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Electronics beginner.

I am trying to follow the layout recommendations for this part, which is a high PSRR LDO.

For the SET pin, one can read the following (P.15):

Any leakage paths to or from the SET pin create errors in the output voltage... Minimize board leakage by encircling the SET pin with a guard ring operated at a potential close to itself — ideally tied to the OUT pin. Guarding both sides of the circuit board is recommended.

Then the manual goes on to show this image of a "guard ring":

enter image description here

First Question: where is the guard ring here, from this question, I thought it was supposed to be a ring of conductive material surrounding the pin we want to protect.

In the evaluation board the OUT pin does not even seem to surround the SET one (see following image):

enter image description here

Second question: Concerning the elusive "Guarding both sides of the circuit board is recommended" sentence: What does this mean and how do I do it? I don't see them doing it on the evaluation board...

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  • \$\begingroup\$ What accuracy and stability do you require? \$\endgroup\$ Sep 12, 2023 at 16:13
  • \$\begingroup\$ @Tim Williams , this is literally the first PCB I am making, the LDO comes after a DC DC converter, I have not thought much of the accuracy requirement, as I am still a beginner in this field, I just thought I would follow the instructions as carefully as possible. \$\endgroup\$ Sep 12, 2023 at 16:51
  • \$\begingroup\$ If you have no other constraints, then why choose such an expensive part? Or dedicate more time to making a clean DC-DC converter? \$\endgroup\$ Sep 12, 2023 at 17:03
  • \$\begingroup\$ This probably is due to my lack of knowledge on the subject, I just compared the parts from the same company, and this one had the best specs I thought, since money is not an issue, I am making a few of these PCBs, I just went for this one. \$\endgroup\$ Sep 12, 2023 at 17:05

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From the datasheet:

Application information excerpt highlighted

100nA might be typical leakage of a contaminated PCB, say with mildly conductive flux residue, on a humid day.

0.1% accuracy is extremely precise as regulators go; with 1% initial accuracy, it will have to be trimmed to achieve finer precision, but presumably will be reliable (stable) thereafter.

Since from your comments, you have no critical performance requirements, then this consideration is not important and can be ignored.

As for why they didn't in the eval board, probably because it's an example, not a specific application. They also have control over the assembly, e.g. can specify suitable flux type, and washing to clean it. Whether the board remains as clean after its end users (design engineers) have made modifications, is entirely up to them [the users].

They may also simply only care about demonstrating the low noise performance, the absolute DC voltage of which doesn't matter.

Or they may simply not care. Eval boards and appnotes are notoriously dubious in quality; they are, at best, starting points for further development. As the name suggests, "eval board" is for evaluation purposes; it's not a finished product, and need not demonstrate any particular features of the devices used. (Presumably it'll be made to cover some, most even, but almost certainly not all, features.)


As a beginner, even if cost is no object, going for "maximum performance" is generally ill advised, especially when you have no apparent way to show benefit from whatever nebulous variables are being "performed".

Now, in this particular case, from a quick scan I don't see any obvious pain points, but generally speaking, performance comes with a cost.

Let me illustrate through automotive example: consider all those video clips of idiotsdrivers wiping out and crashing their, you know, Corvette, Mustang, Porche, whatever. Maybe they're new drivers, not used to how the vehicle handles; maybe they're in fact experienced drivers and just made a mistake. Whatever the case: sure they're performance cars, but that performance comes at a cost. Hit the throttle too hard, take a corner too tight, you lose traction and it's all over, at the very least clipping a curb damaging wheels/suspension, at worst killing innocent bystanders. There is very good reason why consumer cars are boring to drive: they're also safe to drive. Or more precisely: they're easy to drive safely, and hard to drive unsafely.

So it is with electronics. Choose common, widespread, well understood parts, with the least likelihood of unexpected behavior. An LM7805 (or 317 or etc.) might well do you better (assuming other constraints are satisfied), as it's far less critical of errors in layout or component choice, basically can't be miswired in a nondestructive way (you can only get "so" wrong, with only three pins), and is extraordinarily well documented. They're also easier to solder, not having an invisible middle pad that requires a reflow process.

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  • \$\begingroup\$ I like the part when you say I am an idiot. You are probably right though, i'll see if there are simpler parts I can use. \$\endgroup\$ Sep 12, 2023 at 17:47
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    \$\begingroup\$ Very sorry, I don't mean to imply that! I mean only the drivers. The technical term is "reckless", but given the context of such clips posted to social media (as one might be familiar with; which, granted, is another assumption I've made), such a sarcastic stab might be apt. \$\endgroup\$ Sep 12, 2023 at 17:50

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