TPS63020: Output Voltage Sagging as Load Current Increases

Thanks for posting the schematic and the layout. The schematic looks ok, what effective capacitance do you have at the output?

Regarding the layout, I think it needs to be redesigned. I would recommend to follow the layout guidelines of the datasheet. It is critical to have a good layout to have a reliable design for switching converters. Especially the input and output capacitors should be placed as close as possible to the device. The ground connection should be strong. I also noticed there are no vias under the power pad.

What is the end application?

Sabrina,

Thank you for taking a look.  I have two 22uF caps on the output as was called for in the WeBench design.  I noticed that the datasheet had four caps with the inner-most one being smaller.  I am trying to stick to 0805 parts if I can while I am still doing hand placement and reflow.  Will this be OK?  Also, my board house design rules keep me to 13 mils on the vias.

I realize that my layout was not ideal.  Before I respin the board, would you mind taking a look at the revised layout below? The main issue I saw was a lack of vias to make a strong connection. 

The end application is a solar powered IOT device I am building and selling to local parks.  I have a little board which combines the LiPo / Solar charging, a fuel gauge and the TPS63020 to provide a consistent 4.0V so the IOT device always sees a fully charged battery.

Thanks for taking a look and please let me know if you think I need to make any more changes. 

Chip

One of my fellow TriEmbed members, pointed out that I should not use Thermals.  Also, to answer your question on capacitance, it would be 44uF on the output.

Does this look like it would be an improvement?

I agree with him I would recommend the same regarding the Thermal connections for copper pouring. Now the layout guidelines follows the datasheet recommendation.

Regarding the output capacitance, I would recommend to have 30uF (or more) of effective capacitance. This may not be the case with 2x22uF considering the DC bias of ceramic caps. What is the voltage rating of your capacitors? can you provide the DC derating curves of the capacitors you use?

Sabrina,

Looks like you are on to something here.  I used a Murata 22µF ±20% 16V Ceramic Capacitor X5R 0805.  Now that I look at the datasheet, I see that at a DC bias of 4V, the capacitance drops from 22uF to 12uF.  So, at that bias, my effective capacitance is only 22uF. 

Would you recommend I use a higher voltage capacitor or simply add a third to get to 30uF total?

Thank you! for all your help

Chip

It depends on your design requirements, usually if you have place constraint you can use a higher voltage rated capacitor. Otherwise adding a third capacitor is also an option. In this case I would also add extra vias for the ground

Thank you for all your help.  Decided to add two more caps (can choose to populate 4th or not).  Also, given the drill constraints, I could not fit as many in as I would have liked but, I added many more.  I am going to get this design built and will post the results.