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LM5022: Can LM5022 flyback operate at 6V input?

Neo.Wang
Intellectual 850 points
Part Number: LM5022
Other Parts Discussed in Thread: LM5122, LM5155, LM5157

Hi there,

For LM5022 flyback design, the input voltage may drop to 6V for ~20seconds in our application.

Can LM5022 operates at 6Vin for 20seconds?

Thanks

Neo

  • 0
    TI__Genius 14155 points

    Hi Neo,

    LM5122 is capable of 6V input. It can operate at 6V for long time.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Intellectual 850 points

    Hi Fengji,

    What's the minimum input voltage it can support for long term (~20seconds) operation?

    We need to meet 6V cold cranking requirements, but given the voltage drop from input connector to LM5022 (like voltage drop on fuse, reverse protection diode, common choke, sense resistors and PCB trace), the actual input voltage may less than 6V.

    Please kindly advise.

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    TI__Genius 16675 points

    Hello Neo,

    It is not possible for the LM5022 to operate at voltages below 6V.
    So, either choose a newer controller like the LM5155 / 56 or maybe a coverter with built-in FET, like the LM5157 / 58.
    Or, if you want to use the LM5022, you can use two diodes (one from the battery and one from Vout) to supply the voltage for the VIN pin.
    This way, the converter can run from its own boosted supply when the input voltage drops too low.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 850 points

    Hello Harry,

    We used LM5022 for isolated flyback design so it's not possible to connect the secondary output to primary VIN pin.

    Just for clarification, what's the limitation for VIN not supporting <6V? As i did not see any UVLO on VIN pin. Also, for VCC we have auxiliary winding which can provide external 12V on VCC pin. Please help clarification so that cold cranking support can be rejected.

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    TI__Genius 16675 points

    Hello Neo,

    I am sorry, I did not know about the Aux supply for VCC.
    I see that there are multiple open threads in parallel, but inside this particular thread here it was not mentioned.

    So, when you FIRST power up the converter, the input voltage needs to be above 6V, ideally above 7V to allow for the start-up.
    Then, once it is running, the external 12V Aux comes in to supply the VCC.
    When this is established, the Voltage on the VIN pin can drop down to 3V, as long as VCC is still permanently supplied from the Aux winding.

    You will need to calculate the external components (inductor, Rsense, FET, diode, etc. for the extreme case, where the input voltage falls below 6V.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 850 points

    Hi Harry,

    Sounds good, thanks for confirmation. Let's assume the minimum voltage is 5V at VIN pin during cold cranking.

    To support 5Vin during cold cranking, the UVLO threshold should be tuned to accommodate 5Vin for ~20seconds.

    In this way, during the initial (FIRST) power on, as long as VCC exceeds VCC UVLO rising threshold (Typ 5V, and at this moment VIN is about 5.2V considering 200mV drop on bypass switch), the converter is enabled and starts operation, is this correct? If yes that means we can not ensure VIN>7V for FIRST startup.

    Neo

    Thanks

  • 0 in reply to Neo.Wang
    TI__Genius 16675 points

    Hello Neo,

    > To support 5Vin during cold cranking, the UVLO threshold should be tuned to accommodate 5Vin
    Correct.
    To be honest, I am not certain, if any UVLO setting makes sense here, or if the UVLO pin should maybe better be tied to Vin, so that it can work down to 3V.

    > In this way, during the initial (FIRST) power on, ... at this moment VIN is about 5.2V ... the converter is enabled and starts operation, is this correct?
    No, it will NOT start.
    When you look at the datasheet, you will find the following:
    VIN Range: 6 V to 60 V (operates down to 3 V AFTER start-up)
    As long as VIN is below 6V, the controller will not start-up, even if the voltage on the UVLO pin is above 1.28V.
    This is independent.

    Again: when you FIRST power up the converter, the input voltage needs to be above 6V, otherwise the controller will not start.

    As long as the input voltage is lower than 7V, the VCC voltage is also below its regulation target of 7V.
    But VCC is the supply for the gate drivers.

    Therefore, during the first cycles, the gate drive voltage will be lower than expected.
    So, when starting-up at 6V is required, please be careful with the selection of the MOSFETs.
    This is why I said that VIN should ideally be above 7V.

    The Aux supply for VCC will kick-in after some cycles.
    The gate drivers and the logic inside our controller will be supplied by the Aux voltage.
    From that point on, the input voltage can drop to a voltage which is lower than 6V (down to 3V).

    In an automotive environment, the system should not need to start-up at voltages below 6V, right?
    The input voltage should only go down to 6V or less, after the converter has already started. Correct?

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 850 points

    HI Harry,

    Yes the board can start up normally to 12V or 24V and then drops to ~6V for a few seconds.

    I did experiments today, with UVLO set to ~5V, the system can power up to 12V/24V, and dropping from 24V to 6V is working well (Vout is stable with either slow or fast voltage transition rate).

    However, when dropping from 12V to 6V with >1V/ms voltage transition rate, observed a dip on output voltage. Only when voltage transition rate is lower than 1V/ms the output is stable.

    I don't understand why the 6V is working for 24->6V but not work for 12V-6V transition. Do you have any idea on this?

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    TI__Genius 16675 points

    Hello Neo,

    Good to hear that it is basically working

    I think that there is just more energy stored in the system when the input voltage is higher before it drops.

    Best regards

    Harry

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults” and is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).