RG1 1.8v regulator

Is the LAN9512 EOL'd?  I'm a bit surprised not to find it among Farnell's SMSC LAN9xxx products.

It's available from digikey, mouser, RS etc.. also no obvious indication on the SMSC page that it's EOL, datasheets updated as recently as Feb this year, so I'd be surprised if it was. 

Thanks.

selsinork wrote:

 

are you able to measure the temperature of the lan9512 with this mod in place?

I have an IR thermometer on its way from China which might take a few weeks. I might also be able to dig a thermocouple out of my junk box somewhere. The plan is to take temperature readings in various modes of operation when running the 1.8V plane off each regulator (RG1 and LAN9512) individually. Otherwise there is the finger test. Results to follow.

Quick update before I go to work:

I found a thermistor from the Silicon Chip Multi-Purpose Fast Battery Charger kit from the Feb 1998 issue. Not sure what its specs are if anyone has this issue handy, otherwise I'll have to go digging for the plans or do a calibration. Here is a pic of the test setup:

The pinout for the 4-pin header is: GND, RG1OUT, Pi 1.8V plane, VDD18CORE

This will also allow for current measurements, particularly of interest is the direction of current from VDD18CORE to RG1OUT. Other current measurements in series with the regulator outputs may not be accurate when both regulators are connected due to additional resistance of the multimeter. This may tell us if the LAN9512 is sourcing or sinking current during normal operation. I suspect sourcing due to thermal images showing a cold RG1. The GND pin can be used to connect an external adjustable 1.8V source for further testing behaviour of LAN9512 in this unsupported mode of operation.

Update:

Initial results were not definitive so I took a few more current measurements to maximize the drain on the 1.8V supply. I could get consistently 100-110mA with peaks of 130mA by playing back a 1080p video with omxplayer. I pushed some network traffic through for good measure which did have an observable effect on the equilibrium temperature.

Thermistor resistance at room temperature (about 20 degrees C) was approx 110KOhm, so using a temperature chart for a 100K thermistor we have (roughly, at equilibrium):

Running 1.8V plane from RG1 57.5KOhm (approx 37 degrees C)

Running 1.8V plane from LAN9512 53.7KOhm (approx 39 degrees C)

Holding thermocouple in hands 63.5KOhm (approx 35 degrees C)

These temperatures are fairly rough of course, but give a relative indication of temperature. Not the dramatic effect I was expecting, but it does run hotter as expected with that extra drain through the internal regulator, not blisteringly hot though.

Perhaps in the worst case if VDD18CORE is slightly higher than that of the output of RG1 then not only does it power the 1.8V plane, but leaks back through RG1 to ground. In which case you may be better off removing RG1 so at least the drain on the LAN chip is capped at around 100mA.

Just adding it here for those not following all the different threads..

I asked SMSC tech support:

"I'm evaluating a design using your LAN9512 device. Can you tell me if it's

acceptable to use VDD18CORE to supply 1.8v to external devices and if so what

current can be supplied ?

The datasheet only describes it as "Digital Core +1.8V Power Supply Output" but

offers no details on if it's useable to power external devices or what current

it can supply."

Their reply:

We have received your request for Technical Support.  The following reviews.

SMSC Technical Support Request # 534992-DD1256

31/07/2012 10:06 ISaturle: 

Hi Iain,

 

VDD18 is not intended to power external nets.  Equally, it's not intended to ever be driven by external sources. 

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