RG1 1.8v regulator

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.

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.

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.

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.

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..

John Beetem wrote:

 

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.

Just confirming that does indeed appear to be the case. I popped RG1 off and still measure 1837mV. IC3 still feels about the same temperature wise.

edit: Scratch that last statement. Playing back video over network, IC3 is running considerably hotter, at the limit of my pain threshold, my finger is still sore. Guess RG1 was still providing some relief. Replaced RG1 and it's still a bit warmer than before, might need a touch more solder.

Hi Troy & all

Since this thread and Coder27's thread are actually all about the same issue, can we recombine or something.  Alternatively I'll limit my replies to the one or other so at least we can keep the flow going.

People coming to the forum will recognise a 'hot' SMSC if they have one, so that might be the best place?

Comments / Ideas?

Playing back video ramps the current requirement of the BCM considerably, again with RG1 o/c can you measure the surface temp of the SMSC?

Cheers

Pete

Nice to see you here again, Pete.

My finger is quite toasty now from all the finger tests on my very hot LAN9512.  Looking forward to your input on this!

Morgaine.

I came across a document "Standards for School Premises" at

  http://www.plymouth.gov.uk/standards_for_school_premises_dfes-3.pdf

that says:

   Hot Surfaces   

   83. Some pupils are more at risk than others from hot surfaces. In special

    schools, nursery schools and in teaching accommodation used by nursery

    classes, the surfaces of radiators and exposed pipes located where they

    might be touched by a pupil must not become hotter than 43 degrees C.

This probably isn't directly relevant to electronic devices, but it might suggest

that uncovered electronics with blisteringly hot chips may be considered a

safety hazard in schools.

In California, hot surfaces of 140 degrees F (60 degrees C) are considered

a safety hazard in the workplace.

http://www.dir.ca.gov/oshsb/hot_surfaces_isor.pdf

If you recall our "Finger Contact Survival Time" (:-) testing over in this thread , although initial contact temperature on my Pi was such that pure instinctive body reaction tore the finger away in a fraction of a second to avoid harm, subsequent fingers survived contact for progressively longer periods, indicating that the LAN9512 has low heat capacity and/or high thermal resistivity in its surface material.

Those tests tell me that it's not really a danger in the safety sense.  The temperature is below ignition point for all common materials, and there's not enough of a heat reservoir there to maintain its temperature if coupled to anything that is thermally conducting.  The main danger to humans is probably from the instinctive reaction:  a schoolkid's sudden violent withdrawal of their hand could knock out another kid's eye or break something.

The other thing that those tests tell me is that the main problem doesn't seem to be too much heat being generated but not enough heat being conducted away through the solder connections, because the surface of the device isn't able to conduct nor radiate much heat away at all.

For reference, link to post with thermal image showing the 65.1C peak temperature on LAN9512 .

PS. Because of the low heat capacity and/or high thermal resistivity of the surface material, a thermocouple attached to the surface is likely to indicate a lower temperature than a thermistor.  The thermocouple is entirely metallic and hence very likely to be a good conductor of heat away from the device.  A non-contact measurement like Remy's is best here.

Morgaine.

Yes, I recall the "Finger Contact Survival Time" discussion,

but I'm not sure how relevant it is. As you say, if you pull away

the first finger in a fraction of a second, you can use subsequent

fingers for longer.  But the safety concern is whether the first

finger will get burned if not pulled away in a fraction of a second,

because younger children, older adults, and the disabled are

assumed to have slower reflexes.

Pete,

Here's some additional data.

To hopefully make these reasonably repeatable, here's some detail of the setup I've been using

2012-07-15-wheezy-raspbian.img no firmware/kernel upgrades done.

overscan disabled

1600x1200 Dell 2007FP monitor

all 'idle' readings taken while at a text based login prompt, X has not been started

'playing video' readings taken after running startx and then omxplayer from a terminal to play a 720p h264 mkv file

temperature readings taken with a MCP9803 i2c temperature sensor on top of the lan9512

5v applied via the GPIO header with 16/0.2 cable and I'm arranging that TP1-TP2 reads 5.00v

Ambient, today:

outside 20C

inside 26C

50.3% RH

1005 hPa

at each step I'm giving the board 15mins to reach equlibrium

The blue column at the end is the unloaded voltage reading from RG1 while it's off the board.

Sorry, but I won't be pulling RG1 off that last board, hope you understand why

I have a couple of boards from FN120720xxx FN120722xxx, but as my results are mostly similar I've just picked either end of the range I have along with a couple in the middle.