PMIC choices. - Page 1
Forum section: electronics / projects

author: dmills
author_rank: Super Contributor
timestamp: May 11, 2020, 01:33:47 pm
content: So I have a rather butch fpga, with the associated many, many power rails, because that's how those things roll.My problem is that all the PMIC sand I can find that looks suitable needs configuration over I2C before it will actually work, which means another little processor to set the power subsystem up before I can even boot the main system, I would really like to avoid this (Even the ones with built in eeprom need that configured, so another manufacturing step).What would be ideal is a PMIC that is resistor programmable, instead of relying on the I2C bus for basic setup, I2C is fine for monitoring, but I need to get the power on, at defined voltages without needing to program the damn chip. Something current, available and widely used would be ideal.Has anyone got any good suggestions? Infineon tell me that they can do us a custom pre programmed part, but we don't really do the volume to make that work commercially. I have a vague recollection of a TI part, but can I find the thing now, can I hell.Anyone?Regards, Dan.

author: exe
author_rank: Unknown
timestamp: May 11, 2020, 03:18:47 pm
content: I'm no pmic experts, I'm just curious how many pmics you need, what's the input voltage, output voltage, and what's the current requirement?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2020, 03:59:55 pm
content: Well yeah, if you're shopping for all-in-one digital controlled regulators like what's used on laptops and cellphones, you'll get exactly that. And these usually do have an auxiliary processor, hypervisor, or some sort of low-level startup to do basic control functions like that. If you're using a blind and dumb FPGA alone, yeah, it's not for you.Why not a dumber multichannel (or several duals, or many singles) regulator that's just set with resistors? You'll find these under the regular category, of course.If you aren't optimizing every last watt, you'll likely find it's worthwhile to use a switching reg for the biggest rails (e.g., 5V, 3.3V, 1.0V), and LDO the rest (e.g., 2.5V, 1.8V). Or if one is nearby and doesn't need fantastic regulation, even just a series diode to drop the difference.There's also chicanery that can be done with the regulator topology itself, for example tapped or multi-winding inductors to deliver supplies related by simple ratios. 5 and 2.5V can be reasonable this way; obviously, the diode drops and stray inductance matter: cross-regulation tends to be poor. If you end up with a somewhat too high voltage and LDO it down anyway, well heck, that's just fine.Tim

author: dmills
author_rank: Super Contributor
timestamp: May 11, 2020, 04:33:50 pm
content: Raw input is 24V, which I figure to buck down to 5V as a first step before getting into the annoying shit.1V @ 30A (PL, PS Core, BRAM).1.8V @ 10A.1.5V (DDR3), probably 2A or so.0.75V (buffered divider for DDR Vtt).Then a mess of really weird ones for MGT transceivers, must be really low noise, so probably linear regs, but current requirements for these are proving difficult to track down...1.05V (Yes, really).1.2V1.8V1.8V (Separate reg or noise filter for clock generator).Then the non FPGA stuff,2.7V (To be LDOd to 2.5V for some 12Gb/s reclockers).3.3V for misc shit.15V for a really annoying series led backlight on a front panel.I guess 1.5, 1.3, 2, 2.7, 3.4 would be reasonable (all these are semi low current rails), getting the butch 1V and 1.8V as well would be a bonus, but I think those are going to wind up being some sort of polyphase thing probably from TI, Empirion, or AD (Who have managed to fuck up Linears web site in remarkably short order, grumble).I know roughly what I need, it is filtering down the ways to get there that is causing me a headache.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2020, 04:39:36 pm
content: Ah, and what tolerances? How many of those can overlap? Like the filtered 1.05 might possibly be Vcore with some LCs.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2020, 05:00:35 pm
content: Many of those "strange" voltages go to just a few closeby power pins (or even just one), right? In that case "having" to use single- or dual-channel parts (switching or LDO) isn't necessarily a downside, it may work for the layout really well, fewer power planes to route, you may be able to even cut the number of layers, just have the regulators right next to the power pins of the consuming devices. Thinking about those weird MGT voltages.You may have success even looking under the "voltage reference" section, instead of "linear regulators", if some of those strange, accurate voltages is just tens of mA.Are there requirements regarding power sequencing?15V LED backlight, do it using a constant current buck LED driver IC from the 24V source directly.Do everything else from 5V.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2020, 05:39:13 pm
content: Quote from: blueskull on May 11, 2020, 05:27:29 pmYou are right and that would have an added benefit of nothing depending on the +24V input, but only on +5V, so it would be easily changed to run from +5V source if ever needed for any reason.

author: dmills
author_rank: Super Contributor
timestamp: May 13, 2020, 11:27:10 am
content: It is a pair of little 2.4 inch screens, control and current regulation is already on the front panel card as a simple BJT current source and a current mirror for the second screen (The original product had only one screen, hence the current mirror).There is most emphatically no need for anything clever here.Thanks guys, that's given me some ideas. I think this is going to be a couple of AP6200 for the medium current rails and a whole mess of TPS748 for the LDOs to produce the low voltage twiddly bits (They have disappointing PSRR at high frequency, but there are ways to fix that).Regards, Dan.