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BQ25570: MPPT reference selection

Kawsar Ali
Intellectual 320 points
Part Number: BQ25570
Other Parts Discussed in Thread: MSP430F4270, , TS5A4624, TPS22860

I am trying to do an external MPPT in the bq25570 EVM using the DAC module of MSP430F4270 microcontroller. To switch between the MPP reference source I am using the TS5A4624 switch in the configuration as shown in the figure.

The idea is to not do the MCU-assisted MPPT until the VBAT (CBAT > 30uF) bus has been charged to say 3V. However, since we know that the 16-second-periodic-sampling cannot be disabled in bq25570, I see bad interference on my VREF signal. It is randomly over-riding the DAC-generated VREF and going to 80% MPPT. I am guessing this could be because the NC pin in my TS5A4624 switch is floating.

Any suggestion on what to connect NC pin? or, do you see any other obvious issue here? Thanks!

  • 0
    Intellectual 320 points

    Oops! Sorry, I swapped NC and NO pins in the figure. This is the correct figure:

    And accordingly, the question should be: What do I connect NO pin to?

  • 0 in reply to Kawsar Ali
    TI__Guru*** 148885 points

    Hi Kawsar,

    I support the BQ25570 not the TS5A4624.  If you remove the switch and connect the DAC (I'm assuming the 80% at NC) directly to VOC_SAMP, do you get the result that you want? Is VOC_SAMP floating or is the resistor divider there. I am not certain that this configuration will work. If your DAC has some drive capability, you might want to connect its output directly to VREF_SAMP.  I have tested that and it works.

    I will forward your question about the TS5A4624 to the correct group.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 320 points

    Thank you Jeff.

    I think I need to make my setup a bit more clear. Please see the figure below.

    As you said, DAC is connected to VREF_SAMP. I have put the switch TPS22860 in between, because it's a power-saving application. What do you think the problem is, now?

    Best regards,

    Kawsar

  • 0 in reply to Kawsar Ali
    TI__Guru*** 148885 points

    Hi  Kawsar,

    I apologize for the late followup.  I do not understand the purpose of the TS5A624.  If you are driving the VREF_SAMP capacitor to a voltage that is higher the MPP being set external resistors and the DAC has enough drive, I do not see the need for TS5A624.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 320 points

    Thanks for the followup Jeff!

    I am not trying to drive CREF to a voltage higher than 80% or 50%, or voltage set by a resistive divider. My aim is to do a more dynamic MPPT (as opposed to fractional-open-circuit-voltage MPPT).

    Starting from cold-start phase until VSTOR (and VBAT) reaches 3V, the MPPT reference is given by 80% of VOC. At VSTOR=3V the MCU is turned ON by the help of VBATOK and takes over the MPPT reference setting. This change over is done using TS5A4624.

    Also, I am trying to save power, and therefore not leaving the DAC ON. It periodically sets the VREF in a few milliseconds and turns OFF. Now, to prevent the discharge of CREF, I am using TPS22860 as shown in the last figure.

    I hope this setup makes sense.

    To repeat, the problem is that I see bad interference on my VREF signal. It is randomly over-riding the DAC-generated VREF and going to 80% MPPT. I am guessing this could be because the NO pin in my TS5A4624 switch is floating.

    Best regards,

    Kawsar

  • 0 in reply to Kawsar Ali
    TI__Guru*** 148885 points

    Kawsar,

    The IC itself automatically updates the MPP on CREF every 16s.  If you aren't synced with that, and I can't think of a way for you to be, it seems to me that every 16s there will be interference, unless the voltage source that you using to drive CREF is stronger than the internal driver (which is very weak).

    Regards,

    Jeff