11
SLUUBD3D – September 2015 – Revised September 2018
Copyright © 2015–2018, Texas Instruments Incorporated
Basic Measurement System
Chapter 2
SLUUBD3D – September 2015 – Revised September 2018
Basic Measurement System
2.1
Introduction
NOTE:
For this section, refer to the
bq769x0 3-Series to 15-Series Cell Battery Monitor Family for Li-
Ion and Phosphate Applications Data Manual
(
) for further details.
The bq78350-R1 reads the bq769x0 companion AFE registers that contain recent values from the
integrating analog-to-digital converter (ADC) for current measurement, and a second delta-sigma ADC for
individual cell and temperature measurements. The bq78350-R1 also has the capability to measure the
battery voltage through an externally translated voltage.
2.2
Current and Coulomb Counting
The integrating delta-sigma ADC in the companion bq769x0 AFE measures the charge/discharge flow of
the battery by measuring the voltage drop across a small-value sense resistor between the SRP and SRN
pins. The 15-bit integrating ADC measures bipolar signals from –0.20 V to 0.20 V with 15-µV resolution.
The AFE reports charge activity when VSR = V
(SRP)
– V
(SRN)
is positive, and discharge activity when VSR =
V
(SRP)
– V
(SRN)
is negative. The bq78350-R1 continuously monitors the measured current and integrates the
digital signal from the AFE over time using an internal counter.
To support large battery configurations, the current data can be scaled to ensure accurate reporting
through the SMBus. The data reported is scaled based on the setting of the
SpecificationInfo()
command.
The data reported through the
Current()
can also have a deadband applied to it. This removes any noise
or offset that has not been calibrated out from being reported as real current. This value is programmed in
Deadband
with a default configured for mA scaling in
SpecificationInfo()
. If the
SpecificationInfo()
IPSCALE is set to 10 mA or 100 mA, then it is strongly recommended to set
Deadband
to 1.
2.3
Voltage
The bq78350-R1 updates the individual series cell voltages through the bq769x0 at 250-ms intervals. The
bq78350-R1 configures the bq769x0 to connect to the selected cells in sequence and uses this
information for cell balancing and individual cell fault functions. The internal 14-bit ADC of the bq769x0
measures each cell voltage value, which is then communicated digitally to the bq78350-R1 where it is
scaled and translated into unit millivolts. The maximum supported input range of the ADC is 6.075 V.
The bq78350-R1 also separately measures the average cell voltage through an external translation circuit
at the BAT pin. This value is specifically used for the gas gauge algorithm. The external translation circuit
is controlled via the VEN pin so that the translation circuit is only enabled when required to reduce overall
power consumption. VEN requires an external pullup to VCC, typically 100 k, to operate correctly.
In addition to the voltage measurements used by the bq78350-R1 algorithms, there is an optional auxiliary
voltage measurement capability via the VAUX pin. This feature measures the input on a 250-ms update
rate and provides the programmable scaled value through the
VAUXVoltage()
SMBus command. The data
can be enabled to influence selected fault recovery features. See
General Protections Configuration
[VAUXR]
, for further details.
The VEN pin will go high 2 ms prior to the BAT being measured if
DA Configuration [ExtAveEN]
= 1,
and then return low unless
DA Configuration [VAUXEN]
= 1, which will cause VEN to remain high for a
further 2 ms prior to making the VAUX measurement. This results in VEN possibly being high for up to
40 ms per second in NORMAL mode.
