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WEBENCH® Tools/LM43602: strange results (if any)

Leo Potjewijd
Prodigy 90 points
Part Number: LM43602
Other Parts Discussed in Thread: LM20333

Tool/software: WEBENCH® Design Tools

When opening the design with the required values (VSON package, 5.95V≤Vin≤27.6V, Vout=5.12V, Iout=1A, 30°C), Webench simply states that this is not possible with the LM43602 ; no explanation is given. If I change the package to PWP a solution is found (along with 122 others): strange, since the datasheet lists no other difference between the packages than the thermal resistance...
When entering the advanced options (Fsync=1070kHz and power good), only the LM20333 comes up; if the power good checkmark is removed 20 results show up (of which 6 show a power good signal !), but the LM43602 is not among them.
The 'parts not found' list is surprisingly empty in all cases...

And that is where I am lost: the LM43602 datasheet states a power good signal, an input voltage from 3.5 to 36V (wider than required), an output current of 2A (more than required) and a synchroniszation capability between 200kHz and 2.2MHz (where 1.07MHz is required). It also states a minimum on-time of 250ns max, resulting in an absolute minimum duty-cycle of 17.6% at 1.07MHz which should be sufficient to regulate 5.12V from a 27.6V input; with the described frequency foldback feature the minimum input voltage should also pose no problem.
To be perfectly clear: I need synchronization with an integer multiple of 134.08kHz.

Any help and/or insight is welcome.

  • 0
    TI__Guru**** 191445 points
    LM43602 cannot support those requirements at 1.07 MHz due to minimum off time restrictions. Webench should still have found a solution at a lower frequency below about 540 kHz or so, an in any case should have returned a reason why the design did not work. I will move this post to the Webench forum for clarification.
  • 0 in reply to JohnTucker
    Prodigy 90 points
    I don't get it: maximum specified for the minimum off time is 250ns, that would be a dutycycle of 17% at 1.07MHz.
    TTBOMK that would suffice to get down to 5.12 from 27.6V. Unless of course my understanding of the buck regulator is incorrect, in which case I would gladly be educated....
  • 0 in reply to Leo Potjewijd
    TI__Mastermind 26710 points

    Leo,

    You should be able to see the reason under "Show why other parts were not found". I see the reason as that the external sync frequency is higher than the max frequency limit that the device can work with for your input conditions. In WEBENCH, the frequency is limited to around 482kHz due to min toff limitation. 

    Fsmax due to min toff = (1-Vout/VinMin*1.05)/(200ns). We are using the typical min off-time and adding 5% margin to duty cycle calculations done at VinMin. This leads to around 482kHz for VinMin of 5.95V with Vout=5.12V. It comes to around ~385kHz with max off-time of 250ns. You can lower your sync frequency to within this range to ensure the device works properly. Let me know if this helps. 

    Here is the shared design link with 268160Hz (2*134080Hz) as Fsync: webench.ti.com/.../SDP.cgi

    Regards,

    Amod 

  • 0 in reply to Amod Vaze
    TI__Guru**** 191445 points
    I tried this with just the standard Webench inputs of Vin range, Vout and Iout as listed above from the product folder page. Webench did not return a design and the "show why parts were not found" was blank.
  • 0 in reply to JohnTucker
    TI__Mastermind 26710 points

    I just tried again from the product folder page (www.ti.com/.../lm43602) and it came up with a 323KHz design from there.  You cannot set sync Fsw from there though.

    For the "Show why other parts not found" explanation, from the regular flow (ti.com panel entry) after entering the SYNC Fsw as 1070kHz and enabling "Power good" check box, as Leo mentions, you only see the LM20333 but when you check the reason, you should see the failure reason. If this is not reproducible for both of you, then we might have to take a deeper look.

  • 0 in reply to Amod Vaze
    TI__Guru**** 191445 points
    Yes it worked for me just now. I tried it this morning from outside the TI network and it did not. Maybe that has something to do with it, or maybe Webench was just "down". I'll try it again tonight when I get home. Or maybe the OP could try it again.
  • 0 in reply to JohnTucker
    Prodigy 90 points

    I did, and it worked here too.
    It may have had to do withthe fact that we are running Linux here, or with some security settings in FireFox/Noscript.

    As for the other stuff: it appears that I suffered from a severe cafeine deficiency and mixed up the on and off timings, sorry about that.
    The designs come up just fine now.