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Original question:

LMR33610: Current limit spec

LMR33610: Current Limit

Joshua Stump
Prodigy 140 points
Part Number: LMR33610
Other Parts Discussed in Thread: LMR33620, , LMR33640

I have seen in other posts the datasheet does not include current limit information, but it is the same as LMR33620. However, I have a doubt about this. Could you help me understand?

If they have the same current limit behavior, then their current at startup should limit at the same level, but that is not what is shown on the screenshots in the datasheets. 

For example, in the LMR33620 datasheet, looking at Figure 8-4, the startup current through the inductor seems to peak at ~2.5A, and bottom out at roughly 1.5A, centered around 2A. In the LMR33610 datasheet, looking at Figure 10, the startup current through the inductor seems to peak at ~1.4A, and bottom out at roughly 0.6A, centered around 1A. However, it is my understanding that if this part has the same current limit characteristics of LMR33620, then this start-up current would be higher? I understand their is a soft-start function, but that seems to be separate?  What am I missing? How can both of these parts have the same current limit specifications?

  • 0
    TI__Mastermind 30430 points

    The device is not hitting current limit during startup.

    Please noted the IOUT specification. Average value of inductor current would be just that as you noticed, and peak and valley would be dictated by inductor selection (ripple).

    LMR33610 and LMR33620 share same current limits. I was the one who reported this after talking to product designer.

  • 0 in reply to Marshall_Beck
    Prodigy 140 points

    Marshall,

    Thanks for your prompt response. As a follow-up, if it isn't a current limit limiting the device during startup, what is? I understand the IOUT spec is 1A for the LMR33610 and 2A for the LMR33620, but during startup on the waveform in the datasheet, the current seems "clamped" to 1A. This is just purely being limited by the soft-start functionality?

  • 0 in reply to Joshua Stump
    TI__Mastermind 30430 points

    Device current limit is active during startup. It is not hitting current limit evident by peak inductor current and the fact the device is not entering hiccup mode.

  • 0 in reply to Marshall_Beck
    Prodigy 140 points

    Marshall,

    Here is figure 10 from the LMR33610 datasheet.

    At point 1, the unit regulator starts. At point 2, current is increased to approximately 1A. At point 3, the regulator reaches full regulation and settles in. My point of confusion is number 2. However, I believe this all has something to do with the soft start feature, which is sparsely mentioned in the datasheet. 

    I found this paragraph in the datasheet under section 8.3.2, which I believe resolves my confusion:

    "The uses a reference-based soft start that prevents output voltage overshoots and large inrush currents as the regulator is starting up. Figure 10 shows a typical start-up waveform, indicating typical timings. The rise time of the output voltage is about 4 ms (see the Electrical Characteristics section)."

    So there is some soft-start magic inside this IC that is limiting the current in this fashion. If there wasn't, what would be preventing the current to go to the current limit during turn on? Can you confirm this?

    The reason I asked this question to start with, was because I could not find a SPICE model for this part "LMR33610". Under this part number, their is a spice file, but it is listed as the 4A version "LMR33640". If the LMR33620 is in fact comparable to the LMR33610, then I can use that spice model for simulation, since it is included on the website. However, simulations do not show this startup feature. So maybe the soft-start functionality is not captured in the model?

  • 0 in reply to Joshua Stump
    TI__Mastermind 30430 points

    hello Joshua.

    Peak current is limited by soft start.

    Current demand during start up (assuming un loaded) would be primarily from output cap. i(t) = c dv/dt. If you increase dt, i(t) will be limited. Soft start thus controls peak current.

    FB is ramped from 0 to 1V.

    If my memory serves me well, soft start is often not modeled as performance is quite predictable.

  • 0 in reply to Marshall_Beck
    Prodigy 140 points

    Thank you for confirming.

  • 0 in reply to Joshua Stump
    TI__Mastermind 30430 points

    Sure thing. Follow up if you find you need additional support on this device or other TI device. Please make a new thread though to ensure the appropriate person gets notified of your request as I personally only support a small portion of TI portfolio.