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cascade BQ25890/2 and BQ25895 PMID and VBUS

John Newell
Intellectual 280 points
Other Parts Discussed in Thread: BQ25895, BQ25892, BQ25890

SYS is quoted as 3.5 to 4.5V. I assumed this was provided by the converter circuit from the battery. Am I correct  in saying this is only true if VBUS is present. i.e. SYS in the absence of VBUS is simply the battery voltage less a FET RdsON IR drop. If the battery voltage is 3.0V, then SYS will be less.

If OTG is enabled, so providing 5V on either VBUS or PMID, then is SYS still present - and is it the battery voltage lesss IR drop still?

Finally, is the PMID pin voltage the same as the VBUS when in  charge mode. so can be as high as 14V. How does the BQ25895 PMID pin avoid this - not a good idea for the OTG to go that high.

FYI I need a >3.3V to 4.2V main SYS rail and a low power (200mA) 5V. The OTG seemed to be a way to achieve this. I also need to be able to very fast charge a 5300mA Li-ION pack so used the 9A cascade system.

John

  • 0
    TI__Guru**** 166626 points
    Yes, SYS in the absence of VBUS is simply the battery voltage less a FET RdsON IR drop. If the battery voltage is 3.0V, then SYS will be less.

    OTG will only come if VBUS voltage is not present. The buck converter works in reverse as a boost converter with the battery current flowing through the BATFET to SYS then through the boost circuit to VBUS (or only PMID for 895).

    VBUS connects to PMID through the reverse blocking/current limiting sensing FET Q1 so PMID sees the same voltage as VBUS. When in OTG mode, there should be no voltage on VBUS.

    You can only be in either charge mode or OTG mode, not both at the same time.
  • 0 in reply to Jeff F
    Intellectual 280 points

    Jeff,

     If there is a voltage on VBUS ( 14V say) how is this prevented from appearing on the PMID pin of the BQ25895? Is the reverse blocking diode turned off if OTG is not enabled. If so, how does the voltage appear on PMID on the BQ25890 to feed the BQ25892. Different logic in the RFET control? Your answer implies that on the BQ25890/2. PMID also has the OTG voltage appearing at VBUS. I note there is a external diode between SW and PMID in the 895 EVM, which is not on the 90/2.

    Regards

    John

  • 0 in reply to John Newell
    TI__Guru**** 166626 points
    OTG output is at PMID for 895 and VBUS for 890/2. On 895, when OTG is enabled, FET Q1 is turned off but its body diode would apply whatever voltage is at VBUS on PMID. The external diode between SW and PMID is to improve efficiency when in boost mode.
  • 0 in reply to Jeff F
    Intellectual 280 points
    More directly - on the 895 - does VBUS ever appear on the PMID pin? If so, then I cannot use it without some other device to isolate a connected load.
    Regards
    John
  • 0 in reply to John Newell
    TI__Guru**** 166626 points
    Yes. In charge mode, VBUS connects to PMID through a FET used as a switch.