Project with multiple voltage regulators - Page 1
Forum section: electronics / projects

author: sentry7
author_rank: Regular Contributor
timestamp: March 09, 2016, 10:10:01 pm
content: Hey guys,Until now, every project that I've done has had only one rail, but I'm working on a project now where I need two power rails. Is there anything that I should be aware of when using multiple regulators in one project?

author: Howardlong
author_rank: Super Contributor
timestamp: March 09, 2016, 10:53:41 pm
content: Quote from: sentry7 on March 09, 2016, 10:10:01 pmSequencing possibly, depending on the application. Otherwise, there are a million and one ways to skin this cat depending on requirements. It's not uncommon for some devices to have a dozen or more power domains.

author: uncle_bob
author_rank: Unknown
timestamp: March 09, 2016, 11:21:46 pm
content: Quote from: sentry7 on March 09, 2016, 10:10:01 pmHiAre some positive and some negative? Do they both go to the same part(s)? If so do those parts specify a start up sequence? If not will parts that are driven "early" have a problem?If it's a couple of positive rails, one easy solution is to cascade them. Then you know pretty much how they come up. Depending on caps and current load, you might also be able to guess how they go down.Bob

author: sentry7
author_rank: Regular Contributor
timestamp: March 09, 2016, 11:45:10 pm
content: Thanks for the heads-up, guys. Because this is the first time I've had multiple power taps, I'm not familiar at all with the associated problems. From what it sounds like, power sequencing is when certain parts need to be powered on in a certain order? Also, cascading regulators would just be feeding the output voltage of one into the input of the other?For more information, both rails are positive; they are 5 and 3.3V. The 5V is going to an op amp and the 3.3V is going to the rest - the rest being low-power TTL parts. There really isn't an issue of powering things in order since the only thing the op amp is doing is amplifying signals from the 3.3V parts.

author: uncle_bob
author_rank: Unknown
timestamp: March 10, 2016, 12:05:49 am
content: Quote from: sentry7 on March 09, 2016, 11:45:10 pmHiYes, cascading is just running the 3.3 off the 5.0. That way you can be sure the 5 us up first. If all the current drain is on the 3.3, it's a pretty good bet it dies before the 5 or at least it dies at the same time as the 5V.Bob

author: sentry7
author_rank: Regular Contributor
timestamp: March 12, 2016, 02:05:14 am
content: So this is the diagram for the power section of the project I'm working on. Any suggestions?

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 02:10:26 am
content: Quote from: sentry7 on March 12, 2016, 02:05:14 amHiDiode on the second regulator?Bob

author: sentry7
author_rank: Regular Contributor
timestamp: March 12, 2016, 02:24:01 am
content: Quote from: uncle_bob on March 12, 2016, 02:10:26 amThe datasheet for that one doesn't mention input short protection; is it because it's common sense?https://www.sparkfun.com/datasheets/Components/LD1117V33.pdf I see on page 16/27 the diode on the input, does this apply even though I not designing a battery backup?

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 02:33:19 am
content: Quote from: sentry7 on March 12, 2016, 02:24:01 amHiThe issue is the stuff connected to each supply. If 5V comes up first and 3V second that may be ok for your parts. If 5V goes away first and 3V hangs around for a long while, that may (or may not) be ok for your parts.It's not just the regulators, but the stuff they power and the signals between those parts ....Bob

author: sentry7
author_rank: Regular Contributor
timestamp: March 12, 2016, 02:48:37 am
content: Ah, I see; that makes sense. Also, I think the filter caps may be too heavy. When the input voltage goes away, wouldn't the resulting current overload the shunt diodes? Should I put down series resistors with D2 and D4?

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 02:54:05 am
content: Quote from: sentry7 on March 12, 2016, 02:48:37 amHiWhen the wall power goes away, there (likely) no current out the power plug. That voltage simply decays slowly as the 5V regulator drops out. The current from the 3.3V back to the 5V is never going to be more than the drain of the parts that run off of the 5V supply. Unless you have many amps of 5V and a 1A diode, this should not be a problem.Bob

author: sentry7
author_rank: Regular Contributor
timestamp: March 12, 2016, 07:45:43 pm
content: What would be some of the differences in doing them in series like this versus parallel?

author: bson
author_rank: Unknown
timestamp: March 12, 2016, 08:13:29 pm
content: Quote from: sentry7 on March 12, 2016, 07:45:43 pmA cascaded regulator is just another load on the upstream. It will give improved regulation of the lower voltage at the expense of adding ripple and noise to the upstream. Typically a lower voltage like 1.8V is more sensitive to ripple so cascading off a 3.3V or 5V supply isn't a bad idea, especially if the load is minuscule. Conversely might not be so great to cascade it off a higher voltage used to power a low-noise amplifier.

author: Howardlong
author_rank: Super Contributor
timestamp: March 12, 2016, 10:44:29 pm
content: Another reason to cascade is if the higher voltage domain is relatively lightly loaded but the lower voltage one more so. By cascading you can distribute the thermal stress.