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WEBENCH® Tools/BQ24640: shuts down at below set power

Emmanuel Roble
Intellectual 405 points
Part Number: BQ24640
Other Parts Discussed in Thread: LM5155

Tool/software: WEBENCH® Design Tools

Hi,

I used the BQ24640 spreadsheet calculator to to design the supercap charger for 9 Vinput, 7.5V 5A output. But my prototype shuts down at just above 2A. I dont think i am already at max current because it should be constant current at high load condition.

Attached circuits for reference. Please comment if I may have missed anything.

Thank you,

Emman

  • 0
    TI__Mastermind 24075 points

    Hi Emmanuel,

        What is your measured VCC voltage when charge current is increased to 2A, and just before part shuts down. Sweep ISET voltage, and on a scope capture, capture VCC, SRN, charge current, and PH. I suspect that the impedance between your input supply and VCC node causes a voltage drop which increases with increasing charge current (causes increased input current). As you already have a small headroom between input voltage and output voltage, you could be entering SLEEP mode. Do you observe oscillation between charging and not charging? This would be the part entering and exiting SLEEP mode.

  • 0 in reply to Kedar Manishankar
    Intellectual 405 points

    Hi Kedar,

    No the supply is in complete shutdown after 2A load. Will it be safe to just increase the input voltage?

    Will get the scope shots and update you after.

    Thank you,

    Emman

  • 0 in reply to Emmanuel Roble
    TI__Mastermind 24075 points

    Hi Emmanuel,

       Please double check that you don't have a current limit on your power supply.

  • 0 in reply to Kedar Manishankar
    Intellectual 405 points

    Hi Kedar,

    If it triggers the UVLO, will it not auto restart?

    Thank you,

    Emman

  • 0 in reply to Emmanuel Roble
    TI__Mastermind 24075 points

    Hi Emmanuel,

        If there is a huge current draw then yes, once UVLO is hit (most likely SLEEP threshold will be hit before this) the input current will be disabled, causing input voltage to rise again, and you will have the part oscillate between on and off.

    What are your power supply specifications? (input voltage and if there is a constant current limit?) Can you get a scope capture of VCC, SRN, PH, and ACDRV when the power supply shuts down? Also are you connecting an external load of 2A, or are you referring to the fact that you have charge current set for 2A?

  • 0 in reply to Kedar Manishankar
    Intellectual 405 points

    Hi Kedar,

    My final configuration is 8V 3A output at 10V input. The input is now 10V 4.5A. After compensation adjustsment on 3.3V-10V LM5155 boost, the circuit is now stable and I am getting around 25W at the output. I can also get 2.75A short circuit current.

    I am now using it to charge my 133F capacitor. I think it is normal that when the capacitor is reaching the charger's output voltage, the charging current gets lower and lower. Can you advise if there is a way to have the maximum current even at close to top up voltage?

    Thank you very much for your assistance.

    Regards,

    Emman

  • 0 in reply to Emmanuel Roble
    TI__Mastermind 24075 points

    Hi Emmanuel,

      Glad you were able to solve the initial problem. Yes, what you are referring to is the CV (constant voltage) charging portion of the charging algorithm. In the constant current phase, charge current is regulated, but as you get close to set charge regulation voltage, the CV loop kicks in to accurately regulate the output voltage, while allowing the charge current to taper down until it finally hits the termination threshold. This is important, as if there is any impedance in the path between the supercap/battery and where the output voltage is sensed by the charger, then a voltage drop will be developed across this (charge current * impedance) tricking the charger into thinking supercap/battery is fully charged. As output voltage is regulated with tapering current, we can ensure accurate charging voltage accuracy.

    CV loop is inbuilt and non configurable.