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LP87524B-Q1: Unused buck

Guy Shemesh
Prodigy 155 points
Part Number: LP87524B-Q1
Other Parts Discussed in Thread: AWR1642

Hello, 

In my application, only outputs 1-3 are being used, output 0 (the default 3.3V generated on SW_B0, FB_B0, VIN_B0) is not used since out VIN may become lower than the required 3.8V. 

Can the external components for buck 0 be eliminated ? (input / output capacitors, inductor and RC snubber filter)

Or else, can you please recommend how to connect output 0 in this case?

Thanks

Guy

  • 0
    TI__Genius 11020 points
    Hello Guy,

    If you can disable the Buck0 through I2C bus before starting the bucks then it is possible to remove the components. Input pin need to be connected to supply voltage anyway, and FB would be good to connect to GND. If you cannot disable the buck and you try to activate the buck without inductor, it will generate short circuit fault since the output voltage does not start rising at buck activation. So the PGOOD/INT will be pulled low until the fault is removed.

    Thanks.

    Regards,
    Tomi Koskela
  • 0 in reply to Tomi Koskela
    Prodigy 155 points

    Hello Tomi, 

    Thank you for the detailed answer. 

    In our board the PMIC (= LP87524B) drives the AWR1642, and there are no other processors / MCUs on the board. The 3.3V rail is generated for the `1642 by an LDO and Buck0 is unused as I mentioned before. So we need the PMIC to provide correct voltages on buck1/2/3 for the '1642 to boot. 

    Now, looking at our schematics, PMIC's PGOOD is not connected and PMIC's INT is connected as a GPIO for the `1642. In addition the I2C bus is connected between the PMIC and the `1642.

    Is it safe to assume that bucks 1/2/3 will operate and generate correct voltages while buck0  external components are eliminated? and that after boot the `1642's software can clear the fault condition by I2C ? 

    Generally, I wouldn't try such tricks as this is a first revision PCB, but we also have a board space issue and we are at layout stage, I'm trying to see if we can regain some space in this way for other circuits that need to enter. 

    If you believe that this a dangerous attempt for first revision please also let me know. 

    Thank you

    Guy

  • 0 in reply to Guy Shemesh
    TI__Genius 11020 points
    Hello Guy,

    Bucks1/2/3 will generate correct voltages in this case, the short circuit fault of Buck0 affects only Buck0. At the startup the Buck0 will try to turn on, but will generate SC fault when the FB voltage does not rise above 0.35V in 1us or so. If the INT and PGOOD are not used as a reset signal for the SoC/MCU, the device will start correctly.

    The only problem with this approach is that the LP87524B has not been validated/verified with this kind of use case and it is little unknown what will be long time effects of the 'unpopulated' buck. So even though in theory it is ok, there is still risk involved and therefore I would not recommend leaving the components out.

    Hope this helps.

    Thanks.

    Regards,
    Tomi Koskela
  • 0 in reply to Tomi Koskela
    Prodigy 155 points

    Thank you very much Tomi, yes - this helps alot. 

    I accept your recommendation, and I found enough board space elsewhere, therefore I will keep buck0 components for this revision (as unused).

    Best Regards

    Guy