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LP8733: Unused Buck1 DC/DC

tom
Prodigy 210 points
Part Number: LP8733

Hi,

On our board we do not have a 5V powersupply rails, so I want to power the LP87334D PMIC directly with 3.3V.

Therefor we do not need the buck1 converter of the PMIC. (5V->3.3V)

How should the buck1 pins be connected?

I assume we can leave VIN_B1, SW_B1 and FB_B1 all unconnected.

Then GPO (15 ms enable delay) will be used to give power ready to the CPU. (since PGOOD cannot be used).

Can you let me know this is correct?

best regards,

Tom

  • 0
    TI__Expert 4985 points

    Hi Tom,

    BUCK1 must be disabled through I2C before the EN pin can be pulled high. If this is done then SW_B1 and FB_B1 can be left unconnected. VIN_B1 must be connected to the input voltage.

    BUCK1 must be disabled because otherwise the PGOOD will be constantly low.

    BR,

    Samuli

  • 0 in reply to Samuli Piispanen
    Prodigy 210 points

    Hi Samuli,

    When using PGO (which is just 15ms delay on EN) it should also work without disabling buck1?

    Based on the power sequence all powersupplies start based on EN pin, so unused buck1 should not prevent other supplies from starting.

    I understand PGood will be constantly low, but if not used this should not matter?

    We don't have any other CPU on our board, so sending i2c commands before boot is not an option.

    best regards,

    Tom

  • 0 in reply to tom
    TI__Expert 4985 points

    Hi Tom,

    Using GPO as enable for the CPU doesn't provide the information if the voltages are at the correct levels though. Also note that the nINT pin would also be active (low because it is active low) constantly since BUCK1 will detect a short circuit and disable itself.

    TI recommendation is to disable the buck before EN pin is high. BUCK1 PVIN pins must be connected to the input voltage even if the buck would be disabled.

    BR,

    Samuli

  • 0 in reply to Samuli Piispanen
    Prodigy 210 points

    Hi Samuli,

    Ok clear.

    What is the minimum voltage head-room for the buck1 converter?

    Can a voltage divider in FB_B1 be applied to reduce output voltage, so buck1 seems to start-up normally?

    best regards,

    Tom

  • +1 in reply to tom
    TI__Expert 4985 points

    Hi Tom,

    The output voltage window is +50 mV / -40 mV. I guess it could be possible in theory to have a voltage divider on the FB pin to get the buck to think that it is at correct voltage. However, this is something that we have not validated and therefore cannot recommend.

    BR,

    Samuli

  • 0 in reply to Samuli Piispanen
    Prodigy 210 points

    Hi Samuli,,

    Thanks for your clear explanation.

    We will validate this ourselves.

    best regards,

    Tom