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BQ25504EVM-674: BQ25504EVM-674 , question about initial startup, UV resistor adjustment and MPPT%

LzBone
Intellectual 770 points
Part Number: BQ25504EVM-674
Other Parts Discussed in Thread: CC2540, BQ25504

Hi, 

I am new to this evaluation board. I would like to ask following questions.

Our setup as follows:
Input : ~1V DC source to BQ25504EVM-674
Battery : More than UV
Load : CC2540 MCU

Q1. As per BQ25504EVM user guide (page 8) Figure 5, does it means, once EVM has input power, it will start to provide power to MCU (Vstor), at the same time, starting to charge the battery (Vbat)? If that the case, will it cause unintended behaviours to MCU, due to low voltage from (Vstor) at initial start-up ?

--------------------------------------------------------------------------
Q2. As for under voltage (UV) adjustment, we have to add variable resistor at R18 right.
However, by adding variable resistor, it will be in parallel with 0 ohm resistor (R15). Any resistor in parallel with 0 ohm resistor will be almost zero too.
So, how does it works?

--------------------------------------------------------------------------
Q3. For wireless energy harvesting application, do you have any recommended MPPT % ?

Thanks a lot.

Regards

  • 0
    TI__Intellectual 2050 points

    LzBone said:


    Q1. As per BQ25504EVM user guide (page 8) Figure 5, does it means, once EVM has input power, it will start to provide power to MCU (Vstor), at the same time, starting to charge the battery (Vbat)? If that the case, will it cause unintended behaviours to MCU, due to low voltage from (Vstor) at initial start-up ?

    VSTOR will ramp up as soon as sufficient input power is present, this also means that the MCU will see the voltage on VSTOR. However, the battery only begins to charge once VSTOR rises above the VBAT_UV threshold.

    What is the supply voltage for your MCU? The only unintended behaviour I can think of is the MCU turning on and off. As soon as VSTOR rises above VBAT_UV, VSTOR and VBAT are internally connected and the battery can help supply the load if the input is not sufficient enough.

    LzBone said:

    --------------------------------------------------------------------------
    Q2. As for under voltage (UV) adjustment, we have to add variable resistor at R18 right.
    However, by adding variable resistor, it will be in parallel with 0 ohm resistor (R15). Any resistor in parallel with 0 ohm resistor will be almost zero too.
    So, how does it works?

    Yes you are right and for this you will want to depopulate the 0 ohm resistor. The zero ohm is there to provide a path to ground if R18 is not populated.

    LzBone said:

    --------------------------------------------------------------------------
    Q3. For wireless energy harvesting application, do you have any recommended MPPT % ?

    Can you share more info on your wireless harvester?

    You will want to characterize your harvester and determine the I vs. V behavior. Once this is known, we can determine where the maximum power point is.

    -Joe

  • 0 in reply to Joe Stephan
    Intellectual 770 points

    Hi Joe,

    Thanks a lot for your reply.

    As for my MCU, it needs about 2.6V and 0.01A for the operation I need to perform.

    As for wireless harvesting, I will look further into I vs V behaviour as per your suggestion.

    I still have further questions about this PM module that I would like to verify. They are as follows:

    --------------------------------------------------------------------------

    Question 1:

    Can I measure VBAT_OV at TP6?

    --------------------------------------------------------------------------

    Question 2:

    For OV adjustment, it is controlled by following formula right, then, may I know what is VBIAS voltage?

    I have seen the "bq25504_Design_Help_V1_2.xlsx" excel file. Inside that file, VBIAS is used as 1.25V.

    The reason I ask is because, yesterday, I have desoldered R14 (0 ohm resistor) and added 500k variable resistor for R13. So now, my (ROV2 = R4) and my (ROV1 = R3 + R13). Even though I adjust variable resistor R13 value, VBAT_OV seems to stay only at 1.36V only. I measure VBAT_OV at TP6. And I also made sure that R13 resistor legs are correctly connected to gnd and one side of R3. So I am not sure why VBAT_OV doesn't move. Is it because my VBIAS is too low or am I missing something?

     

    Thank you.

    Regards,

    LB

  • 0 in reply to LzBone
    TI__Intellectual 2050 points
    LB,

    Q1: Yes you can measure the VBAT_OV reference threshold at TP6.

    Q2: VBIAS is typically 1.25V and is internal to the device. Is the 1.36V what you are reading on TP6? You need to keep in mind that this is a divided down voltage from the VRDIV pin. The voltage is used to compare to the internal VBIAS refernce voltage. Based on the comparison the IC will set the VBAT_OV voltage, which is also what VSTOR and VBAT will be regulated to.

    So if you want to measure the VBAT_OV voltage and not reference threshold, measure VSTOR.

    -Joe