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BQ78350-R1A: LED displaying SOC

KIM
Intellectual 550 points
Part Number: BQ78350-R1A
Other Parts Discussed in Thread: BQ76930, , BQSTUDIO, BQ78350

Hi,

I am using the BQ76930 EVM.

I want to press the display button and show the battery SOC on the 5 LEDs.

Can you tell me how to do that (press the button and turn on LED showing SOC)?

Thanks!

  • 0
    TI__Guru 54715 points

    Hi Kim,

    For settings on the BQ78350-R1A, see the technical reference manual.

    The LED_EN is not set by default on the BQ78350-R1A.  Power the EVM at a voltage which will give an adequate SOC for display.  You can check the SOC values on the Registers screen in BQStudio.

    In BQStudio Data Memory, settings, Manufacturing, set the LED_EN bit.  See the technical reference manual  section 12.1.1 Manufacturing Status Configuration.  You can read the data memory to be sure your setting is stored.

    The LED_EN requires a reset to take effect. Send a reset to the part with the RESET command or cycle power and boot the board.

    Press the display button.

  • 0 in reply to WM5295
    Intellectual 550 points

    Hi, refering to bq78350evm, there is no resistor to limit the led current. 

    can i know how the Led current Is limited bu BQ78350 led pins?

  • 0 in reply to KIM
    TI__Guru 54715 points

    Hi Kim,

    The LED pins are current sink outputs shown in the electrical characteristics table with VOL at 3 mA, https://www.ti.com/document-viewer/BQ78350-R1A/datasheet/electrical-characteristics-i-o-slusb487225#SLUSB487225 

    Figure 4 in the typical characteristics https://www.ti.com/document-viewer/BQ78350-R1A/datasheet/typical-characteristics-slusas31503#slusas31503 shows the typical sink current from the pins.

  • 0 in reply to WM5295
    Intellectual 550 points

    Hi,

    Can i say that the maximum sink current of LEDx is 3mA? I need to use 3 LEDs. Need to calculate the power dissipation of BJT (see if can use smaller BJT).

    Also, can i connect the BJT collector to other cell voltage e.g. C5 instead of C10? to have lower voltage to reduce the power dissipation of BJT? Please refer to figure below.

    On the EVM schematic, what is the low voltage and low current operation that allow us to use the smaller BJT?

    Do we need to place a resistor at BJT collector?

    Thanks!

  • 0 in reply to KIM
    TI__Guru 54715 points

    Hi Kim,

    The BQ78350-R1A data sheet does not give a tolerance on the 3 mA sink.  You might provide a good margin for variation, perhaps 4 mA maximum per output.

    Connecting the current path for the collector provides the LED current from the top of the pack so that it does not imbalance the pack.  This might be more of a consideration if the LEDs are on for some time. 

    The voltage would generally be how many cells are used and the current how many LEDs.  For 6 cells the voltage would be 60% of a 10 cell design.  With only 3 LEDs the nominal current when all are on would be 9 mA vs. 15 mA if all LEDs were used.  The power for a 6 cell 3 LED system should be only about 36% of a 10 cell 5 LED design.

    The resistor in the collector dissipates some power at the resistor which would otherwise be in the transistor.  It would not be necessary but will allow spreading the heat on the board and may allow a lower power transistor.    Of course if you have a 3.3V or similar from a switching supply for other circuitry in the battery you could use that for the LED power and avoid the transistor and its dissipation.

  • 0 in reply to WM5295
    Intellectual 550 points

    Hi,

    Can we use the BQ76930 CAP1 3.3V supply to power the 3 LEDs?

    Or it is not advisable to do so as it will increase the power dissipation of BQ76930?

    Thanks!

  • 0 in reply to KIM
    TI__Guru 54715 points

    Hi Kim,

    CAP1 is not specified to provide any current external.  The base current for the NPN is negligible and is ok.  Trying to drive LEDs with CAP1 would affect the power in the IC as you note, it could cause imbalance of the pack and malfunction of the measurement system.

  • 0 in reply to WM5295
    Intellectual 550 points

    Hi,

    We only want to use 1 LED for SOC and RCA alarm indication.

    We shall set the LED Thresh 1 to 0% and LED thresh 2,3,4,5 to 100%, right?

    Thanks!

  • 0 in reply to KIM
    TI__Guru 54715 points

    Hi Kim, 

    Yes, that sounds right.  Always test settings to be sure they do what you intend.