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BQ25505: Question on the difference in RDS(ON) between BQ25505 and BQ25504

Kai Cheong Tam
Prodigy 40 points
Part Number: BQ25505
Other Parts Discussed in Thread: BQ25504

Hi,

I have a question about the ON state resistance of the MOSFET between VSTOR and VBAT for BQ25505 and BQ25504. In the datasheet of BQ25504, only the maximum value is given (2 ohm) while in BQ25505, the typical (~1 ohm) and maximum (1.5 ohm) are given. From this data it appears BQ25505 is slightly better but are there also any typical value for BQ25504 as well? In my application where the Supercap has to power a WiFi module for a few seconds, this seems to be a very significant loss. I would like to know if there is a way to minimize such loss, such as bypassing this transistor or using an external MOSFET instead. Your suggestions are much appreciated. Thank you.

Eric

  • 0
    TI__Guru*** 143880 points

    Hi Eric,

    Typical will also be 1ohm for the BQ25504.  the PFET between VSTOR and VBAT is only used for undervoltage protection.  It opens if the battery discharged to 2.2V.  If you don't need that protection, then you can short VSTOR to VBAT.

    Regards,

    Jeff

  • 0
    Prodigy 40 points

    Hi Jeff,

    Thank you for your clarification. I am a bit confused by your answer though. The diagram below shows the application circuit for BQ25505 but I presume the PFET between VSTOR and VBAT(_SEC) of BQ25505 and BQ25504 in the circuit is similar.

    Please correct me if I am wrong. I thought the PFET between these two pins turn on whenever VSTOR is above VBAT_UV + VBAT_UV_HYST and only turns off when VBAT drops below VBAT_UV. Thus for both chips, the system load is designed to directly connect to VSTOR. BQ25505 offer an additional external control PFET to turn off the system load by /VB_SEC_ON.

    If it is not the case, does it rely solely on the body diode to connect between the two?

    In both cases, I think the R(DS) (or Vbi of the body diode) on this PFET will incur quite significant loss when the system load is high.

    Another unrelated question based on your answer though. If I use supercap instead of battery, will it not be better to short the VSTOR and VBAT so that I can make better use of the storage capacity?

    Thank you.

    Eric

  • 0 in reply to Kai Cheong Tam
    TI__Guru*** 143880 points

    Hi Eric,

    Your explanation is correct.  The PFET body diode also allows current to flow from VSTOR to VBAT to charge the battery. If using a super cap or a LiIon battery pack with protection, I recommend shorting VSTOR to VBAT.

    Regards,

    Jeff