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From: John Larkin on 21 Nov 2009 15:07 On Sat, 21 Nov 2009 10:04:18 -0800, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> On Fri, 20 Nov 2009 17:26:06 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> don wrote: >>>> John Larkin wrote: >>>> >>>>> ftp://jjlarkin.lmi.net/First.JPG >>>>> ftp://jjlarkin.lmi.net/Swapped.jpg >>>>> >>> Hey John, you wasted lots of real estate there. Wish I could have some >>> of that. Doing an EMC fix on a client design right now and I can't even >>> shove one more 0402 part in there :-( >>> >> >> We have lots of space for this one. We're replacing an >> older-generation board that is about 6x or so of our board area. >> >> You will be not-pleased to know that the switcher section has its own >> rectangular ground plane section that is connected to the rest of the >> plane through a number of thinnish slivers. ... > > >Interestingly, one of the line items in my recommendations for this one >is to pepper a similar isolated plane with vias to the ground plane. For >EMC purposes, since this one must pass much stricter rules than the >usual class B. I'm trading off potential switcher-harmonic noise (EMI test hazard) against allowing switcher fundamentals to creep into other parts of the board, where they would make birdies in my spectra right in the region of interest. I have no idea whether any of this will work. > > >> ... The rows of inductors, >> incoming and outgoing, straddle the plane gaps. The idea is to keep >> the various circulating currents in the switchers from leaking into >> the main ground plane where the analog stuff is. I did the >> spread-spectrum thing on all the switchers, too. >> > >Oh, how I wish I could do spread spectrum. But with the EMC measures so >far I am already at a full 100% of available real estate. Unless someone >knows a self-contained oscillator in an SC75 package :-) I'm using one tiny logic schmitt as an RC oscillator. The resulting triangle, around 1 volt p-p, gets squirted into the Fset pin of each of the switchers through a pretty big resistor, and that FMs the switcher frequencies. That's not a lot of parts, but then I have a lot of area available on this one. John
From: John Larkin on 21 Nov 2009 15:15 On Sat, 21 Nov 2009 18:44:44 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>OK, I just got the first board from production this morning, for this >>spectroscopy controller thing. >> >>ftp://jjlarkin.lmi.net/First.JPG > >I assume The Brat can kiss her Christmas invitation goodbye this year >:-) But then maybe I couldn't drive her jeep! I did take the board home at night to do some of the placement and routing myself, for the analog front end, some fast stuff, and the switcher region. So it may well be my fault. She's learning fast, so I suspect this will go on her evolving checklist and it's not likely to happen again. There are probably 30 to 50 things one should check before releasing gerbers, and it's easy to forget a couple. We need a checklist. All the supplies are working now. On Monday I'll have them put two new BGAs on the board and see what happens next. Bringing up a new board of this complexity is always, well, tense. John
From: John Larkin on 21 Nov 2009 15:20 On Sat, 21 Nov 2009 13:35:01 -0600, krw <krw(a)att.bizzzzzzzzzzz> wrote: >On Sat, 21 Nov 2009 18:59:49 +0000, Raveninghorde ><raveninghorde(a)invalid> wrote: > >>On Fri, 20 Nov 2009 16:22:15 -0800, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>OK, I just got the first board from production this morning, for this >>>spectroscopy controller thing. >>> >>>ftp://jjlarkin.lmi.net/First.JPG >>> >>>It gets 12 volts in, which runs an LTM8023 switcher brick to make 3.3 >>>volts. The 3.3 runs most of the logic on the board (including a >>>Spartan 6 and a PLX PCIe bridge, both BGAs) and also drives four >>>secondary switchers and some LDOs to make 1.2, 1.5, 1.8, 2.5, and -5 >>>for various uses. >>> >>>So when I powered it up everything went nuts. The PLX chip was >>>obviously fried. After that was pulled, the Xilinx was running hot, >>>and the 3.3 volt supply was bogged down to about 2.6. The LTM >>>regulator was hot. >>> >>>Pulled the Spartan BGA next. >>> >>>Now the 3.3 volt rail wants to run at 5 or so. >>> >>>After much head scratching, I discovered this: >>> >>>ftp://jjlarkin.lmi.net/Swapped.jpg >>> >>>The resistor that's screened "R127" is actually R129. And vice versa. >>>So the switcher was programmed wrong, told to run at an absurdly low >>>frequency and an absurdly high voltage. The ref designators somehow >>>got misplaced during layout. We usually check for this. >>> >>>Apparently our production people, when semi-auto placing dense parts, >>>double-check the ref designator and plop the part into the "correct" >>>place, even if the machine coordinates are a little off. I'll have to >>>warn them to be suspicious about cases like this, especially on first >>>articles. >>> >>>TGIF >>> >>>John >>> >>> >> >>Experience has taught me to power up new boards on a bench psu by >>winding up the voltage from zero while monitoring the supply rails and >>input current. > >With switchers this rarely does anything and often makes things even >worse. Yes. The only thing to do is to isolate the power section from the loads and bring up the supplies unloaded. That requires jumpers or whatever. I do have LC filters between the supply pours and the main pours, and in retrospect I should have removed the inductors and tested the supplies. Given the consequences of power supply failure, I'm leaning more towards always incorporating transzorbs/clamps/crowbars on things like this. John
From: Raveninghorde on 21 Nov 2009 15:22 On Sat, 21 Nov 2009 11:06:33 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Raveninghorde wrote: >> On Fri, 20 Nov 2009 16:22:15 -0800, John Larkin >> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> OK, I just got the first board from production this morning, for this >>> spectroscopy controller thing. >>> >>> ftp://jjlarkin.lmi.net/First.JPG >>> >>> It gets 12 volts in, which runs an LTM8023 switcher brick to make 3.3 >>> volts. The 3.3 runs most of the logic on the board (including a >>> Spartan 6 and a PLX PCIe bridge, both BGAs) and also drives four >>> secondary switchers and some LDOs to make 1.2, 1.5, 1.8, 2.5, and -5 >>> for various uses. >>> >>> So when I powered it up everything went nuts. The PLX chip was >>> obviously fried. After that was pulled, the Xilinx was running hot, >>> and the 3.3 volt supply was bogged down to about 2.6. The LTM >>> regulator was hot. >>> >>> Pulled the Spartan BGA next. >>> >>> Now the 3.3 volt rail wants to run at 5 or so. >>> >>> After much head scratching, I discovered this: >>> >>> ftp://jjlarkin.lmi.net/Swapped.jpg >>> >>> The resistor that's screened "R127" is actually R129. And vice versa. >>> So the switcher was programmed wrong, told to run at an absurdly low >>> frequency and an absurdly high voltage. The ref designators somehow >>> got misplaced during layout. We usually check for this. >>> >>> Apparently our production people, when semi-auto placing dense parts, >>> double-check the ref designator and plop the part into the "correct" >>> place, even if the machine coordinates are a little off. I'll have to >>> warn them to be suspicious about cases like this, especially on first >>> articles. >>> >>> TGIF >>> >>> John >>> >>> >> >> Experience has taught me to power up new boards on a bench psu by >> winding up the voltage from zero while monitoring the supply rails and >> input current. > > >That often doesn't not help. Many switcher do nothing intil the UVLO >threshold is exceeded, and then they step on it with gusto. You may not >have enough time to rip out the banana plug before phssseee ... *POOF* This depends on the topology of the switchers. I normally use buck regulators but I admit that it doesn't work with other topologies.
From: krw on 21 Nov 2009 15:40
On Sat, 21 Nov 2009 20:22:06 +0000, Raveninghorde <raveninghorde(a)invalid> wrote: >On Sat, 21 Nov 2009 11:06:33 -0800, Joerg <invalid(a)invalid.invalid> >wrote: > >>Raveninghorde wrote: >>> On Fri, 20 Nov 2009 16:22:15 -0800, John Larkin >>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> OK, I just got the first board from production this morning, for this >>>> spectroscopy controller thing. >>>> >>>> ftp://jjlarkin.lmi.net/First.JPG >>>> >>>> It gets 12 volts in, which runs an LTM8023 switcher brick to make 3.3 >>>> volts. The 3.3 runs most of the logic on the board (including a >>>> Spartan 6 and a PLX PCIe bridge, both BGAs) and also drives four >>>> secondary switchers and some LDOs to make 1.2, 1.5, 1.8, 2.5, and -5 >>>> for various uses. >>>> >>>> So when I powered it up everything went nuts. The PLX chip was >>>> obviously fried. After that was pulled, the Xilinx was running hot, >>>> and the 3.3 volt supply was bogged down to about 2.6. The LTM >>>> regulator was hot. >>>> >>>> Pulled the Spartan BGA next. >>>> >>>> Now the 3.3 volt rail wants to run at 5 or so. >>>> >>>> After much head scratching, I discovered this: >>>> >>>> ftp://jjlarkin.lmi.net/Swapped.jpg >>>> >>>> The resistor that's screened "R127" is actually R129. And vice versa. >>>> So the switcher was programmed wrong, told to run at an absurdly low >>>> frequency and an absurdly high voltage. The ref designators somehow >>>> got misplaced during layout. We usually check for this. >>>> >>>> Apparently our production people, when semi-auto placing dense parts, >>>> double-check the ref designator and plop the part into the "correct" >>>> place, even if the machine coordinates are a little off. I'll have to >>>> warn them to be suspicious about cases like this, especially on first >>>> articles. >>>> >>>> TGIF >>>> >>>> John >>>> >>>> >>> >>> Experience has taught me to power up new boards on a bench psu by >>> winding up the voltage from zero while monitoring the supply rails and >>> input current. >> >> >>That often doesn't not help. Many switcher do nothing intil the UVLO >>threshold is exceeded, and then they step on it with gusto. You may not >>have enough time to rip out the banana plug before phssseee ... *POOF* > >This depends on the topology of the switchers. I normally use buck >regulators but I admit that it doesn't work with other topologies. Often doesn't work with buck regulators either. Depending on what they're driving startup can get quite nasty. |