Datasheet

Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
bq77PL900
SLUS844B – JUNE 2008 – REVISED JANUARY 2009 ...................................................................................................................................................... www.ti.com
Set CAL2 = 0, CAL1 = 0, CAL0 = 1, CELL[4:1] = 0, VAEN = 1
Measure the output voltage of the differential amplifier at 0-V input (both inputs of the differential amplifier are
connected to GND). The output voltage includes the offset and is represented by:
Vd
OUT
(0V) = measured output voltage of differential amplifier at 0-V input
(This value includes an offset voltage (V
OS
) and a reference voltage.)
Set CAL2 = 0, CAL1 = 1, CAL0 = 1, VAEN = 1
VREF is trimmed to 0.975 V or 1.2 V within ± 2%. Then measure internal reference voltage VREF directly from
VOUT:
VREF_m = measured reference voltage (0.975 V or 1.2 V)
Set CAL2 = 0, CAL1 = 1, CAL0 = 0, CELL[4:1] = 0, VAEN = 1
Measure the scaled REF voltage through the differential amplifier.
Vd
OUT
(VREF_m) = The output voltage, including the scale factor error and offset
= VREF + (1 + K) × VOS – K × VREF
= VREF_m + (1 + Kd
ACT
) × V
OS
– Kd
ACT
× VREF_m
where: VREF_m + (1 + Kd
ACT
) × V
OS
= Vd
OUT
(0V)
Kd
ACT
= ( Vd
OUT
(0V) – Vd
OUT
(VREF_m)) / VREF_m
= (measured value at step 1 – measured value at step 3)/ measured value at step 2
Calibrated differential voltage is calculated by:
Vdout = VREF + (1 + K) × V
OS
– K × Vdin
= Vd
OUT
(0V) – Kd
ACT
× Vdin
Where: Vdin = input voltage of differential amp lifier
Set CAL2 = 1, CAL1 = 0, CAL0 = 0, CELL[4:1] = 0, VAEN = 1
Measure scaled REF(2.5V) though differential amp,
Some TI-Benchmarq gas gauges cannot measure 2.5 V directly, because the ADC input voltage is 1 V. So to
measure the 2.5-V internal reference voltage, use a differential amplifier as a method to scale down the
measurement value.
Vdout(2.5V) = measured differential amp output voltage at the 2.5-V input
Already, differential amplifier calibration was performed in steps 1, 2, and 3.
So VREF_2.5V is presented by
VREF_2.5V = { Vd
OUT
(0V) – Vdout(2.5V)}/Kd
ACT
Set CAL2 = 1, CAL1 = 0, CAL0 = 1, CELL2 = 0, CELL1 = 0, VAEN = 1
Vout(0.975V or 1.2V) = Measure scaled REF (0.975-V or 1.2-V) output voltage S/H and differential amplifier.
Set CAL2 = 1, CAL1 = 1, CAL0 = 0, CELL[4:1] = 0, VAEN = 1
Vout(2.5V) = Measure scaled REF (2.5-V) output voltage S/H and differential amp.
Scale factor
K
ACT
= – (V
OUT
(2.5V) – V
OUT
(0.975V or 1.2V)/(VREF_2.5V – VREF_m)
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