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bq78350-R1

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SLUSCD0B – AUGUST 2015 – REVISED NOVEMBER 2018

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Copyright © 2015–2018, Texas Instruments Incorporated

9.2.3.1 Measurement System

9.2.3.1.1

Cell Voltages

The bq78350-R1 device is required to be configured in the AFE Cell Map register to determine which cells to

measure based on the physical connections to the bq76920 AFE. The cell voltage data is available through

CellVoltage1()…CellVoltage5(). The cell voltages are reported as they are physically stacked. For example, if the

device is configured for 3-series cells connected to VC1, VC2, and VC5 per the AFE Cell Map, then the cell

voltages are still reported via CellVoltage1(), CellVoltage2(), and CellVoltage3(), respectively.

For improved accuracy, offset calibration is available for each of these values and can be managed through the

bqStudio tool. The procedure for calibration is described in the bq78350-R1 Technical Reference Manual

(SLUUBD3) in the Calibration chapter.

9.2.3.1.2

External Average Cell Voltage

This is enabled by default (DA Configuration [ExtAveEN] = 1) and uses the external resistor divider connected

to the VEN and BAT pins to determine the average cell voltage of the battery pack. The average cell voltage is

available through ExtAveCellVoltage().

CAUTION

Care should be taken in the selection of the resistor and FETs used in this divider

circuit as the tolerance and temperature drift of these components can cause

increased measurement error and a gas gauging error if CEDV Gauging Config

[ExtAveCell] = 1 (default = 1).

For improved accuracy, offset and gain calibration is available for this value and can be managed through the

bqStudio tool. The procedure for calibration is described in the bq78350-R1 Technical Reference Manual

(SLUUBD3) in the Calibration chapter.

9.2.3.1.3

Current

Current data is taken from the bq76920 and made available through Current(). The selection of the current sense

resistor connected to SRP and SRN of the bq76920 is very important and there are several factors involved.

The aim of the sense resistor selection is to use the widest ADC input voltage range possible.

To maximize accuracy, the sense resistor value should be calculated based on the following formula:

RSNS(min) = V(SRP) – V(SRN) / I(max)

(1)

Where: |V(SRP) – V(SRN)| = 200 mV

I(max) = Maximum magnitude of charge of discharge current (transient or DC)

NOTE

RSNS(min) should include tolerance, temperature drift over the application temperature,

and PCB layout tolerances when selecting the actual nominal resistor value.

When selecting the RSNS value, be aware that when selecting a small value, for example,

1 mΩ, then the resolution of the current measurement will be > 1 mA. In the example of

RSNS = 1 mΩ, the current LSB will be 8.44 mA.

For improved accuracy, offset and gain calibration are available for this value and can be managed through the

bqStudio tool. The procedure for calibration is described in the bq78350-R1 Technical Reference Manual

(SLUUBD3) in the Calibration chapter.

9.2.3.1.4

Temperature

By default, the 78350 uses an external negative temperature coefficient (NTC) thermistor connected to the

bq76920 as the source for the Temperature() data. The measurement uses a polynomial expression to transform

the bq76920 ADC measurement into 0.1°C resolution temperature measurement. The default polynomial

coefficients are calculated using the Semitec 103AT, although other resistances and manufacturers can be used.