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LMZM23600: Output voltage limiting factor

Mikhail Kharkine
Prodigy 10 points
Part Number: LMZM23600
Other Parts Discussed in Thread: TPSM63610, LMR36501, LMR54406

Tool/software:

Why is the output voltage limited at 15V? Is it because integrated 10uH inductor could lead to high ripple?

I need adjustable 3-18V output at 100mA. The input is 24V. I tried LMZM23600 in this configuration. It works, ripple is acceptable. What are potential problems?

  • 0
    TI__Mastermind 18465 points

    Hello,

    Since this device is a module (inductor inside the package) it is optimized for the recommended conditions in the datasheet. Operating outside of the recommended operating conditions listed in Section 7 of the datasheet, the device may not behave properly. This may be as the result of excessive current ripple, loop instability  or other factors.

    In your application, you are violating an absolute maximum rating on Vout. See the 16V abs max rating in Table 7.1 of the datasheet. Operating outside of the absolute maximum ratings may result in device damage. This device is not suitable for the proposed Vout=18V application.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 10 points

    So, is it loop instability (not acceptable obviously), excessive ripple (which is acceptable in my case) or other factors? :)

    I hoped for exact answer from TI. Not just guessing and pointing at obvious things.

  • 0 in reply to Mikhail Kharkine
    TI__Mastermind 18465 points

    Hi Mikhail,

    The max output voltage is limited by subharmonic oscillation. Being that there is a max switching frequency and a fixed inductor, you cannot get a higher output voltage than what the device is rated for. The 18V output is in violation of the absolute maximum specification. TI cannot advise that the device will function properly or at all when a design is in violation of the absolute maximum specification.

    You will need to use a device such as the TPSM63610 which is a module rated for Vout=20V. Otherwise, a converter is more suitable to this application.

    For 100mA, the LMR36501 is a good starting point.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 10 points

    I am not going to use anything violating manufacturer's specs in production. Just trying to understand why respectable TI designers selected topology with such restrictions. It should be something really good about it. 

    BTW, I can not reproduce subharmonic oscillations. Vout 18V Iout 0-200mA. The inductor is exposed so the access to the switch output is easy. Whatever I do there is no intermittent wide-narrow pulses.

    The output voltage of LMR36501 is limited at 16V while Vim max is 65V. And there is no integrated inductor to blame. Definitely somebody in TI likes this topology.

    TPSM63610. I don't think the expensive device optimized for 8A is a good choice for 100mA circuit.

    LMR54406 is probably  the best fit.

  • 0 in reply to Mikhail Kharkine
    TI__Mastermind 18465 points

    Hello,

    The LMR54406 or the LMR36501 are both good options for this application. 

    For the LMR36501, the 16V abs max rating rating for the Vout/FB pin does not set the maximum possible output voltage. The Vout/FB pin is the feedback node of the buck converter. If you are using the adjustable output mode, then the resistor divider (set for Vref = 1V) on the output will allow for an 18V output. Since your application would require adjustable output, the below figure shows the necessary configuration for an 18V output.

    If the device was used in the fixed output mode, then you would connect the Vout/FB pin directly to the output of the buck (after the inductor). The device is capable of sensing the feedback path to determine which mode you are operating in.

    The LMR36501 is rated for the application you have described with a 100mA output. Please refer to Section 8.3.6 of the datasheet for reference on calculating the necessary resistor values for an adjustable output application. 

    Otherwise, the LMR54406 would work as well.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 10 points

    What is the maximum Vout for LMR36501? :)

  • +1 in reply to Mikhail Kharkine
    TI__Mastermind 18465 points

    Hello,

    The maximum possible Vout can be calculated for your application using the minimum on and off times, the duty cycle, and desired frequency.

    Best regards,

    Ridge