BQ25708: Can not make it work without battery.

Part Number: BQ25708

Hi all.

I have some trouble with BQ25708 design. Added schematic.

It work good with connected battery. Battery charge ok.

But I can not make it work without battery.
It not started, I have about 1V at Vsys, so my 5V converter not worked.
(but I have 6V at REGN and VDDA)

Chip in default state, no write to registers, only CELL_BATPRESZ pin connected to GND (for case without battery)

Thanks advance.

AnatolyBQ25708RSNT - test board.pdf

3 Replies

  • Anatoly,

    Actually,

    DC/DC should operate whatever battery existed or not. So could you help me to double check below signal?

    1. What is voltage in REGN pin?

    2. Could you measure input voltage, SYS voltage and system current waveform to check?

    From your circuit, I find there some items need to modify.

    3. please change R67/R43/R38/R39 resistor value from 100ohm to 10ohm

    4. Change C12/C29 capacitor value to 47nF

    5. Please keep your input voltage max value is 24V because of our recommend operation max voltage is 24V

    6. what is your JP2/3/4 jumper status?

    Thanks

    Regards,

    Paul

    Texas Instruments

    Battery Charger Solutions

    Please click "This Resolved my issue" button if this post answer your question

  • In reply to Paul Yeh:

    Hi Paul.

    Thanks for reply. We changed resistors and capacitors according points 3 and 4

    But this no have fundamental difference. It still not work as needed. We have tested several boards 

    "1. What is voltage in REGN pin?"

    6V at all boards

    2. Could you measure input voltage, SYS voltage and system current waveform to check?

    Added waveform from good board (Blue - 24V, yellow - Vsys) . At other board it all flat, nothing really moving.

     

     

    "3. please change R67/R43/R38/R39 resistor value from 100ohm to 10ohm

    4. Change C12/C29 capacitor value to 47nF"

    This done

    "5. Please keep your input voltage max value is 24V because of our recommend operation max voltage is 24V"

    Sure

     

    "6. what is your JP2/3/4 jumper status?"

    For waveform that was done. Only JP4 set (battery not connected). Remove JP4 stop oscillating (Goes dead). Move to JP3 - still dead (without battery)
    after add battery, starts up.

    I was thinking that problem can be in 24V source, but even if we using external adjustable power supply (tried 24V and 18V) nothing changed.

    Additional question, what must be default Vsys voltage? (without battery, JP4 set, after reset state)?


    Thanks.

    Regards, Anatoly

  • In reply to Anatoly Dobreen:

    Anatoly,

    For 4cell design and no battery condition, the min system voltage should be 12.288V.

    Do you isolate system loading, just no load condition to see does system voltage ramp up or not?

    BTW, could you also send your register setting log file to me I would like to check your initial setting?

    Thanks

    Regards,

    Paul

    Texas Instruments

    Battery Charger Solutions

    Please click "This Resolved my issue" button if this post answer your question