10 Measurement Subsystem
10.1 Voltage Measurement
The BQ76942 device integrates a voltage ADC that is multiplexed between measurements of cell voltages,
an internal temperature sensor, and up to nine external thermistors, as well as performs measurements of
the voltage at the VC10 pin, the PACK pin, the LD pin, the internal REG18 LDO voltage, and the VSS rail
(for diagnostic purposes). The BQ76942 device supports measurements of individual differential cell voltages
in a series configuration, ranging from 3-series cells to 10-series cells. Each cell voltage measurement is a
differential measurement of the voltage between two adjacent cell input pins, such as VC1–VC0, VC2–VC1,
and so forth. The cell voltage measurements are processed based on trim and calibration corrections, and then
reported in a 16-bit resolution using units of 1 mV. The raw 24-bit digital output of the ADC is also available for
readout using 32-bit subcommands. The cell voltage measurements can support a recommended voltage range
from –0.2 V to 5.5 V. The voltage ADC saturates at a level of 5 × V
REF1
(approximately 6.06 V) when measuring
cell voltages, although for the best performance, it is recommended to stay at a maximum input of 5.5 V.
10.1.1 Voltage Measurement Schedule
The BQ76942 device's voltage measurements are taken in a measurement loop that consists of multiple
measurement slots. All 10 cell voltages are measured on each loop, then one slot is used for one of the VC10
or PACK or LD pin voltages, one slot is used for internal temperature or V
REF
or VSS measurement, then up to
three slots are used to measure thermistors or multifunction pin voltages (ADCIN functionality). Over the course
of three loops, a full set of measurements is completed. One measurement loop consists of either 12 (if no
thermistors or ADCIN are enabled), 13 (if one thermistor or ADCIN is enabled), 14 (if two thermistors or ADCIN
are enabled), or 15 (if three or more thermistors or ADCIN are enabled) measurement slots.
The speed of a measurement loop can be controlled by settings. Each voltage measurement (slot) takes 3 ms
(or 1.5 ms depending on setting), so a typical measurement loop with 15 slots per loop takes 45 ms (or 22.5 ms
depending on setting). If measurement data is not required as quickly, the timing for the measurement loop can
be programmed to slower speeds, which injects idle slots in each loop after the measurement slots. Using slower
loop cycle time will reduce the power dissipation of the device when in NORMAL mode.
10.1.2 Usage of VC Pins for Cells Versus Interconnect
If the BQ76942 device is used in a system with fewer than 10-series cells, the additional cell inputs can be
used to improve measurement performance. For example, a long connection may exist between two cells in a
pack, such that there may be significant interconnect resistance between the cells, such as shown in
between CELL-A and CELL-B. By connecting VC7 close to the positive terminal of CELL-B, and connecting VC8
close to the negative terminal of CELL-A, more accurate cell voltage measurements are obtained for CELL-A
and CELL-B, since the I·R voltage across the interconnect resistance between the cells is not included in either
cell voltage measurement. Because the device reports the voltage across the interconnect resistance and the
synchronized current, the resistance of the interconnect between CELL-A and CELL-B can also be calculated
and monitored during operation. It is recommended to include the series resistance and bypass capacitor on cell
inputs connected in this manner, as shown below.
Note
It is important that the differential input for each cell input not fall below –0.3 V (the absolute maximum
data sheet limit), with the recommended minimum voltage of –0.2 V. Therefore, it is important that the
I·R voltage drop across the interconnect resistance not cause a violation of this requirement.
SLUSE14B – DECEMBER 2020 – REVISED DECEMBER 2021
34
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