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TPS7A57: Unexpected sine wave at the output

Part Number: TPS7A57
Other Parts Discussed in Thread: LMS3635-Q1, LM1084


I'm using TPS7A5701 chip in compination with LMS3635-Q1 DC-DC. The idea was to create a voltage follower with clean output.
The problem is that I see a sine wave with the frequency of LMS3635 under load. Without load the results a good (at least under resolution of the oscilloscope)

At this pictture you see the output curve under 1A resistive load. The bigger the load is, the bigger is the sine noise.

What I've already tried to do:
1. using external/internal bias
2. adding/changing type of an additional output cap
3. removing and adding 0.1uF cap parappel to REF
4. Moving ground sense (Rsense GND) connection outside of the board made the signal only a bit better.

R2 in now 39k. I get 4.9V after DC-DC.

The board is 4-layer 1.0mm board.

I also have almost the same board with combination of LMS3635 and LM1084 and everything works good. Any ideas?

PS I understand that I violated some PCB design considerations but I doubt that this could cause such performance issues.

  • Hey Maksim, 

    Are R23 and R29 Testpoints? 

      I've noticed in your layout the nets lead to a header connection through a rather long trace. 

     SNS and ref pins are sensitive to noise coupling in though traces functioning as antennae's

    They show up as not connected on your schematic, did you happen to populate those points?. 

    Also, your scope  probes may not have the resolution to get as clear of a signal at a 1mV scale:

    Your pk-pk is showing up as 800uV, and at a 1.95V output is a .1% deviation, well within the 1% accuracy spec of this device. 

    please see figure 50 for acceptable performance. 



  • I have got something similar. I think I've solved the problem.

    The coils on the PCB must have the certain direction. Despite there is no marking regarding winding direction, the influence in significant. I have the coils with flat winding and 1 winding layer. The lowest winding must be closer to the DC-DC switching node. And also for the ciol of L-C filter

    The results are here.