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LMR14203 webench Ton error incorrect?

Thomas Lucas
Prodigy 110 points
Other Parts Discussed in Thread: LMR14203, LMR14206

I question whether the webbench model for the LMR14203 is accurate. it fails when trying to use it for a design which the literature shows as typical. it errors as failing min TOn time. there should be no problem with min on time with the typical example. I was initially checking the part out with: 10-30 VDC in 3.3 VDC @0.075 A out when using webbench for the parameters above it allows the LMR14206 which is nearly identical. frequency is slightly different and it has a greater current capacity. ]

[Steps Needed to Recreate Problem:

model the parameters:
4.5-42 vdc in
3.3 vdc @ 300ma out.

  • 0
    TI__Prodigy 235 points

    Hello Thomas,

    Thank you for contacting us regarding this issue. I will take a look at the LMR14203 to see what is causing the failure. 

  • 0
    TI__Prodigy 235 points

    Hi Thomas,

    The reason why both devices are failing with your given inputs ( 4.5 - 42 Vin, 3.3Vout, @ 300mA) is because it is failing the minimum duty cycle check. The duty cycle check for these devices is the following:

    Vout /( (min on time) * (Fsw) ) > Vin Max

    For your numbers, this translates to:

    3.3 /( ( 100^-9) * (1.2e^6) ) > 42
    27.5 > 42, which is an untrue statement, and thus fails.

    This means the only way to pass this check is to lower your max Vin. The datasheet does state an alternate switching frequency of 350KHz when Vfb = 0, however that switching frequency is not currently modeled in WEBENCH. We decided to fix the frequency of these devices to 1.2MHz in order to allow the use of small external components while keeping the Vout ripple as small as possible.

    The alternate frequency would make the above equation 94.285 > 42, which is a true statement. The device can operate over the range you (and the datasheet) specified, it is simply not yet modeled in WEBENCH.

    Please let me know if you have any more questions.

  • 0 in reply to Jazmine Livingston - WEBENCH Design Center
    Prodigy 110 points

    Jazmine,

    So this is just a limitation of Webench then correct? I say this because the datasheet has examples that I assume work and the demo board that is sold has values that fail that calculation.

    Data sheet 

    4.5 - 42 VDC in, 5.0 VDC out

    4.5 - 42 VDC in, 3.3 VDC out

    4.5 - 12 VDC in, 0.8 VDC out

    http://www.ti.com/lit/ds/symlink/lmr14203.pdf

     

    demo board 4.5-42 VDC in 1.2VDC out

    http://www.ti.com/lit/ug/snva502b/snva502b.pdf

  • 0 in reply to Thomas Lucas
    TI__Prodigy 235 points

    Hi Thomas,

    We are digging deeper into this issue to figure out why these devices are failing when the datasheet seems to say they should not. I will let you know once we have gotten to the root cause. 

  • 0 in reply to Thomas Lucas
    TI__Prodigy 235 points

    Thomas,

    We have looked deeper into this and did some bench testing. The datasheet seems to have mistakenly carried over example designs from a device that this was adapted from. I am working with the group to get these erroneous designs removed/corrected and a new updated datasheet released. It should be out within the next several weeks.  

    The 4.5V - 42Vin, 5Vout example you referenced above  is correct and will pass in WEBENCH.

    In the mean time, please feel free to plug in your parameters into WEBENCH in order to find an alternative device.

    If you have any more questions or concerns, please reach out to me again.