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BQ24295 PMID and SYS pins

Other Parts Discussed in Thread: BQ24295

Hi

I see that charger chips like the BQ24272 have the PMID pin but it's not recommended to be connected to an external load.

However, BQ24295 has a PMID pin as well but it's designed to boost the voltage for an external load. That said, it seems that it's active only when VBUS is inactive (8.3.1.3) - or is it? Does that mean PMID is inactive during charge mode?

But looking at the block diagram, it looks like VBUS passes current to PMID.

If the device is to pull current regardless of the state of VBUS, how should it be hooked up? It'd be in vain if it's connected to PMID alone (similar to the example circuit) as there'd be no power when the input source is present. Isn't the load usually connected to SYS? They call SYS the 'system connection point' whilst PMID the 'battery boost mode output voltage'.

In fact, what does PMID actually stand for?

Thanks

  • Hi Steve,

    PMID is used for Power Bank applications where power is transferred from battery to PMID in OTG mode. During OTG, VBUS is not connected (no voltage output).

    When in charge mode, PMID can draw power from VBUS. Even though it seems that system can be connected through PMID since it always has power output on any mode, system should be connected to SYS pin because it will have priority in any situation, making sure the system does not crash.

    If you change from charge mode to OTG mode, there is a transition time where there is no voltage on PMID. Same when going from OTG mode to charge mode. This transition time might crash system, which is not desirable.

    Thanks,
    Steven
  • Hi

    For Power Bank applications, the voltage at SYS isn't sufficient to power up a USB device - right? Its max is only 4.42V, which may be enough for "low power devices" but 4.75V is needed for "high power devices", according to the USB spec. Do you mean that only for Power Bank applications, PMID should be used (as the load already has a battery of its own so it won't crash), but if the BQ24295 is integrated in a system, then SYS should be used instead?

    Cheers
  • Hi Steve

    System load should always be at the SYS pin, and battery connected at BAT. If you want to power up a USB device, unplug VBUS, enable OTG and then PMID will be your output power for the USB device.

    Regards,
    Steven

  • Hi Steven,

    So if I want to power up a USB device whilst charging the Power Bank, will PMID still be the output power for the USB device even when VBUS is connected to an input source? That is, if the USB device is initially connected to PMID whilst in OTG mode with VBUS inactive and subsequently VBUS becomes active, the chip will automatically switch to charge mode from boost mode and continue to provide 5V at PMID with a transition time.

    Cheers
    Steve
  • Hi Steve,

    Yes that is correct. You can power up something connected to PMID while still having input source at VBUS. However, the current passing through PMID will not be available to system and battery, as it is counted towards the input current limit.

    During the transition there is a down time. Just so you be aware of that.

    Thanks,
    Steven