Datasheet

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Cell Voltage
Calibration of Cell Voltage Monitor Amplifier Gain
bq29312A
SLUS629A – JANUARY 2005 – REVISED AUGUST 2005
FUNCTIONAL DESCRIPTION (continued)
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.
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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