are not used at all. Voltage measurements for all inputs are reported in a 16-bit format (in units of mV), as well as
32-bit format (in units of raw ADC counts), irrespective of whether they are used for cells or not.
10.1.3 Cell 1 Voltage Validation During SLEEP Mode
In rare cases, an invalid
Cell 1 Voltage()
reading has been observed to occur in some devices taken during
SLEEP mode.
While the device is in SLEEP mode, each result obtained from reading the
Cell 1 Voltage()
must be validated
before it can be considered valid. During SLEEP mode, current is below programmable thresholds, so the pack
is typically not being charged or discharged with any significant level of current. Thus, the cell voltages will
generally not be changing significantly.
In order to determine if a measurement of
Cell 1 Voltage()
taken during SLEEP mode is valid, it is necessary to
compare each measurement to measurements taken before and after the particular measurement. It is important
that these three readings represent three
separate measurements
for the
Cell 1 Voltage()
. If the reading is
significantly different from the separate readings taken before and after, then that reading is considered invalid
and should be discarded.
In order to ensure the three measurements read from the device are truly
separate measurements
, the host
can read the measurements at intervals exceeding
Power:Sleep:Voltage Time
while the device is in SLEEP
mode. This is necessary to avoid the host reading an existing measurement multiple times, before a new
measurement has been taken and is available for readout.
An invalid
Cell 1 Voltage()
reading may result in an SUV PF Alert being set but does not result in an SUV PF
status fault if the SUV Delay is set to 1 second or longer. It also does not trigger a Cell Undervoltage (CUV)
Protection alert or status fault, since this protection uses a comparator for its detection. If a reading reported by
Cell 1 Voltage()
is below the
Protections:CUV:Threshold
level and the CUV protection is enabled, but the CUV
Alert is not triggered, this also can be used as an indication the reading is invalid.
This validation process is necessary to ensure that valid
Cell 1 Voltage()
results are measured.
10.2 General Purpose ADCIN Functionality
Several multifunction pins on the BQ76942 device can be used for general purpose ADC input (ADCIN)
measurement, if not being used for other purposes. This includes the TS1, TS2, TS3, CFETOFF, DFETOFF,
HDQ, DCHG, DDSG, and ALERT pins. When used for ADCIN functionality, the internal bandgap reference is
used by the ADC, and the input range of the ADC is limited to the REG18 pin voltage. The digital fullscale range
of the ADC is effectively 1.6667 × VREF1, which is approximately 2.02 V during normal operation.
The BQ76942 device also reports the raw ADC counts when a measurement is taken using the TS1 pin. This
data can be used during manufacturing to better calibrate the ADCIN functionality.
10.3 Coulomb Counter and Digital Filters
The BQ76942 device monitors pack current using a low-side sense resistor that connects to the SRP and
SRN pins through an external RC filter, which should be connected such that a charging current will create
a positive voltage on SRP relative to SRN. The differential voltage between SRP and SRN is digitized by an
integrated coulomb counter ADC, which can digitize voltages over a ±200 mV range and uses multiple digital
filters to provide optimized measurement of the instantaneous, averaged, and integrated current. The device
supports a wide range of sense resistor values, with a larger value providing better resolution for the digitized
result. The maximum value of sense resistor should be limited to ensure the differential voltage remains within
the ±200-mV range for system operation when current measurement is desired. For example, a system with
maximum discharge current of 200 A during normal operation (not a fault condition) should limit the sense
resistor to 1 mΩ or below.
The SRP and SRN pins can also support higher positive voltages relative to VSS, such as may occur during
overcurrent or short circuit in discharge conditions, without damage to the device, although the current is not
accurately digitized in this case. For example, a system with a 1-mΩ sense resistor and the Short Circuit in
Discharge protection threshold programmed to a 500 mV level would trigger an SCD protection fault when a
discharge current of 500 A was detected.
SLUSE14B – DECEMBER 2020 – REVISED DECEMBER 2021
36
Copyright © 2021 Texas Instruments Incorporated
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