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bq24298 - PG and SYSOUT are not valid if the adapter is connected under load, with no battery installed

Greg Berlin
Prodigy 40 points

I am experimenting with a 24298EVM board. I am using this in standalone mode.

I have a load connected to the SYS output (3ohms).

PSEL is pulled low via JP1.

R19 is turned completely counterclockwise so that ILIM is set to max (3A).

If I have a charged battery connected to the IC, the system will start up normally when I plug the adapter in (PG goes low, and the SYS output goes to 4.15V). The appropriate current draw from the adapter occurs (~1.7A).

If I remove the battery and then plug in the adapter, PG stays unlit and SYS output does not go active. There is a 100mA draw on the adapter. It does not start up properly,

If I remove the load from SYS, it will start up properly when I plug in the adapter. I can then connect the load and everything works OK.

Why won't it start up without a battery? Yes, it will normally have a battery in it, but why is it required?  Even if it were installed, it might be completely drained and disconnected from the circuit via its PCM - this is the state it will be in if the battery dies and the user lugs it back in.

  • 0
    Prodigy 40 points
    I tried some different load values on SYS to see if a light enough load would allow it to startup OK with no battery.

    11.5 ohm load on SYS starts up OK (0.31A 5V, 0.35A SYS/4.17V)
    10 ohm load on SYS does not start up OK. If I disconnect the load it will start up, and when I then reconnect the load it remains OK (0.35A 5V, 0.40A SYS/4.17V)
  • 0 in reply to Greg Berlin
    TI__Guru 57321 points
    Referring to page 7 of the d/s, the input current limit is 100mA typically when SYS voltage is less than 2.2V. So please do not apply hearvy load until SYS goes above 2.2V when battery is not present. Thanks!
    IIN_START Input current limit during system start up VSYS < 2.2 V 100 mA
  • 0 in reply to Ning T
    Prodigy 40 points
    That seems like an unreasonable constraint. I have set ILIM and PSEL correctly to remove such a limitation. I can't easily control the load to less than 100 mA. The load is what it is. How long until I can apply the full load?
    This makes it unuseable. Do the other battery manager ICs have this same limitation?
  • 0 in reply to Greg Berlin
    TI__Mastermind 25800 points
    Hey Greg,

    This constraint can also be eased/worsened by the amount of output capacitance on Vsys. Is this a custom board or our EVM?

    If it is a custom board, could you provide the output capacitance you have placed on Vsys?

    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 40 points

    I was doing load testing with a resistive load only.  I am using your DEV board.

    For our particular application, I suppose we can put circuitry in place to delay our load, but this seems like a burden put on the customer that could have been avoided.  How long of a delay is required after SYS goes higher than 2.2V?  

  • 0 in reply to Greg Berlin
    TI__Mastermind 25800 points
    Hey Greg,

    This isn't meant to be a burden to the customer as much as it is a protection of the IC when the converter starts up. This is a conservative feature to prevent a lot of inrush current to the converter as Vsys ramps up and also account for external capacitance on Vsys.

    If you have the ability to disable the load in your real application (which I assume is not a purely resistive load), then you could use the /PG pin with some delay circuitry to enable the load once Vsys ramps up.

    Another option I recommended is using a load switch in series with Vsys.


    Hope this helps,
    Regards,
    Joel H