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BQ25570: VOUT_EN never high/Output always at ~500mV

Felix Mark
Prodigy 10 points
Part Number: BQ25570

Hey there!
I am having trouble with my BQ25570 based PCB.
The charging of my super capacitor works fine, the digital part of the circuit after the BQ25570 works fine as well on it's own, but it never gets powered by the buck converter.
I have tied VBAT_OK to VOUT_EN to enable the buck converter, when the Voltage of the supercap is high enough.
But no matter the voltage of the supercap, the buck converter never outputs 1.8V, but always around 500mV.
Furthermore I feel like the Supercap Voltage is decreasing really fast, until it reaches my VBAT_UV.
I append the schematic and also pictures of my soldering job. (Also under a microscope if needed)
I hope you can help.
With kind regards,
Felix

  • 0
    TI__Guru*** 143980 points

    HI Felix,

    Unless I misread or miscalculated, I see VBAT_OV=5.5V, VBAT_OK range = 2.5-1.8V and VOUT=1.8VV?  VBAT_OK range down to 1.8V is not helpful since the IC doesn't exit cold start until VSTOR > 1.9V.  Also, with VBAT_OK = 1.8V, the buck converter does not function as intended with about 200mV of headroom.  I recommend setting VBAT_OK = VOUT+200mV and VOUT_OK_HYST higher than that.  

    Is there a square-ish waveform at LBUCK pin when VSTOR>1.8V+200mV?  If not, then there could be a solder short that is causing a problem.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff,

    that is good to know, I thought that VBAT_OK could go down to V_OUT!
    I will try setting VBAT_OK to 2V.
    These were my settings up until now:

    VBAT_OV = 5.5V
    VBAT_OK = 1.8V
    VBAT_OK_HYST = 2.5V
    VOUT = 1.8V

    Thank you so much and I will keep you updated!
    Felix

  • 0 in reply to Felix Mark
    Prodigy 10 points

    Hey Jeff Slight smile

    I now removed the old resistors and soldered on two new ones to have:
    VBAT_OV = 5.5V
    VBAT_OK = 2V
    VBAT_OK_HYST = 2.5V
    VOUT = 1.8V

    I took better pictures of the PCB as well.
    If I power the ATTiny directly with 1.8V it works as intended but I guess the Buck Converter of the BQ25570 never gets enabled or something, since I can only read the 500-600mV at the output. Unfortunately I don't have an oscilloscope at home to measure the output and see, if it has repeated cold-start issues.
    I am not 100% sure if the VBAT_EN pin is soldered on correctly, even though it looks okay. Would this cause this behaviour as well?
    Here are the pictures I took (I can also send microscope pictures if needed) as well as a video of the externally powered ATTiny.


    With kind regards,
    Felix

  • 0 in reply to Felix Mark
    TI__Guru*** 143980 points

    Hi Felix,

    Can you use an ohmmeter to measure the impedance of VOUT to GND) when no power is applied to the board?  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hey Jeff,

    I did and the resistance from VOUT to GND is around 19MOhm sinking down slowly to 13MOhm and very slowly further.

    Regards,
    Felix

  • 0 in reply to Felix Mark
    TI__Guru*** 143980 points

    Hi Felix,

    Does VOUT_EN go high?  Even if VOUT_EN is high, I would not expect the buck to output 500mV.  The buck cannot regulate below 1.2xV so it is not the known issue about the parasitic resistors created by residual solder flux.

    Can you order a BQ25570EVM, reconfig the resistors and connect to the load?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hey Jeff,

    I measured and VBAT_OK, which is directly connected to VOUT_EN never goes high, even when VBAT = 3V which is greater than VBAT_OK..
    I will resolder the BQ25570, since a test setup with it has worked as intended, so I think a soldering error might be the cause.

    Regards, Felix

  • 0 in reply to Felix Mark
    TI__Guru*** 143980 points

    Hi Felix,

    If BAT_OK is not tripping at the correct voltage, I suggest removing OK resistors (assuming they are sized correctly), cleaning the board and resoldering the resistors.  

    Regards,

    Jeff

  • +1 in reply to Jeff F
    Prodigy 10 points

    Hey Jeff,

    I resoldered the resistors as well as the BQ25570 and everything seems to be working except for one thing, which is that VOUT is working fine for some seconds but then it suddenly collapses for just a really small timeframe, before being stable again. In this timeframe the uC (ATTiny) is not powered and therefore resets, which is a problem.
    The voltage on VSTOR is around 4.5V, just as VBAT is, when VOUT collapses.
    The load draws a maximum of 1 to 2 mA at 1.8V.
    I am not 100% sure if the max. current of the inductors I used are sufficient, would this cause such behaviour?
    Or.. may other parts cause this behaviour?
    Also VOUT may be a bit close to the minimum Voltage of the uC so I will test setting it a bit higher to be slightly above 1.8V.

    Thank you so much for your help so far!

    Regards,
    Felix

  • 0 in reply to Felix Mark
    TI__Guru*** 143980 points

    Hi Felix,

    The buck inductor needs to have ISAT slightly greater than 205mA and the boost inductor slightly greater than 285mA.

    Is VSTOR=VBAT falling below VBAT_OK = VOUT_EN and therefore turning off the buck converter?

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hey Jeff,

    thank you! I believe I found the error and will report back to you soon!

    VSTOR=VBAT was not falling below VBAT_OK, but I think the uC might get underpowered because VOUT is very close to Vmin of it and therefore goes into brownout and is restarting.

    Regards,
    Felix

  • +1 in reply to Felix Mark
    Prodigy 10 points

    Hi Jeff,

    I have made some measurements and came to the conclusion that the problem I am now facing is a duplicate of THIS POST.

    Thank you so much for all your help!
    I will mention you in the project I am working on, if you are fine with that (it is an open source/open hardware project on Github) over HERE :)

    Have a wonderful day,
    Felix