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TPS5450: Ringing and 2V p-p noise on output

Charlie Jeff59
Prodigy 20 points
Part Number: TPS5450
Other Parts Discussed in Thread: TPS54540

Hi,

I'm working on a design based on the WEBENCH and TPS5450 converter. My issue is that I'm getting >2v p-p noise on output. I used the simulator but my real world results are troubling.

Specs

- 24V input
- 3.5A output
- 5V output

Components

- L: 12uH/23mR/ - SRR1260-120ML
- Cin: 20uF/20mR - GRM32ER71H106KA12L x 2
- Cout: 330uF/17mR - 6SVPC330M
- D: 5A/60V/100pF - SBR5E60P5-13

Circuit

Problem

I'm getting 2V p-p noise on the 5V output. Also visible on inductor. Please see waveforms:


Solutions attempted

- Adding a 24R in series with Cboot
- Adding a snubber network (5R/110pF)
- Changing D to a B340AF-14

This helped but nothing like what it should. I wonder if it is the PCB but in all honesty have never experienced such an issue with a switching converter. Can it really be that sensitive?

Thanks for the support

Kind regards,
Charlie.

  • 0
    TI__Expert 6245 points

    Hello Charlie,

    You have two potential issues:

    1) Measurement:  It is likely that you have both cross talk in your oscilloscope – for this try disconnecting the probe measure the PH node – and grounding is not quite right – see the figure that I am attaching for a good measurement method.  In this picture the TPS54540 is shown but the technique is the same.   

    2) Longer than needed input loop:  I have marked up your layout screen shot to show the converter’s ground loop.  Notice that the high AC current path is through the ground island connecting to the catch diode, not the VIN to chip ground loop bypassed in your layout.  See Figure 17 in the datasheet.  You can move one of the input capacitors to bypass the catch diode island, which should reduce ringing.  

    Also, the input trace is thin despite the high current that it carries.  If you redesign your board, it should be beefed up.  

    Regards, Robert

  • 0 in reply to Robert Blattner
    Prodigy 20 points

    Hi Robert,

    Thank you for your response. I have looked into these and updated the PCB design. Would you mind confirming this is along the right lines before I commit it to the next revision please?

    +24V (in)/5V = dark red
    GND = pink
    Lx (PH pin) = bright red

    Kind regards,
    Charlie.

  • 0 in reply to Charlie Jeff59
    TI__Expert 6245 points
    Hello Charlie,

    Your new layout looks excellent. If you would like a look-ahead test, try flipping the closer cap on your current design to bypass the ground island next to the catch diode.

    Regards, Robert