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LM5163: LM5163 and LMR38010 current entering SW pin when disabled

Alex L
Prodigy 110 points
Part Number: LM5163
Other Parts Discussed in Thread: LMR38010

Hello,

I was wondering:

what is the behaviour of LM5163 and LMR38010 SW pin when some current is injected on Vout (from 0mA up to 50mA), with Vout=0
i.e. when Vout rail is pulled up by an external mechanism (such as a protection diode+pullup resistor).

NOTE: the behaviour herein described is inherent to protection devices only, meaning not a proper voltage regulator placed in parallel to LM5163/LMR38010.

Some pullup current may enter SW pin under these two separate conditions:

1) when Vin > Vin_min but Ven < enable_threshold

2) when Vin < Vin_min (e.g. Vin=0) and (of course) Ven < enable_threshold

Note: FB pin would be powered as well, but through an equivalent high impedance resistor (>10KOhm)

I assume current would be injected backwards to Vin through High Side MOS intrinsic diode,
or is HS a proper hardware switch (e.g. two n-MOSes with common source), and hence Vout is floating when regulator isn't enabled ?

If currents is injected backwards to Vin,
are the related parameters of the HS MOS fully specified ? (i.e. intrinsic diode parameters, and in particular diode continuous / pulse forward current)

Would this current damage Vds comparators (LM5163/LMR38010) or Zero-crossing comparator (LM5163)?

Thank you very much in advance

Alex

  • 0
    TI__Mastermind 25260 points

    Hi Alex,

    Current would normally flow into the SW node if VIN was also lower than VOUT. Otherwise the body diode will be biased off. The body diode characteristics of the MOSFET are not usually characterized.

    The LM5163 should support pre-bias startup, so I think it should be ok.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 110 points

    Dear Richard

    that means when Vout > Vin current will flow through the body diode of LM5163 HS MOS (LMR38010 as well) to the capacitors on Vin. Right?

    I perceive this as a potentially dangerous situation, especially if there are big capacitors on Vin to replenish. What do you think?
    And what about sensitive electronics in this situation, such as comparators etc. (e.g. if Vin<Vin_min) ?

    When Vout < Vin, MOS will be positively biased (LMR38010 as well), and no back-current will flow. Right?

    Best regards

    Alex

  • 0 in reply to Alex L
    TI__Mastermind 25260 points

    Hi Alex, 

    Just wanted to bring this back on track. Usually the case you see will happen if VIN drops very quickly compared to VOUT, or for some reason, is much lower than VOUT in the first case.

    This is usually not the norm in application.

    If you are concerned, you may

    1) Control operation via EN pin

    2) add a diode at the input to prevent current from flowing back.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 110 points

    Hi Richard

    Ok for the series diode entering Vin pin.

    Regarding EN pin, I am not sure this will have any effect, since from datasheet EN only controls the logic,
    while the HS switch (nMOS) is connected directly between Vin and SW pin
    (not taking into account the 'peak/valley current limit' which would probably be a shunt or something like that - or maybe the voltage is read directly on the MOS)

     Regards

    Alex

  • 0 in reply to Alex L
    TI__Mastermind 25260 points

    Hi Alex,

    I just meant that from the application section of the datasheet, you can see that in figure 20, EN toggles the behavior of the device, and you can see that in this case, it is not really possible for current to flow back at all as long as you have VIN larger than VOUT. 

    Thanks,

    Richard