)}/K_(ACT) – {V_O(4-5) – V_(CELLOUT)}/K_(ACT)
For improved measurement accuracy, V_OS(ACT) for each cell voltage should be measured.
• Set CAL1=0, CAL0=0, CELL1=0, CELL0=1, VMEN=1
• Set CAL1=0, CAL0=0, CELL1=1, CELL0=0, VMEN=1
• Set CAL1=0, CAL0=0, CELL1=1, CELL0=1, VMEN=1
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