Author Topic: Adding solder jumper on MOTEINO PCB for LDO ?  (Read 1527 times)

Uncle Buzz

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Adding solder jumper on MOTEINO PCB for LDO ?
« on: June 15, 2017, 04:41:58 AM »
Hi Felix,
As I understand, the LDO you choosed is the best compromise for low cunsumption, but as a compromise, it's not the best choice for all cases. If we want to use an extern 3.3V source, we have to remove LDO.
Is it possible to add in your PCB a solder jumper so we only have to cut it to deactivate reverse current in LDO without removing it, and just solder the jumper to reactivate LDO without dealing with SOT-23 component ?
Thks.
« Last Edit: June 15, 2017, 04:45:47 AM by Uncle Buzz »

Felix

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #1 on: June 15, 2017, 09:29:57 AM »
Not really. A few reasons:

- we'd need at least 2 pairs of 3 way jumpers to bypass and link the VIN/3v3, plus remove silkscreen to make room for that, and even so i dont think there's enough space
- the fuses would need to be different and set to the internal oscillator - assuming in this mode the moteino needs to run at less than 3.3v

I actually tried it once and it was a mess. At that point I decided if this ever happens it's going to be another board with no LDO and preset fuses so the users don't have to do more work.

Uncle Buzz

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #2 on: June 15, 2017, 09:46:16 AM »
I'm not sure to understand, with the actual configuration, we use the VIN pin if > 3V3, the pin 3V3 is a power output from LDO
If I want to apply an extern 3V3 supply, I have to unsolder the LDO, and apply 3V3 on 3V3 pin as an power input.
So, from schematic point of view, if I apply extern 3V3 supply on 3V3 pin, VIN pin is not used, so if we cut the Vout of LDO from 3V3, it will be sufficient to isolate LDO with only a link to GND, VIN and VOUT not connected, right ?
I don't ask to link VIN with 3V3, just directly put 3V3 on 3V3 pin
If i'm not wrong, only one jumper with 2 pads connected by default is needed, if there is room for it.
« Last Edit: June 15, 2017, 09:59:36 AM by Uncle Buzz »

Felix

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #3 on: June 15, 2017, 10:18:24 AM »
To bypass the LDO we need to disconnect both the VIN and 3v3 from it so it's completely out of circuit. Don't forget that the VIN is also on the FTDI side which will be connected when you upload a sketch.

Uncle Buzz

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #4 on: June 15, 2017, 10:31:22 AM »
If LDO is still connected to VIN, but disconnected from 3V3, FTDI won't be able to supply the moteino and we need to supply 3V3 from 3V3 pin even when connected to FTDI.
Is there a problem if LDO is connected on VOUT and GND without VIN ? LDO will regul 3V3 on its VOUT pin disconnected, or connected to its caps depending where is the jumper.
Apart from LDO runing without load, there is no difference with removing LDO and supply 3V3 from 3V3 pin even with FTDI, right ?
Quiescent current in LDO is not a concern when FTDI is connected, on moteino wihtout USB of course.

Felix

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #5 on: June 15, 2017, 12:51:33 PM »
Let me try to explain:

- we need 3 way jumpers to redirect the board VCC from either 3v3 or LDO
- we need another jumper to disconnect VIN from LDO
- FTDI VIN can stay connected to the LDO, but since the LDO output is disconnected by above jumper - its pointless to use this as VIN and would require separate power from FTDI anyway

So with all these issues and the limited board space, plus the support amount for each user that will go ??? when they try to figure out all these cases without extensive documentation (let alone debug problems when they fry it by screwing up the jumpers). Then there's users that want 3.3v input which can use existing fuses, then those who want to use down to 1.8v which requires different fuses etc, endless problems and use cases. I'd rather just make another LDO-less board, with proper fuses, proper everything, that's my take.

Uncle Buzz

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #6 on: June 16, 2017, 03:25:00 AM »
Hi Felix,
I still don't understand the need to use VCC pin since right now we have to use 3V3 pin when removing LDO and it seems much simpler for me, but maybe you have in mind more universal cases than I have... Anyway, your solution with a MOTEINO without LDO makes sense for me, so even if I don't like to not understand things, I'm happy with your plans.
Thanks for your answer and your time, keep up the good work !

Felix

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #7 on: June 16, 2017, 09:59:09 AM »
I still think its too difficult to make the Moteino work this way (ie the added flexibility/benefit vs complexity is not worth it for me). Maybe if we find the solution, you and 5 more people would understand it, the rest of the users would just be confused and who's going to support that?

I tried something quick, with this solution you could cut/resolder the jumper to power from the 3V3 port, but in that case you have to power the board from the 3v3 pin even when you program the board. In which case the LDO is a complete waste, the effort to just remove it is far less complicated than producing a whole set of new boards to accomodate this change, IMHO. I think the effort to make new boards would be better spent on making a dedicated board that comes with proper fuses and perhaps a different format which can accomodate a coin cell holder too.



Could you perhaps provide a schematic showing your idea with a single jumper? Maybe it's much better than what I had envisioned?
« Last Edit: June 16, 2017, 10:52:31 AM by Felix »

Uncle Buzz

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #8 on: June 16, 2017, 10:48:22 AM »
Maybe I misunderstood something in moteino schema...
With moteino (mine si R4 but I think there's no difference in power supply with R6), if I want to use an extern 3V3 power supply (or other supply less than 3V3), I unsolder the LDO, then I connect my extern 3V3 to 3V3 pin analogside header.
PWR from analogside header is connected to PWR pin from FTDI header, but disconnected from everything else. So to upload a sketch, we need to supply 3V3 on 3V3 pin even if FTDI is powered by USB. But apart from that, there's no difference.
In normal use, PWR pin becomes useless.
So, to reproduce this state with LDO still on PCB, if we cut the VOUT pin of the LDO , we will be in the exact state as if LDO was removed, apart from LDO will run without load when FTDI apply power on PWR pin. In normal use, if we let PWR pin on anologside disconnected, LDO will be connected to GND, but nothing else.
So, apart from 3V3 pin which become a power input instead of a power output, I don't see where is the risk for the MOTEINO, apart from applying 5V on 3V3 pin, but it's already the case.
But anyway, a moteino without LDO is ok, it's less universal to my mind but I probably miss the problem you guess.

On your exemple, I don't isolate 3V3 pin header from VCC, so 3V3 pin is still an power output with LDO, with a simple jumper, but if you think it's a bad idea, it's probably than I miss something, so don't worry, if you want to spent some time to explain to me the problem, you're welcome, but I guess you have more intersting things to do.

Felix

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #9 on: June 16, 2017, 11:10:08 AM »
Are you talking more something like this?


Uncle Buzz

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #10 on: June 16, 2017, 12:25:50 PM »
Yes, it's exactly what I have in mind, here or anywhere after the caps depending of where it's easier to place on PCB. So it's easier to disactive LDO without unsloder it, and also even easier to reactivate it than handsolder back the MCP1703.
Do you see any disadvantage with this configuration ?

Felix

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #11 on: June 16, 2017, 02:58:28 PM »
Ok, that would allow you to keep the LDO in place and bypass power via 3v3.
Maybe I can do this

Do you see any disadvantage with this configuration ?

Yes, some which I mentioned:
- I think most users that wanted to remove the LDO was because they wanted to run from lower voltage (2xAA, coin cell). So the board layout will not be coin cell friendly, but maybe not that important.
- You still have to set fuses for anything if you want to run from less than 3v - i think in most cases you need to set the fuses to internal oscillator even if you start with two full AA batteries which are close to 3.3v, but then they decrease towards 2V as they deplete. (brain fart: i wonder could we change the clock fuse from the program based on the voltage of the VCC? maybe not since what would happen in that no-clock-land transition period between clock sources ??? maybe someone knows the answer)
- you cannot power the board from FTDI - you'd need to keep it powered from battery. The 328p datasheet says that signals (from FTDI, 3.3v) should not exceed the VCC, so you are overloading them if your board runs from 2.4v for example. Not sure how bad this would be or what kind of 'wear' it would put on the serial pins in the long run but definitely something to consider.
On the other hand if a dedicated board would have VIN same as 3.3v pin without LDO, then when you program you could use 3.3v sources and even if a battery is attached, it would "charge" a little during programming :)

Uncle Buzz

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #12 on: June 16, 2017, 05:25:59 PM »
For low voltage, if it's supplied from 3V3 pin, where is the problem ? I thought that's what people did, do they really shunt VIN to VOUT to supply power to MOTEINO from PWR pin ?
For the others points, they are the same when remove LDO, right ?

Felix

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #13 on: June 17, 2017, 11:11:41 AM »
I completely forgot you need a new bootloader too.

The problem is ... there are too many assumptions to get right, it's too complicated to use it the right way. You obviously understand how you would use it, but others might not, and the support falls back on me, or perhaps you are ready to volunteer for that and writing all the documentation?

I think i will leave the Moteino alone and make a new board rather than allow the user to believe the board should just magically work at 8mhz by cutting a solder jumper.

Uncle Buzz

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Re: Adding solder jumper on MOTEINO PCB for LDO ?
« Reply #14 on: June 17, 2017, 11:40:32 AM »
Do as you planned, I just asked if it was possible, I thought it will be more universal but since my goal is only to replace LDO for power saving point of view (some mA, so quiescent current is less important than efficiency), not dealing with non standard configuration, I'm not aware of all the problems you could have to face, so if you guess it will be misunderstood by some users, that's fine for me.
Thanks to spent time to reply.