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LMR36520: Impedance of SW pin when EN is low

Oz Kocoglu
Intellectual 270 points
Part Number: LMR36520

Hi all,

I have a question regarding this P/N: can I assume that, when EN pin is low (< Ven-VCC_h), the SW pin is “high impedance”?

I would need to understand if I can apply a voltage to it, without shorting out. I guess I can, since VCC regulator should be disabled so there’s no way that the low side output mosfet is on.

What I need to do, basically, is to short together the two outputs of LMR36520ADDAR-based buck converter (after the inductor, I mean) for a “redundant” power supply (one from +48V, one from 7.2V battery).  Both are set for 3.3V output, and they are interlocked in the sense that they cannot be enabled simultaneously.

Thanks,

Oz

  • 0
    TI__Mastermind 23845 points

    Hi Oz,

    Your understanding is correct. the SW pin will be high impedance when EN pin is low. 

    Normally for power supply redundancy, we put a diode at each 3.3V output before they both being shorted. This way it will prevent a backflow of current from the 2nd power supply to the input of the 1st supply. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 60 points

    Hello Arief,

    thanks for the reply. I have a couple more questions:

    1) about the diode for ORing, what can you do to prevent the diode forward voltage to eat the voltage margin of the 3.3V rail? Can you take feedback after the diode?

    2) due to leakages, I expect the output caps of the "off" 3.3V power supply to charge up, perhaps to 3.3V. Is LMR36520 capable of turning on in this condition when the other power supply is switched off?

    Thanks and best regards,

    Giaime

  • 0 in reply to Giaime Ugliano
    TI__Mastermind 23845 points

    Hi Giaime,

    1) For your first question, unless you are able to guarantee that there is always current in the diode, it is not recommended to take the feedback for regulation after the OR-Ring diode. Here is one article that may be useful

    https://www.ti.com/power-management/power-switches/ideal-diodes-oring-controllers/products.html

    You may be able to use a MOSFET based approach with a controllers above. 

    2) For your 2nd question, yes the LMR36520 would be able to startup with a pre-bias voltage at the output. Meaning if there is excess voltage at the output capacitor, the part should be able to startup ok

    Thanks

    -Arief