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LM5185-Q1: Unable to get desired output voltage of 12V

SndrS
Prodigy 75 points
Part Number: LM5185-Q1
Other Parts Discussed in Thread: LM5185, , LM5181-Q1

Tool/software:

Dear All,

We have designed a board with the LM5185. VIn=14-21V. The desired (single) output needs to be 12V 2A. See calculation below:

It nicely switches on at 11.5V and off at 10.5V. However the output is ~24V! (not 12V). As suggested in the calculator below, we are using Rfb=127kΩ and T1 has a 1:1 ratio, we have be trying to change T1 1:2 and lowering Rfb to 100kΩ but no effect.

Using the following FlyBack Transformer: Wurth 750318701, https://www.we-online.com/components/products/datasheet/750318701.pdf

Is there anything you can suggest? Please advise! Thanks a lot. 

Sander

  • +1
    TI__Guru* 76745 points

    Hi Sander,

    Thank you for designing with the LM5185.-Q1.  Do you have load applied?  Note that the circuit has a minimum load requirement.  Please refer to the last paragraph of the datasheet Section 7.3.2:

    If you load can become open, you should install the dummy load Rdmy, or the output Zener clamp device, of both.

    Hope this helps.

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 75 points

    Dear Youhao,

    Thanks for your prompt reply! Yes! This was way too easy... another great example of thinking too difficult Slight smile

    Yes, our load can becomes open indeed. However there has been a Dz on our output. It's a SML4749A with a Vz of 24V...

    I know it might be hard to judge from your side, but the LM5185-Q1 now outputs 12.8V under a additional load of 200mA! It remains still 0.8V over our desired output. Can this be caused by the feedback? 

    Thanks!

    edit: part number updated to LM5185-Q1.

  • +1 in reply to SndrS
    TI__Guru* 76745 points

    Hi Sander,

    The Dz should be close to the Vout setting point.  We usually take a Zener of about 5 to 10% greater than the Vout target.  

    Sorry is it a typo?  You started the thread with LM5185-Q1, but know you are talking about the LM5181-Q1.  Could you clarify?

    Anyway, please reduce RFB resistor from 127k to 124k as the calculator suggested, in order to get Vout closer to your 12V target.  

    Hope this clarifies. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 75 points

    Hi Youhao,

    Thanks for your reply again. Yes, Dz is already close to the Vout point. Need to update that part. But your load suggestion made us double check the Dz specs. Thanks again.

    Yes thanks, we are aware of the 127 vs 124kΩ. Will lower the Rfb. Issue has been solved.

    Thanks!!
    Cheers, Sander

    ps: Sorry, yes was a typo!! Still talking about the same LM5185-Q1 (shall update my prev message).