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TPS61175: Output voltage drops with increasing load

Vincent Rigolet
Prodigy 120 points
Other Parts Discussed in Thread: TPS61175, TPS61175EVM-326

We did a PCB using the webench results (attached file), with the Vishay Dale IHLP2020BZER6R8M01 instead of the Bourns SRN8040-6R8Y. (This point was the subject of another post).

The output voltage is ok (12.6V) @ no load, but it's decreasing when output current becames higher than 60mA. It's only 5.5V with a load that should draw 0.5A .

Do you have an idea of what is happening?

Here a a view of the pcb (double sided) and the webench results:

http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/196/1817.Webench-tps61175-IHLP.pdf

 

 

  • 0
    TI__Guru*** 137730 points

    Thank you for making a new thread.  My previous reply still applies.

    You will need to debug your design by looking at the waveforms of the various pins of the IC and in the circuit.  Based on what you describe and since you did your own layout, it sounds like noise is affecting the regulation.  You can always order the EVM to get a known good circuit to test.

    Based on your layout, some of the traces are much too narrow.  Vin and SW will carry amps and yet they appear to be 10 mil wide traces.

  • 0 in reply to Chris Glaser
    Prodigy 120 points

    I do have an EVM. I'm going to get in the same configuration (as webench), to see if our routing and traces are the problem cause.

  • 0 in reply to Vincent Rigolet
    Prodigy 120 points

    Ok, it's working well on the EVM.

    What would you suggest to make our PCB work? Add "thick" wires on the Vin and SW to compensate their thin traces?

  • 0 in reply to Vincent Rigolet
    TI__Guru*** 137730 points

    Yes, I think you could try adding thicker wires from Vin of the board to the inductor and from the inductor to the SW node and diode.

    I'm not sure where you are drawing the output power from (is it just that one via under the diode?), but thicker wires might help here too.

    I also noticed that the ground of your input cap does not connect to the board GND anywhere near the IC, so you might add a wire from its ground to IC GND (across the SW trace).

    Finally, C8 looks very small.  What part number did you use for that one?

  • 0 in reply to Chris Glaser
    Prodigy 120 points

    Chris,

     

    C8 is a 10µF X5R ceramic capacitor in 0805 footprint. It worked well on the EVM. I did the modifications you suggested me (thick wires on Vin, Vout and SW nodes, and wire add on input cap). It much improved the performances, but I still can't draw more than 0.18A, instead of 0.5A. Below these loads the voltage drops and the regulator becomes noisy.

    One last point, I'm not sure the powerpad is connected to the ground, (the chip was hand-soldered) could it be a possible cause of problem?

    Would you have other ideas to get the 0.5A before doing a new pcb?

  • 0 in reply to Vincent Rigolet
    TI__Guru*** 137730 points

    Yes, if the power pad is not fully soldered this can limit the IC's power dissipation by raising its temperature.

    Make sure you are measuring Vin and Vout at the IC's pins, so as not to include IR drops on the PCB.

    Webench called out a 1210 package for the output cap, so just make sure that you have the voltage rating high enough.  The smaller package will have worse dc bias performance, so you might need to adjust the compensation values.  Do this if the output voltage is oscillating.

  • 0 in reply to Chris Glaser
    Prodigy 120 points

    There's a 0805 footprint for the output cap but we used the webench specified cap for the tests, thus eliminating this cause.

    We will try to resolder the TPS61175 taking a special care to the power pad. We'll do a new layout if we don't succeed in getting the right output power. Anyway thanks for your useful advices.

    Can we eventually submit our new layout to you, to be sure not doing some big mistakes?

  • 0 in reply to Vincent Rigolet
    TI__Guru*** 137730 points

    You could just copy the gerbers from the EVM, located here: http://www.ti.com/tool/tps61175evm-326