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LP87745-Q1: Inquiry about LP87745-Q1 Configuration: Impact of Leaving BUCK2/BUCK3 SW Floating and FB Grounded on Other Channels

Simon wang
Prodigy 190 points
Part Number: LP87745-Q1
Other Parts Discussed in Thread: LP87745

Tool/software:

Hello, expert.
I'm using the LP87745 PMIC, but I only need the BUCK1, LDO and BOOST functions. I don't need BUCK2 and BUCK3. When I designed the circuit, I left the SW of BUCK2 and BUCK3 floating and connected the FB to ground. Will this design trigger any safety mechanisms and cause other BUCKs to fail?

  • 0
    Prodigy 190 points

    Can I configure the LP87745 via SPI when the ENABLE pin is at a low voltage level?

  • 0 in reply to Simon wang
    TI__Genius 16950 points

    Hi Simon,

    Are you using LP877451A1RXVRQ1 device ? 

    If yes, then BUCK2 and BUCK3 are enabled by default and you cannot just leave BUCK2/3 SW pin floating. You need to connect properly with external inductor and minimum one output capacitance to each buck and FBx connected to Vout. After then you can leave it as is and do not use it but if you leave them like this PMIC would sense error as SW pin is open and then there would be reaction and PMIC would not function properly. 

    Regards,

    Ishtiaque 

  • 0 in reply to Ishtiaque Panhwar
    Prodigy 190 points

    This sounds frustrating because the PCB fabrication is already completed, and we didn’t reserve space for capacitors or inductors for BUCK2/3. We previously checked the datasheet, which stated for unused BUCK channels: "SW: floating; FB: ground." This seems contradictory to your explanation.

    my classmate mentioned that the PMIC currently has no output voltage—only the VOUT_V LDO is active. Does this match the behavior you described?

    Are there any other fixes? For example, if we externally supply 3.3V to the VIO_LDO pin, can the PMIC’s SPI still function? Could we disable BUCK2/3 via SPI software configuration to make the PMIC work properly?

  • +1 in reply to Simon wang
    TI__Genius 16950 points

    Hi,

    I am sorry to hear that but that's not how it should be interpreted. The data sheet says about the connection use when BUCK2/3 is not used. When you are not using BUCK2/3 then that means you need to disable the those in the NVM by default which is not the case if you are using   LP877451A1RXVRQ1 device. 

    Simon wang said:
    Are there any other fixes? For example, if we externally supply 3.3V to the VIO_LDO pin, can the PMIC’s SPI still function? Could we disable BUCK2/3 via SPI software configuration to make the PMIC work properly?

    I am sorry but you probably need to have work around that I mentioned earlier.  VIO LDO may work but you would face resets because of not having external components for BUCK2/3. 

    Regards,

    Ishtiaque