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LM5163: Output voltage is to high

Roger Ludwig
Prodigy 20 points
Part Number: LM5163

Tool/software:

Hi,

We are using a LM5163DDAR in a new design. It is used to generate 5 V / 0.3 A from 20...50 VDC, the schematic looks like this:

Using the values in the schematic the output voltage was not very stable and jumped between 5.1 and 5.9 V depending from the load current. So I have optimized the values as follows:

L201 = 100uH

C205 = 100pF / 50 V

R201 = 43k

Now the output voltage is more stable in a range from 5.09...5.22 V but depending from changes in the output current it rises up to about 5.4 V. If the current then falls again to old value the voltage usually drops also but sometimes it stays at the higher value.

For me it is unclear why the voltage is significant higher than the calculated maximum of 5.15 V? And why does the voltage make such high jumps?

Because of the limited layout space our layout is not 100% as specified from TI. Could this be a layout problem e.g. from the feedback connection? 

Thanks in advance.

Best regards,

Roger

  • 0
    TI__Guru 51081 points

    Hello Roger 

    The VOUT average is increased by adding the CFF capacitor (C205) since the converter regulates the valley of the VOUT ripple via the FB pin. 

    In addition with that, I think the switching is unstable now and increasing the output volage. Would you please send me the inductor current waveform together with VOUT when VOUT is ~ 5.4V? 

    -EL

  • 0 in reply to EricLee
    Prodigy 20 points

    Hi Eric,

    Thank you for your answer.

    I have reduced the value of CFF to 100pF and then getting the ~5.4V at VOUT. With the 2.2nF VOUT was ~5.9V.

    To measure to inductor current I've inserted a 0.05 ohm shunt resistor at the cross in the image below. 

    With this shunt resistor VOUT was stable at ~5.12V! So I was not able to measure the inductor current @5.4V VOUT. Btw. I'm using a inductor from Würth Electronic 744787101.

    But I have made the following measurement @5.4V VOUT:

    Ch A: Voltage on VOUT

    Ch B: Voltage on SW U200.8

    Hope this helps you.

    Best regards,

    Roger

  • 0 in reply to Roger Ludwig
    TI__Guru 51081 points

    Hello Roger 

    Loos like the converter is not stable and I think you need more ESR.

    Based on my rough calculation, you need ~ 100mOhm ESR to make the system stable. 

    Please download the Quick Start calculator from the product folder and double check the ESR requirement.

    -EL

  • 0 in reply to EricLee
    Prodigy 20 points

    Hi Eric,

    Thanks for your hint. I will try this probably next week and then give you feedback.

    Roger

  • 0 in reply to Roger Ludwig
    TI__Guru 51081 points

    Hello Roger 

    Please share some waveforms after making the changes next week. 

    -EL 

  • 0 in reply to EricLee
    TI__Guru 51081 points

    I will be out of office and I will not be able to reply this E2E post anymore.
    If you are still trying to resolve the issue, please don't reply on this post, but create a NEW post to get a help from other engineers quickly.
    -EL