10 Measurement Subsystem
10.1 Voltage Measurement
The BQ769142 device integrates a voltage ADC, which is multiplexed between measurements of cell voltages,
an internal temperature sensor, and up to nine external thermistors, and also performs measurements of the
voltage at the VC14 pin, the PACK pin, the LD pin, the internal REG18 LDO voltage, and the VSS rail (for
diagnostic purposes). The BQ769142 device supports measuring individual differential cell voltages in a series
configuration, ranging from 3-series cells to 14-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
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 × VREF1 (approximately 6.25 V) when measuring cell voltages,
although for best performance, it is recommended to stay at a maximum input of 5.5 V.
10.1.1 Voltage Measurement Schedule
The BQ769142 voltage measurements are taken in a measurement loop that consists of multiple measurement
slots. All 14-cell voltages are measured on each loop, then one slot is used for one of the VC14 or PACK or LD
pin voltages, one slot is used for internal temperature or Vref 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 18 (if no thermistors or
ADCIN are enabled), 19 (if one thermistor or ADCIN is enabled), 20 (if two thermistors or ADCIN are enabled),
or 21 (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 21 slots per loop takes 63 ms (or 31.5 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 Using VC Pins for Cells Versus Interconnect
If the BQ769142 device is used in a system with fewer than 14-series cells, the additional cell inputs can be
utilized 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, as shown in
Pins for Interconnect Measurement
, between CELL-A and CELL-B. By connecting VC10 close to the positive
terminal of CELL-B, and connecting VC11 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. Since 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 does not cause a violation of this requirement.
SLUSE91A – SEPTEMBER 2020 – REVISED FEBRUARY 2021
34
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