LMR36503-Q1: LMR36503-Q1 Stability Calculations

David Pastl1

Part Number: LMR36503-Q1
Other Parts Discussed in Thread: TPS62933, LMR36503

Hello,

I'm trying to design a buck regulator with a 20V output, 0.1A current, using the LMR36503-Q1. I cannot find any guides on how to calculate the loop stability for a design like this, the datasheet specifically says to just use the reference designs for 3.3 and 5 volt outputs. Webench is glitching for me quite a bit and I don't really trust the data I'm getting from it. Simulating it in PSpice isn't sufficient since I need to know how stable it is at various operating points, to ensure it'll work in all conditions.

Are there tools, such as workbooks or simulations, that can evaluate the stability of my design? I was able to find one for a USB PD chip you make and that was really incredibly useful as part of the design process.

Thanks!

2 months ago

0 Ben C 2 months ago

TI__Mastermind 35905 points

HI David,

Here is a link that can help you with your stability analysis.

https://www.ti.com/lit/an/sluaag4/sluaag4.pdf

Ben

0 David Pastl1 2 months ago in reply to Ben C

Prodigy 20 points

Hello Ben,

Thanks for the link. That's an interesting read through, however it's specific to the TPS62933 part and includes data not present in the datasheet for the TPS62933 that is needed to do the analysis. What is interesting is that the TPS62933 datasheet includes calculations for the output capacitance, where the datasheet for the LMR36503 does not.

The LMR36503 datasheet also does not contain the same information required for those calculations (DC gain, poles) so that's a non-starter as well. At least I can't find it.

With so little design information available for the LMR36503 I don't really understand how it would be possible to use it for any design that isn't a 3.3v or 5v output regulator.

+1 Ben C 2 months ago in reply to David Pastl1

TI__Mastermind 35905 points

Hi David,

My suggestion is to order our EVM and measure the gain and phase margin (bode plot) on the bench. Below is a link that shows how it is done:

https://www.ti.com/lit/an/snva364a/snva364a.pdf

Ben

0 David Pastl1 2 months ago in reply to Ben C

Prodigy 20 points

Thanks Ben, we're looking at investing the equipment needed to do that now. So far we've gotten away without doing that but I think it's time we do so we can make sure our designs are functional with sufficient margins of safety. It's so easy to make something that works, but so much harder to make something that always works and that you can prove will always work.

0 Ben C 2 months ago in reply to David Pastl1

TI__Mastermind 35905 points

Hi David,

Sounds like a good idea. Best of luck to you.

Ben

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LMR36503-Q1: LMR36503-Q1 Stability Calculations