SparkFun Forums

[houta69]

I think the problem is that you're assuming everybody knows what they're talking about.

This is true to some extent. I know that I don't chime in on something unless I felt confident in what I am saying (bad information is worse than no information), so I assume that most others behave accordingly (at least in technical forums such as this).

I don't have any insight into the members on this site, but I am fimilar with Leon from the microchip forums and I've never seen him steer anyone wrong. Again, this doesn't mean that I discount what you are telling me (I'm "assuming" that you're knowledgeable), this just tells me that I need to do more homework and now I am a little better prepared to do that.

Thanks

[maokh]

I have the exact situation .. one of my components likes 2.8V, the other likes 3.8V. There is really no way around it. the board works fine.

[rdpzycho]

there will be times that 'yes' the regulators will conflict..

but doing precautions and using caps around them usually work, all the time..

a lot of IC will be doing an internal regulation too (to somewhere around 2.++ volts)..

even industrial and commercial PSU's do use regulators in series with pre-regulators and other regulators..

it should just be noted that the load of the regulator is also a regulator in which the control should be able to compensate..

on the ground plane, try reading this..

http://www.hottconsultants.com/pdf_file ... signal.pdf
http://www.hottconsultants.com/pdf_file ... _plane.pdf

using planes or using fat traces won't be of any good effect if layout is not taken into consideration..when doing layout always try to look at the current return path of each signal..it should be as close as possible to the signal path so loop areas are minimized..the proper use of ground plane instantly makes the loop area almost zero..

then always remember to put a dividing line where the RF, Analog, Digital, and Power sections of your are..you may use a single ground plane for all of them but you can never put them exactly where the other ones are..

one more thing, don't use 90 degree corners on RF and high speed signals..a circular corner is ideal but 45 degree corners are sufficient..

good luck on your first CAD..

[NleahciM]

there will be times that 'yes' the regulators will conflict..

but doing precautions and using caps around them usually work, all the time..

a lot of IC will be doing an internal regulation too (to somewhere around 2.++ volts)..

even industrial and commercial PSU's do use regulators in series with pre-regulators and other regulators..

it should just be noted that the load of the regulator is also a regulator in which the control should be able to compensate..

I think everybody is missing the point of what I was saying: there is no advantage to him on this board to put the two regulators inline with each other. There are, however, potential disadvantages. We are not talking about every single possible situation. We are talking about this one dude's PCB.

[maokh]

I think everybody is missing the point of what I was saying: there is no advantage to him on this board to put the two regulators inline with each other. There are, however, potential disadvantages. We are not talking about every single possible situation. We are talking about this one dude's PCB.

He needs 5VDC for the RF device and 3.3VDC for the PIC.

Instead of saying "there is no advantage to him doing xyz", and ending your paragraph, why not suggest the possible alternative so everybody knows what you are talking about?

The alternative would be to put both regulators in parallel, as both his selected regulators can handle 12VDC.

6 of 1, half-dozen of the other. You could do it either way.

[leon_heller]

there will be times that 'yes' the regulators will conflict..

but doing precautions and using caps around them usually work, all the time..

a lot of IC will be doing an internal regulation too (to somewhere around 2.++ volts)..

even industrial and commercial PSU's do use regulators in series with pre-regulators and other regulators..

it should just be noted that the load of the regulator is also a regulator in which the control should be able to compensate..

I think everybody is missing the point of what I was saying: there is no advantage to him on this board to put the two regulators inline with each other. There are, however, potential disadvantages. We are not talking about every single possible situation. We are talking about this one dude's PCB.

You still haven't provided any evidence for the potential disadvantages, or specified exactly what they are, apart from mentioning 'weird oscillations'. Are both regulators affected, or only one of them, and what causes them?

Leon

[rdpzycho]

I think everybody is missing the point of what I was saying: there is no advantage to him on this board to put the two regulators inline with each other. There are, however, potential disadvantages. We are not talking about every single possible situation. We are talking about this one dude's PCB.

I am just saying that it's not wrong to do so..

I, myself, would have tied the 3.3V regulator to the 12V input..

one potential problem that is more critical (than the series loaded regulators) is that the 3.3V regulator is directly connected to a supply for an RF module..their loops must be well localized in the PCB so the RF load doesn't affect the 3.3V regulator..

[NleahciM]

You still haven't provided any evidence for the potential disadvantages, or specified exactly what they are, apart from mentioning 'weird oscillations'. Are both regulators affected, or only one of them, and what causes them?

So I know I've read in various app-notes or datasheets to not inline vregs - but so now you're forcing me to actually think about why this advice was given. Damn you for making me think!

Anyways, the way I picture it - I see multiple control loops driving each other. That's a recipe for badness. It'd be a much bigger deal with DC/DC converters. I agree that it would almost never be a problem - and have done so in the past on some boards when I didn't have a choice.

My point here is that the OP does have a choice. There are no advantages to inlining them for him while there are potential disadvantages. Probably some that I'm leaving out that a better analog guy could fill in.

[houta69]

The AME1117's have an absolute maximum Vin of 12V whereas the 7805's Vin can go to 35V. I have a bunch of 12V wall-warts laying around that I had planned to use to power the board(s) with which is why I thought it was best to run the LDO off of the 7805. Perhaps I would be better off picking up a smaller supply so that I'm not pushing the limits of the 1117 if I decided to supply it directly.

To continue my original line of thinking, I thought that using the second layer of the board as a ground plane and running all of my traces on the top layer (which I ought to be able to do since there are so few components) would eliminate or reduce to an acceptable level any negative effects of the RF module.... but then I saw so many differing opinions on grounding that I begain to have a lot of doubts. That mixed signal document that rdpzycho linked (thanks btw) is the most concise that I've seen yet and reinforces the "single plane is best in nearly all cases" therory.

[leon_heller]

Using the bottom layer as a ground plane is a good approach. I often make RF boards with through-hole parts, with the top layer left unetched with clearance round holes that are not connected to ground.

Leon

[NleahciM]

The AME1117's have an absolute maximum Vin of 12V whereas the 7805's Vin can go to 35V. I have a bunch of 12V wall-warts laying around that I had planned to use to power the board(s) with which is why I thought it was best to run the LDO off of the 7805. Perhaps I would be better off picking up a smaller supply so that I'm not pushing the limits of the 1117 if I decided to supply it directly.

To continue my original line of thinking, I thought that using the second layer of the board as a ground plane and running all of my traces on the top layer (which I ought to be able to do since there are so few components) would eliminate or reduce to an acceptable level any negative effects of the RF module.... but then I saw so many differing opinions on grounding that I begain to have a lot of doubts. That mixed signal document that rdpzycho linked (thanks btw) is the most concise that I've seen yet and reinforces the "single plane is best in nearly all cases" therory.

Your schematic says LM1117. That part (http://www.national.com/ds/LM/LM1117.pdf) is rated for Vin of up to 15V while your schematic says your VIN is 12V.

[houta69]

Your schematic says LM1117. That part (http://www.national.com/ds/LM/LM1117.pdf) is rated for Vin of up to 15V while your schematic says your VIN is 12V

Yep, that's my bad for not fixing it on the schematic that I uploaded, I used a predefined component and failed to update it, but I did specify the exact component in my 2nd post.

[NleahciM]

Your schematic says LM1117. That part (http://www.national.com/ds/LM/LM1117.pdf) is rated for Vin of up to 15V while your schematic says your VIN is 12V

Yep, that's my bad for not fixing it on the schematic that I uploaded, I used a predefined component and failed to update it, but I did specify the exact component in my 2nd post.

Hmm - don't do that on your schematics - that's just a recipe for problems.

[khearn]

Don't trust an unregulated 12V wall wart to give 12V. I've got one that I'm using that claims 12V and 1000mA. At low current draws it is usually giving me over 15V. So if the 3.3V regulator has a max of 15V, you probably don't want to hook it up directly to a "12V" wall wart.

Keith

[houta69]

Hmm - don't do that on your schematics - that's just a recipe for problems.

Hmmm I didn't expect you to notice that level of detail. Even though I provide my name (which you don't), you've referred to me as "dude" (my fav) and "op". Find another thread to play in kid.