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BQ25570EVM-206: EVM not working down to min. VIN_DC figure in User's Guide

Danny Di Giacomo
Prodigy 30 points
Part Number: BQ25570EVM-206
Other Parts Discussed in Thread: BQ25570

Hi, I'm not having success using the EVM with a source that produces about 295mV DC constantly at up to 20uA. However, if I apply 350mV or greater (at around the same current) into VIN_DC via different source, I notice some pulsing (using a DSO) at TP2 (LBOOST) and a step-up in voltage across C6 (CBAT). Below this, no activity is seen at LBOOST. Jumpers are set as in Figure 11 in the User's Guide. Changing VOC_SAMP from 80% to 50% doesn't make any difference. Also, I haven't made any modifications (e.g. changing resistors) on the board. I chose this EVM as it seemed to be the only one with a VIN_DC rating as low as 130mV, however I'm now starting to suspect this could be an error. Any help would be much appreciated. I do have facilities and skill to rework the SMD components onboard. Thanks, Danny.

  • 0
    TI__Guru**** 166176 points

    Hi Danny,

    With a charged storage element at VSTOR >1.8V, the boost charger works down to 130mV at VIN_DC.  If VSTOR<1.8V, the cold start circuit now requires at least VIN_DC=600mV up from 300mV to function.  The datasheet was recently revised but we may have missed the users' guide.  Thanks for pointing that out.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Hi, thanks for the fast reply. It wasn't a good idea to quickly skip across to the "Charging a Super Capacitor from Buck Converter Output" section and expect it to just be plug and play! I noted your info above while having a thorough read of the General Description section. Also, the step-up in voltage I mentioned previously occurred at VOUT, not C6 (CBAT).

    I was hoping to harvest at least 3V into a 0.22F 5.5V super cap (Panasonic EECS0HD224H). Would you suggest I connect this to VSTOR, or VOUT (as in Figure 11) after reconfiguring VOUT via R9 (ROUT2) and R10 (ROUT1)? For the latter, I expect I'd also have to add a smaller value (100uF?) capacitor to VSTOR, and either way, N and P FETs to isolate VSTOR to charge to >1.8V before connecting my source.

    I've just started reading sections 7 and 8 on the bq25570 datasheet which should give further insight.

    Edit: thinking about it further, I guess I could apply >600mV to VIN_DC before connecting my source to bring VSTOR up to >1.8V.

  • 0 in reply to Danny Di Giacomo
    TI__Guru**** 166176 points

    Hi Danny,

    If you connect the super cap to VSTOR, the charger boost converter is heavily loaded and it will take a long time to charge.  If you connect to VOUT, the buck converter's soft start and under-voltage lockout prevents VSTOR from collapsing and a provides smoother and faster charge profile for the super cap.  I suggest setting the VSTOR VBAT_OV at least 200mV above the buck converter VOUT voltage setting.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Hi, so I would still need to apply >600mV to VIN_DC before connecting my source to bring VSTOR up to >1.8V? Would CSTOR be able to retain this with the super cap connected (to VOUT) while changing sources?

  • 0 in reply to Danny Di Giacomo
    TI__Guru**** 166176 points

    Hi,

    Yes.  The cold start circuit needs at least 600mV to start up.  Then when VSTOR gets up to 1.8V, you can lower the input source. With the super cap at VOUT, CSTOR should not collapse below 1.8V while charging the super cap.  However, if you load VSTOR with something else, that load + super cap charging might be enough to collapse VSTOR. 

    Regards,

    Jeff