Buck feeding LDO... best of both worlds or worst? - Page 1
Forum section: electronics / projects

author: nsayer
author_rank: Regular Contributor
timestamp: August 20, 2015, 05:01:19 am
content: I am contemplating a project where a high (~1A @ 3.3v) current power supply is required, but it requires extremely low noise. I'd like to specify a 12 volt input.I'm considering a buck converter set for 4.3 volts feeding an LT1117 LDO. The 1 volt delta is comfortably over the LDO drop-out voltage, but 1A should only result in 1W of dissipated power through the LDO. My big hope is that the LDO sort of regulates out the noise and ripple.I've googled around, and don't see any mention of this idea... Is it just that it's too obvious or too stupid?

author: c4757p
author_rank: Super Contributor
timestamp: August 20, 2015, 05:07:18 am
content: It's a common idea and works well. I'm building a power supply that uses this approach.Keep in mind that it won't really help you too much if you need "extremely" low noise. An LDO isn't going to filter the high frequency crap caused by ringing of parasitic elements in the converter or the wideband crap caused by harmonics of the switching waveform. You still have to filter the everliving hell out of it. But an LDO can be one piece of that filtering equation and also provides a better transient response.

author: ez24
author_rank: Super Contributor
timestamp: August 20, 2015, 07:43:26 am
content: http://www.linear.com/product/LT1117800 ma max is that enough? You say 1 amp wanted.How about LT1965 it is rated at 1.1 amps?Sure wish we could get HKJ to do one of his tests of a buck feeding a LDOhttps://www.eevblog.com/forum/projects/performance-test-and-review-of-20w-dc-dc-step-down-voltage-regulator/I might use this one --> LT1085 because of the high current limitshttp://www.linear.com/product/LT1085I want 2a maxLook at the applications :ApplicationsHigh Efficiency Linear RegulatorsPost Regulators for Switching SuppliesConstant Current RegulatorsBattery ChargersNotice "Post regulators for switching supplies"Let us know the ripple reduction.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2015, 07:58:32 am
content: If the input voltage is relatively fixed, there may be forward convertes as an lower noise alternative to the buck converter. The down side is that they have a rather fixed voltage ratio sometimes with out regulation. A simple form might be a Royer-converter, especially if the current consumption is rather constant.With the LDO there are rather different types - the xx1117 is more a low cost version, and not really low drop. PSRR can be very different between types. As PSSR ist usually not very good at high frequencies, usually extra LC filtering is used between the Buck and LDO.There are also unconvertional types that use a auxillary supply (e.g. 12 V) and than allow for very fast regulation and possibly good PSSR as they internally can use a source follower. They often use external MOSFETS.How low is extremly low noise ?

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 08:06:58 am
content: Why don't you use pi filters, either RC or LC?They work as low pass filters that block any AC but pass DC. Calculate the cuttoff frequiency at about 1 Hz

author: Someone
author_rank: Super Contributor
timestamp: August 20, 2015, 11:07:56 am
content: Between drops in the intermediate filtering and the larger currents drawn, you might find the the headroom you thought you had rapidly disappears. There are some regulators designed for point of load operation that have separate bias and power input pins so the pass elements can get enough drive while still operating with lower headroom on the high current path. Don't forget to check all corner cases of supply and load when building a complex regulator as light loads can remain an issue for the switcher.

author: Delta
author_rank: Super Contributor
timestamp: August 20, 2015, 01:33:43 pm
content: I'd think about upping the buck output voltage a bit and using an NPN linear regulator instead of a PNP LDO for the lower noise and increased stability.

author: c4757p
author_rank: Super Contributor
timestamp: August 20, 2015, 01:39:47 pm
content: Quote from: darkness_electro on August 20, 2015, 08:06:58 amNot so easy. A naively designed pi filter with a 1 Hz cutoff will typically be more of a bandstop with the upper cutoff in the 10-100 MHz range (or worse) due to component parasitics.Quote from: Delta on August 20, 2015, 01:33:43 pmYup, if you have room, I agree. The NPN devices tend to have much nicer-looking outputs.

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 02:27:26 pm
content: Ok, the pi filter may not be the best, i usually design circuits with high voltage. Why don't you try to make a opamp controlled transistor regulator, maybe a nice to-3 serie pass-trough.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2015, 07:32:31 pm
content: Something like an OP based regulator might be an option if extremly low noise is needed. There are also some very low noise regulatiors available, specially for VCO supplys and similar.

author: ez24
author_rank: Super Contributor
timestamp: August 20, 2015, 07:55:00 pm
content: QuoteI assume a LM317 is a NPN so I looked up the datasheets for a LDO (LT1085) and the LM317 and found graphs of ripple rejection at around 60k. The difference is surprising. For my application the LM317 has about 40db MORE rejection than a LDO. So for me a LDO is out.Between the two you have to judge the power loss against the ripple rejection. For example a LDO could have 2 watts less loss and 40 db less rejection (just round numbers). Thanks c4757p for the direction.Looks like it takes power to reject the ripple.

author: nsayer
author_rank: Regular Contributor
timestamp: August 21, 2015, 09:29:01 pm
content: Thanks everyone for the detailed feedback. I have a lot to ponder, but at least it's not a completely stupid idea.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2015, 09:50:25 pm
content: What's up with all these threads?Ref:https://www.eevblog.com/forum/beginners/has-anyone-connected-a-linear-regulator-to-the-output-of-a-switching-reg/?topicseenTim

author: ez24
author_rank: Super Contributor
timestamp: August 21, 2015, 10:07:48 pm
content: QuoteOne was his and one mine. Both will die away in a day or two. I got a lot more of my answers and links on my post.This OP was specific to LDO (it is in the title), mine was not. I plan not to use a LDO, OP does. I want to get away from LDOs. I got enough info from my post to keep me happy.So I will not start another post on this subject (at least for a while)Thanks for keeping me honest.Edit: I hate to ask but what is the difference between a "post" and "thread"? I think a post is a new subject and thread is a comment on a post? Or is a thread any post under the same general subject? Boy am I confused

author: onlooker
author_rank: Frequent Contributor
timestamp: August 21, 2015, 11:44:31 pm
content: You got it backward. A "thread" is a ordered and time varying set of "post"s:thread ={post0,post1,....postn}