Need help with LT8350S - going down a rabbit hole for low noise opamp rails  - Page 1
Forum section: electronics / projects

author: Positron Enthusiast
author_rank: Newbie
timestamp: July 13, 2024, 03:54:25 am
content: I'm trying to design a power delivery network for a low noise analog system, but I think I've gone down the power supply rabbit holeand not sure if I'm taking a reasonable approach.Basically taking in a 24v isolated supply and need a lot of output rails, in addition to retaining the 24v for stepper motors. Something like +/-14.25v, +/-5v, and +3v3 rails. Each of those rails is going to be powered by an RF LDO off either +/-15v switcher-supplied rails.I also know from an existing design that peak current is less than +/-333mA, because I was using a Recom RP10-2415DE as the switcher to feed the LDOs. But from analyzing that design it seems typical peak current is closer to 50mA and quiescent is only ~10mA which isn't reaching the 10% minimum load spec of the switcher. Worst case peak is only like +/-120mA (short circuited opamp outputs).There are a couple other problems with the Recom supply for this application too - EMI filter is huge, switching frequency is in band, and the availability can be hit/miss. And then the price - $60 is nothing to scoff at.So it seems like it's worth designing my own switcher to take it's place. I think I don't need isolation because the 24v is now an isolated supply (it wasn't before), and the first thing the output filter did was undo the isolation. So far it looks like this makes the design much easier. I do want low noise and a simple BOM to maintain though, ideally in a similar total solution size.I've gone through several attempts at a replacement - started with SEPIC but only figured out after a day of work that the loop compensation was going to be touchy. I tried using TI's WEBENCH but the designs don't seem that great, and the negative rail doesn't simulate. It does seem at this point (that is, after several days of research) that I should be able to come up with a stable, reasonable solution that using the same kind of chip to control the positive and negative supplies, but I'm having trouble figuring out the next step.Now I'm trying to figure out ADI's silent switcher products. I looked at the gen 3 stuff but the input range is only 18v - so that's out. Looking at the Gen 2 I've settled on the LT8350S but I'm not sure if that's a reasonable approach or I'm just going to burn another 3 days in simulation.TLDR: For an LT8350S- Does it make sense to use 1 for a +24v to +15v buck and another for Inverting Buck Boost +24v to -15v? Or is it better to pull from the +15 for the -15 IBB to put less stress on the FETs? How do I size the passives for my design? I also really can't figure out where the current limits come from on this topology?

author: moffy
author_rank: Super Contributor
timestamp: July 13, 2024, 05:24:53 am
content: TI makes a range of inexpensive low voltage push pull Tx driver chips e.g SN6507:https://www.ti.com/lit/ds/symlink/sn6507.pdf?ts=1720847873751&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FSN6507%253Fbm-verify%253DAAQAAAAJ_____-KlHnZae57TTpolBo7UGwxUWTjYj_lNXrcrs0fcWkssCIqxUtZz0PcfhhAJSyhkO-EPDdyWFbuQAWw2da-oDJaGaulNLRGfLRzYHWLAZGY7FT47S4S_DMJGwzYbXFTMmuKzEqqFvoS9BrAy0R7QiZyELh3XVA4230-CC7BqR__f7J63uxqCQHH5l-1CpQz3B4nsUxQU3Oq-LGZN_rsezE4AEHD48wmAmYjiNjVOECc83cH_SB44i-J5pdVhOvlJokF5cs8qccy2ixzCL40SbjbP3PXdJe4Q_u2CopnOQyYtWysIt does use a transformer but they too are readily available, and you can get +/- supplies easily. They are open loop so you need a controlled input voltage, so you could generate a single +/- 15V then use your low noise regulators.

author: Positron Enthusiast
author_rank: Newbie
timestamp: July 13, 2024, 06:19:40 am
content: Thanks for the tip moffy, I will take a look. Does open loop in this context equate to completely unregulated? Not much experience designing switchers as you can tell, mostly I've been relying on linear regulators and hybrid modules.Edit: I looked at the datasheet/ reference design. Looks like it might work well. For an isolated topology like this - can I use two of the evaluation board design and just cascade the outputs with a GND in the middle to get my +/- 15v rail?https://www.ti.com/tool/SN6507DGQEVM

author: moffy
author_rank: Super Contributor
timestamp: July 13, 2024, 06:38:28 am
content: Yes, open loop means unregulated, but as long as the input is regulated, and the output cap gets to the square wave level, pretty normal, the output will be reasonably stable with load, the linear regulators then hold everything steady and reduce the noise. These switchers have controlled slew rates so they will produce much less transient noise than a hard switcher.

author: SCSKITS
author_rank: Regular Contributor
timestamp: July 14, 2024, 11:02:04 pm
content: What is the current required for each of the output voltages?Are there any limitations on size? Switching frequency?Can a flyback supply be used? there are some simple isolated flyback controllers available.I recently did a 20V to 40V input supply with three 15V 1A outputs, although I only needed about 0.5A on each.One was set to +15V, one to -15V, and the 3rd to +15V followed by a 5V buck regulator.ed

author: Positron Enthusiast
author_rank: Newbie
timestamp: July 28, 2024, 05:02:55 pm
content: Noise requirements are basically the same as that outgoing recom part.Current draw may be between 1-100mA typical for each pos/neg rail.Flyback is an interesting idea - any tips on finding transformers?I tested out the demo SN6507 board last week and was pretty disappointed by it. Noise was actually really good but the droop was like double what’s estimated in the datasheet (ESR around 2 ohms). And the demo board arrived with a coilcraft TX1 transformer instead of the wurth model they advertised. DC control, slew rate control, and high switching rates don’t work well. Under low loads the part overheats and thermal shutoffs repeatedly. (Same at high loads so it realistically only works between 90-150 mA). Also, there is no sim model, despite what TIs website claims. There is a model for another sn650x but it does not show instability or advanced features. The issues also notably disappear below around 19v Vin so maybe this is a downside of working with a 24v supply. Installing RC dampers on the primary side made it worse, zeners on the output improved things even below Vz.I have noticed that it is a clone of an older LT 15xx chip that Jim Williams wrote a lot of the datasheet for so I might try that one instead. There is also the LT3999 which has a sim model and similar features to SN6507. Interestingly, that sim model does show instability under the conditions I saw from the TI part in the lab.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 28, 2024, 10:41:51 pm
content: Quote from: Positron Enthusiast on July 13, 2024, 03:54:25 amHow many do you expect to make ?Quote from: Positron Enthusiast on July 13, 2024, 03:54:25 am+24 to -15 does not seem like much stress ?Usually you would avoid such skew loading, as it doubles the power thru +15 and increases the ripple by giving it a switcher-input load.Quote from: Positron Enthusiast on July 13, 2024, 03:54:25 amIf your real power needs are lower, why not look at the DC-DC modules closer to your actual powers.They are far more plentiful and lower cost than larger switchers.You can also distribute the noise filtering, today's LDOs can be very good on PSRR and if you are noise paranoid, you can add simple filters.Simple distance also helps, move the noisy bits onto a separate PCB or even a separate enclosure.