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BQ76952
SLUSE13A – JANUARY 2020 – REVISED MAY 2021
10 Measurement Subsystem
10.1 Voltage Measurement
The BQ76952 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 VC16 pin, the PACK pin, the LD pin, the internal REG18 LDO voltage, and the VSS rail
(for diagnostic purposes). The BQ76952 device supports measurement of individual differential cell voltages
in a series configuration, ranging from 3-series cells to 16-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 6.06 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 BQ76952 voltage measurements are taken in a measurement loop that consists of multiple measurement
slots. All 16 cell voltages are measured on each loop, then one slot is used for one of the VC16 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 Usage of VC Pins for Cells Versus Interconnect
If the BQ76952 device is used in a system with fewer than 16-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, such as shown in
between CELL-A and CELL-B. By connecting VC12 close to the positive terminal of CELL-B, and connecting
VC13 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.
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.
36
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BQ76952
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Figure 10-1
Copyright © 2021 Texas Instruments Incorporated
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