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BQ24295: VSYS regulation without battery

Part Number: BQ24295

Hello,
My customer already has device with BQ24295.
Now they want to implement version of device, which can work without accumulator battery.
As I found on e2e in this case BQ24295 will going sleep and waking up 20 times per second. As result any changes in configuration will reset on power-on.

There we have another one aspect of question.
When we set CHG_CONFIG = 1 (charge enabled) we got next oscillogram on SYS:

When charge disabled (CHG_CONFIG = 0) we'll got:

In first case we got average voltage about 4.29 V with ripples. In second - 3.83 V without ripples.
As I understand the ripples are the result of  switching half-bridge (HSFET + LSFET).
In second case 3.8 V it's a voltage from REGN LDO, but I'm not sure about it because I can't found path from REGN LDO to SYS output.
Generally as I understand when battery disconnected and charge disabled we should got 0 V on SYS. But we got 3.83 V. I can't find an explanation for it.

But at the same time this behavior are OK for our application with one refinement - we want voltage on Vout higher that 4 V without ripples. So we should disable charge and raise the voltage. But I don't understand how to set up VSYS in this mode.

Finally there are 2 questions:

  1. It's possible to got voltage above 4V on SYS pin without battery?
  2. How to configure BQ24295 into this mode properly?

  • Hey Vladimir,

    I apologize for the late response:


    When charge is disabled or the charger is in LDO mode, the system voltage is roughly equal to VSYSMIN + 150mV. The maximum settable VSYSMIN voltage is 3.7V, which gives you 3.85V @ SYS.

    When the charger is charging in the switching region (i.e. the battery voltage > VSYSMIN), then VSYS = VBAT + 50mV, and will track with the battery voltage until charging is disabled or terminated.

    Another thing to understand is that when the battery is not present, but charging is enabled, you will see something like waveform (1), which as ripple as the charger will go very quickly between charge cycles. If charge is disabled, then it looks similar to (2) which has no ripple, and is maintained @ 3.83V.

    So in summary, to answer your questions:
    1) It is not possible to get 4V on SYS without the battery. Maximum would be 3.85V (with VSYSMIN = 3.7V).
    2) The charger must be in charge disable mode, and again, maximum voltage would be 3.85V.


    Alternatively, you may add a boost converter at SYS in order to achieve this, but the charger on it's own cannot do so.


    Regards,
    Joel H