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LM5166: Current limit during 100% duty cycle

David Dean
Expert 2315 points
Part Number: LM5166
Other Parts Discussed in Thread: LMR36006

I am considering using the LM5166 in a design where it will sometimes act like an on/off switch for LEDs.  The LED forward voltage is about 9V, so my idea is to set the LM5166 Vout to 12V and allow it to enter 100% duty cycle mode.  The current is limited upsteam, but it can be a little higher than 500mA - maybe 600-700mA.  I am unclear what the LM5166 maximum output current can be in this mode.  Is it 500mA, the typical max output current of the part?  Or, since the part will not be switching, is it 1250mA, the high-side switch limit?  Or somewhere in between?

  • 0
    TI__Mastermind 35210 points

    Hello David,

    Device current limit is defined by peak, HS current limit.

    It does not change in different modes of operation. Use high-side switch limit

  • 0 in reply to Marshall_Beck
    Expert 2315 points

    OK.  My concern was that, even though the HS switch can handle a peak of 1250mA, in normal switching operation the average current (and power dissipation) through that switch is much lower.  But you are saying that the part can handle a constant output current of 1250mA when the duty cycle is 100%?

    In a related question - if operating in 100% duty cycle mode and the current hits the peak current limit, will the part begin switching to limit the output current, even if the input voltage remains below the target output voltage?

  • 0 in reply to David Dean
    TI__Mastermind 35210 points

    Hello David,

    Figure below is for 5V ooutput.

    Average current is based on inductor selection, as peak inductor current defines current limit.

    Please see figure below from D/S to understand device current capability at low VIN/high duty cycle.

    Does this answer the question?

  • 0 in reply to Marshall_Beck
    Expert 2315 points

    Yes and no.  These ratings are for peak switch current, and I do understand how peak switch current determines average output current.  The disconnect is that the high side switch is rated for 1250mA peak current, but section 6.3 states that the max output current is 500mA.  If the part is not switching and I allow 750mA or 1250mA to pass through the part, I am violating the max output current.  When the part is switching the peak current is not the same as the average current, so I fear that this 100% duty cycle mode is an exception and the 500mA max output current is what applies.

  • 0 in reply to David Dean
    TI__Mastermind 35210 points

    Thanks for clarifying.Makes more sense for me.

    I am following up with product definer to confirm on my suspicion before giving you my answer.

    Allow me to follow up with the next couple days.

  • 0 in reply to Marshall_Beck
    TI__Guru 73072 points

    Hi David,

    The LM5166 has two modes of operation: COT (with a resistor at the RT pin) and PFM (RT tied to GND). Also, there is an ILIM pin configuration setting. Take a look at the LM5166 Excel quickstart calculator for more detail - the file is downloadable from the LM5166 product folder.

    As an example, when ILIM is tied to GND and PFM is selected, the peak inductor current is 1.25A. However, in this mode the inductor current always returns to zero (a triangular waveform), so the average current is 1.25A/2 = 0.625A. If COT is selected, the peak current limit is selectable as 0.75A or 0.5A, and there is a corresponding valley current limit as well.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Expert 2315 points

    Thanks Tim.  Yes, I do understand this.  But my question is regarding the maximum allowable current when the part is in 100% duty cycle and is not switching, therefore the current does not return to zero, nor is there a valley current - the LM5166 is essentially acting as an always-on switch with a DC current passing through it.  In that case, can the part handle 1.25A (the peak switch current), 0.5A (the maximum output current), or something else?

  • 0 in reply to David Dean
    TI__Guru 73072 points

    Hi David,

    What's your RT setting - with or without a resistor, i.e. PFM or COT?

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Expert 2315 points

    Hi Tim,

    I have not determined that yet, I was going to base that off of this discussion.  From the datasheet it looks like both modes are able to handle 100% duty cycle operation.  I was probably going to use PFM mode simply to avoid the Resr required for COT mode.  But if one mode or the other is the right choice for what I am trying to do then I would use that one.

  • 0 in reply to David Dean
    TI__Guru 73072 points

    Hi David,

    Yes, the current can go as high as 1.25A in PFM and 100% duty cycle. If this is a concern, then you can choose COT operating mode.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Guru 73072 points

    Hi David,

    You can use a type-3 circuit for COT ripple injection with the LM5166.

    As an alternative, please consider the LMR36006 current-mode converter.

    Regards,

    Tim