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LM5085: input power range smaller than webench design

Harm Lok
Prodigy 145 points
Part Number: LM5085

I have designed a  dc/dc converter from 10-60V to 5.00 V @  1A with LM5085MYX/NOPB

After the build with exact the listed components the supply is working only between 7- 20 Volt

What is the solution our the common design fault  that made this happen ?

webench.ti.com/.../SDP.cgi

  • 0
    TI__Mastermind 23815 points

    Hi,

    Do you have the layout for your design? 

    Generally it is recommended to use similar layout as an EVM especially on the input capacitor, PFET and diode

    http://www.ti.com/lit/ug/snva357b/snva357b.pdf

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 145 points

    Thanks for your reply ,

    I have enclosed the schematic and the pcb layout.

    Everything operates perfect until dc 20V input , then the supply stops.

    Hope you can help ..Celmonitor.pdf

  • 0 in reply to Harm Lok
    TI__Mastermind 23815 points

    Hi,

    It looks like the size of your R15 looks small. Is there anything gets hot when the fault happens?

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    TI__Mastermind 28770 points

    Harm, has the issue been resolved?

    Please let us know if we can be of anymore assistance, or let us know that this issue has been resolved so we dont continue to monitor this thread.

  • 0 in reply to Marshall_Beck
    Prodigy 145 points
    No, the problem still exisst . It took a while to get a temperature scan with a flir device. After the Voltage decrease above 22 Volts there is no change in temperature of any device. We looking forward for any suggestion. But the answer where very common.
  • 0 in reply to Harm Lok
    TI__Mastermind 28770 points

    Arief is trying to understand if it is a thermal issue or a stability issue. 

    You have reported that the device stops to regulate above 20V.

    One experiment you can do for us to better understand the issue is set the input voltage just below the problem voltage (20V - 1V = 19V) Let the device regulate successfully for about 15min. After that time is elapsed, increase the input voltage just above the problem voltage (20 + 1V = 21V). 

    After this experiment, please answer the following:

    Q1: Did the device stop regulaing instantly after increase the input voltage above the problem voltage

    Q2: Did the device stop regulating during the soak time?

  • 0 in reply to Marshall_Beck
    Prodigy 145 points

    Hello Marchall, Arief,

    We have connected the converter first and regulate slowly to the point that the device switched of
    this time the level was 21 volts, the output drops back to 400 mV.
    Then we switch on the converter at 20 Volts and connect it for 15 - 18 Min.
    The converters output voltage keep constant regulated 5.0 V during the whole time.
    After that we have to increase the voltage to 24.5  Volts (3.5  Volts higher) before the converter
    switched of again to 400 mV
    During whole the experiment the temperature was constant. Remarkable is that C17 is the hottest spot 
    with 38.2 degree Celsius this is the capacitor 2n2 which is connected between output and feedback.
    Temperatures are not changing during the experiment.
    We have also checked and measured all the values of resistors and capacitors.
    Hope you can give us some clues.
  • 0 in reply to Harm Lok
    TI__Mastermind 27030 points

    Hi

    Just want to check the output capacitor type, in your wenbench file, it is bulk cap with 0.65ohm ESR which will be helpful to LM5085 stable. if it is ceramic,  it maybe need add external ESR for meet the stability.

    if you can share the SW node waveform when Vo is falling at Vin=21V, it will be helpful for us.

    Thanks

  • 0
    TI__Expert 4555 points

    Hi Harm,

    Do you still this issue?

    Please let us know if we can be of anymore assistance, or let us know that this issue has been resolved so we dont continue to monitor this thread. Thanks!

    Best Regards,

    Ankit Gupta

    Applicaiton Engineer

  • 0 in reply to Ankit Gupta95
    TI__Mastermind 27030 points

    Hi Harm:

    we don't get more feedback from you and we will close this dissusion. if you have further question, you can reopen it.

    Thanks

    Daniel Li