MP2172 step down switcher. Anyone have experience getting one going please?

There's an evaluation board for the 2172 here

https://www.monolithicpower.com/en/ev2172-j-00a.html

That shows a layout in its datasheet.

That's a better starting point than the snippet-of-whatever-it-is in the datasheet. The EVB operates well for sure, or they wouldn't be letting it anywhere near a customer.

Their aims are slightly different to yours - they simply want to showcase the chip to best advantage - but they're still keeping it all fairly compact to do that, so if you understand why they're doing the things they're doing there, it will stand you in good stead for doing your own.

HI jc2048,

I was unaware of the datasheet on the EV Board so thank you for this.
AS you see above I reworked my terrible layout to mimic the 'suggested layout' in the IC's original datasheet, however the EV board layout is different again with components in a completely different placements.
Well my new board is half way through production, so lets see if it works properly. If not you have now pointed me to what should be a 'sure fire' layout on the EV Board.

Great stuff.

Thanks for your help.

Dave

Some other things to think about.

Is the inductor shielded? A semi-shielded part may give you problems here because of the field extending to the sides and down into the tracks/plane areas, particularly with coupling to the feedback resistors. [The usual shielded part has a small airgap for the energy storage, but they put it as a ring on the top surface, so the external part of the small fringing field points up and away from the board planes and components.]
Is the core material good for the frequency you're running it at?
How much headroom do you have on the saturation current (remember that this is a soft ferrite, so the saturation current gets specified at 20% down on the nominal inductance value, so you really don't want to be anywhere near that at all)?
How did you determine the coil value? Did you use their on-line tool or do it yourself from the datasheet? If you're running from a battery, is it single cell? If so, you'll have to be very careful to look at both ends of the voltage range since that's quite a dramatic change in input voltage.

How noisy is your load? Is it something synchronous like a processor that could be sending noise back into your converter. The COT may be prone to resetting cycles early if there are noise spikes.

The converter self oscillates using the voltage ripple, so there needs to actually be voltage ripple clear of noise for reliable operation, so the combined ESR of the output caps is going to be a factor. The datasheet should cover that in detail. Also, if you're calculating your own voltage divider for the feedback, try and keep the loading similar to their examples, as it probably needs a minimum load to work well - don't be tempted to increase the values dramatically [if you think noise is coupling to the feedback, you could try temporarily dramatically reducing the values of the resistors to make it harder for the coupling to occur].

I don't know that you've helped yourself with the second layout. You might have done better to experiment with the first one to determine where the problem lies - there's nothing to stop you adding wires, soldering on bits of copper foil, adding temporary shields between components, and so on.

As to guidance for layout, look at the application notes that all the manufacturers of converter chips produce. They're as good as anything you'll find in books, and much more reliable than random people like me on the internet. But the basic principles are simple. Keep the input and output current loops as tight as possible, and keep the high dv/dt stuff away from the analogue feedback bits. Oh, and it's worth remembering that the designers of chips do think about layout issues and have in mind how the external layout is going to work, so look at the pinout and think your way back to what was going through the designer's head and why they arranged the pins in a particular way.

Some other things to think about.

Is the inductor shielded? A semi-shielded part may give you problems here because of the field extending to the sides and down into the tracks/plane areas, particularly with coupling to the feedback resistors. [The usual shielded part has a small airgap for the energy storage, but they put it as a ring on the top surface, so the external part of the small fringing field points up and away from the board planes and components.]
Is the core material good for the frequency you're running it at?
How much headroom do you have on the saturation current (remember that this is a soft ferrite, so the saturation current gets specified at 20% down on the nominal inductance value, so you really don't want to be anywhere near that at all)?
How did you determine the coil value? Did you use their on-line tool or do it yourself from the datasheet? If you're running from a battery, is it single cell? If so, you'll have to be very careful to look at both ends of the voltage range since that's quite a dramatic change in input voltage.

How noisy is your load? Is it something synchronous like a processor that could be sending noise back into your converter. The COT may be prone to resetting cycles early if there are noise spikes.

The converter self oscillates using the voltage ripple, so there needs to actually be voltage ripple clear of noise for reliable operation, so the combined ESR of the output caps is going to be a factor. The datasheet should cover that in detail. Also, if you're calculating your own voltage divider for the feedback, try and keep the loading similar to their examples, as it probably needs a minimum load to work well - don't be tempted to increase the values dramatically [if you think noise is coupling to the feedback, you could try temporarily dramatically reducing the values of the resistors to make it harder for the coupling to occur].

I don't know that you've helped yourself with the second layout. You might have done better to experiment with the first one to determine where the problem lies - there's nothing to stop you adding wires, soldering on bits of copper foil, adding temporary shields between components, and so on.

As to guidance for layout, look at the application notes that all the manufacturers of converter chips produce. They're as good as anything you'll find in books, and much more reliable than random people like me on the internet. But the basic principles are simple. Keep the input and output current loops as tight as possible, and keep the high dv/dt stuff away from the analogue feedback bits. Oh, and it's worth remembering that the designers of chips do think about layout issues and have in mind how the external layout is going to work, so look at the pinout and think your way back to what was going through the designer's head and why they arranged the pins in a particular way.