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RG1 1.8v regulator

Former Member

Ok, so in a different thread I threatened to remove RG1 and do some current measurements on it's output after seeing those thermal images that show it's not generating any heat...

 

Well, I did it tonight. Some photos here: https://picasaweb.google.com/selsinork/RPi18v

 

The jumper pins in the output let me either just put a jumper on and verify the Pi boots ok, or wire a multimeter in series to get some current readings.

 

The results were interesting to say the least. I had to go back and check I was reading the multimeter correctly, that it wasn't broken etc.

 

On initial power up I see a negative current for a second or so which then reverses to about 0.5mA (yes half a milliamp, that's not a typo) for a few seconds while we get the first sd-card accesses. Once we're booted and sitting at the login prompt the current reading fluctuates from around 0.001mA to maybe 0.04mA. 

 

I'm using the 40mA range on a decent Fluke multimeter, so I've no reason to doubt the results. There's obviously going to be some inaccuracy down at that level due to length of meter leads etc, but the result is fairly clear.  You'll understand why I was checking the meter was working and I was reading it correctly though image

 

 

So from there onto the next test, lets try completely disconnecting RG1 and see if the Pi boots while using the LAN9512 1.8v 'output'.  Yes it does! 

 

I think that's reasonably good indication that jamodio got it spot on, the lan9512 shouldn't be connected to the 1.8v plane and it's heat problems are going to be largely due to supplying current on it's 1.8v filter pin that it was never designed to do.

 

So anyone willing to pull RG1 off a Pi and verify my results ?

Parents

  • Just to expand on my original observations, the negative current I see on the output of RG1 seems to be due to it outputting 1.79v for a few seconds on initial powerup before stabilising at 1.804v. While the LAN9512 appears to power up at 1.805v and stay there.

  • in reply to Former Member

    My second order of Raspberry Pis arrived earlier in the week. A quick check shows one of them is running slightly warmer than the rest, not too much, I can still hold my finger on the LAN chip. I measure 1837mV at RG1 compared to 1793mV on one of the cooler ones (at idle, unmodified boards). Perhaps the LAN9512 core is running over voltage due to the external source and hence a bit hotter in this case. There may be multiple scenarios depending on components.

    I don't expect it will reduce the life of the chip unless it gets really hot, so I'll leave the rest alone unless LAN chips start dying all over the place.

  • in reply to Former Member

    Troy Mackay wrote:

     

    My second order of Raspberry Pis arrived earlier in the week. A quick check shows one of them is running slightly warmer than the rest, not too much, I can still hold my finger on the LAN chip. I measure 1837mV at RG1 compared to 1793mV on one of the cooler ones (at idle, unmodified boards). Perhaps the LAN9512 core is running over voltage due to the external source and hence a bit hotter in this case. There may be multiple scenarios depending on components.

    I don't expect it will reduce the life of the chip unless it gets really hot, so I'll leave the rest alone unless LAN chips start dying all over the place.

    That's consistent with my limited knowledge of voltage regulators.  My understanding is that if two positive linear regulator outputs are connected to the same node, whichever has the higher voltage reference will supply all the current.  The one with the lower reference will shut off, satisfied that it doesn't need to do any work.  RG1 probably has more precise regulation than IC3 (LAN), so it's more likely that +1V8 voltage above 1.800V is sourced by IC3 due to manufacturing variance.  Feel free to correct my understanding if I've got it worng.

  • in reply to johnbeetem

    My understanding is that if two positive linear regulator outputs are connected to the same node, whichever has the higher voltage reference will supply all the current.  The one with the lower reference will shut off, satisfied that it doesn't need to do any work. 

    I'm not convinced that one will shut off. When you look at the equivalent circuit for these lm1117 style regulators in the datasheets there's nothing particularly clever looking in the design.

    Simple ohms law on the other hand gives a good idea that when two are connected in parallel the one with the lowest resistance path will probably end up supplying the most current - at least until a higher voltage offsets the resistance difference.

    When we're talking about something that has a large power/ground plane under it then it's likely going to be a toss-up on manufacturing tolerances - which regulator is a couple of mV higher or lower.. Unlikely anything will ever practically manage to keep at exactly 1.8000000000v across temperature and load variations, so things may even swap over as the board warms up, or when you stress the GPU playing some video etc.  Every board will be different and everyone will have different ambient temperature, peripherals, cpu usage, whatever..

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  • in reply to johnbeetem

    My understanding is that if two positive linear regulator outputs are connected to the same node, whichever has the higher voltage reference will supply all the current.  The one with the lower reference will shut off, satisfied that it doesn't need to do any work. 

    I'm not convinced that one will shut off. When you look at the equivalent circuit for these lm1117 style regulators in the datasheets there's nothing particularly clever looking in the design.

    Simple ohms law on the other hand gives a good idea that when two are connected in parallel the one with the lowest resistance path will probably end up supplying the most current - at least until a higher voltage offsets the resistance difference.

    When we're talking about something that has a large power/ground plane under it then it's likely going to be a toss-up on manufacturing tolerances - which regulator is a couple of mV higher or lower.. Unlikely anything will ever practically manage to keep at exactly 1.8000000000v across temperature and load variations, so things may even swap over as the board warms up, or when you stress the GPU playing some video etc.  Every board will be different and everyone will have different ambient temperature, peripherals, cpu usage, whatever..

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