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WEBENCH® Tools/LM5085: LM5085 load 2A issue

Panagiotis Pentovelis
Prodigy 30 points
Part Number: LM5085


Tool/software: WEBENCH® Design Tools

Hi E2E,

I am using LM5085 with Webench.

VIn = 6 - 75V. The goal is to make a power supply for 12, 24, 48V batteries.
Vout = 4.2V
Iout = 2A

M1 = ZXMP10A18K
D1 = SBRD10200TR

The circuit works when there is no load but with a load of 2A the LM5085 stops runnning or don't work with VIN > 20V.

Can you help to resolve this issue?

Thanks,

  • 0
    TI__Genius 11040 points
    Hello,

    Thank you for sharing the schematic and the layout for your design.

    Would you be able to help clarify the conditions at which your design is not working?
    1. When you say stop working - does the output voltage go to 0V? Or is there an irregular behavior?
    2. Does the design stop working when the input voltage is > 20V even if the output load is 0A?

    Also, would you be able to share waveforms of the SW node, output, and input at no load and 2A load? This way we can understand further what is not working.

    Thank you for your help.

    Best Regards,
    Katelyn
  • 0
    TI__Guru* 75882 points
    Hi Panagiotis,

    I recommend completing the LM5085 quickstart calculator (available by download from the product folder) so we can review current limit values and ripple voltage amplitude at FB.

    Note that for improved efficiency, you can use a 100V Schottky diode (a 3A to 5A current rating is sufficient).

    Regards,
    Tim
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 30 points

    Hi,

    1. The output goes approximatively to 0V

    2. the design works when the input voltage is > 20V with no loads but it stops working when the load is 1A with input voltage is > 20V

    PGATE is in blue.

    VOUT 0A load 18V IN

    SW 0A load 18V IN

    VOUT 82mA load 18V IN

    SW 82 mA load 18V IN

    VOUT 1A load 18V IN

    SW 1A load 18V IN

    SW 82mA load 44V IN

    SW 1A load 44V IN

    VOUT 1A load 44V IN

    Thanks,

    Panagiotis

  • 0 in reply to Panagiotis Pentovelis
    TI__Guru* 75882 points
    Hi Panagiotis,

    I expect current limit is engaging. Does the output recover when the load is reduced?

    Please forward a completed LM5085 quickstart file. Also, measure the inductor current to verify the inductor is not saturating. What is the inductor part number?

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

    Hi,

    yes the output recover when the load is reduced.

    The inductor is SDR1307-270ML 27uH. With the the quickstart file I changed it to a 47uH SDR1307-470ML. Ialso changed the Rsense and Radj according to the quickstart file but I have still the same behavior.

    Thanks,

    Panagiotis

    LM(2)5085_Quick_Start.xlsLM5085_calculation.pdf

  • 0 in reply to Panagiotis Pentovelis
    TI__Guru* 75882 points
    Hi Panagiotis,

    Make sure that the inductor is not saturating (exceeding the sat current spec given in the inductor datasheet), particularly at high operating temperatures. Note that the peak inductor current increases as Vin increases. Also, it's recommended to change to a 100V diode as that will lower the operating temperature and reduce leakage current losses.

    Regards,
    Tim