78 S 05 regulators - any gotchas? - Page 1
Forum section: electronics / projects

author: SL4P
author_rank: Super Contributor
timestamp: December 11, 2016, 12:42:30 am
content: Does anyone here have experience with the LDO version of the venerable 7805 3-pin regulator?Anything to watch out for? Pitfalls, strengths or weaknesses...TIA

author: danadak
author_rank: Super Contributor
timestamp: December 11, 2016, 12:56:20 am
content: Just a general recommendation, look at datasheet recommendations for bypass,especially if there is an ESR limitation for the output cap.Regards, Dana.

author: wraper
author_rank: Unknown
timestamp: December 11, 2016, 02:10:40 am
content: 78S05 is not an LDO. It is 7805 with increased maximum current, nothing more. As what pitfails can be in LDO in general, ceremic capacitor on the output. Unless datasheet states it's ceramic tolerant, assume that it's not and there is a minimum ESR requirement. Therefore generally tantalum capacitor should be used, not ceramic. Otherwise oscillation may occur.

author: SeanB
author_rank: Super Contributor
timestamp: December 11, 2016, 05:51:16 am
content: Be very wary of input voltage range, some of them are not rated for more than 10V input voltage, while the original 78XX was rated for 35V input maximum. Also output capacitance minimum value and minimum ESR is important, most need a 220uF 16v electrolytic capacitor there to be stable over the full current range, and some also recommend a snubber as well, and put the 3 diodes there to prevent the device being reverse powered during turn on and turn off.

author: SL4P
author_rank: Super Contributor
timestamp: December 11, 2016, 06:48:29 am
content: Thanks - got all those in my design!Yes - sorry, its not an LDO, but an optimised (sort-of-switching) version of the old 78XX series. The internal block diagram is interesting.78Sxx provides wider Vin range, and other benefits - as well as higher Iout - which I don't really need.

author: Monkeh
author_rank: Super Contributor
timestamp: December 11, 2016, 06:56:53 am
content: Quote from: SL4P on December 11, 2016, 06:48:29 amuuh.. so far as I can see, they're nothing of the sort, merely a raised current limit. Identical to ST's standard 78XX, which is functionally identical to every other 78XX.QuoteThe same Vin as.. every other 78XX?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 11, 2016, 07:29:52 am
content: Quote from: SL4P on December 11, 2016, 06:48:29 amWat!?No.

author: mrpackethead
author_rank: Super Contributor
timestamp: December 11, 2016, 08:38:08 am
content: there are parts that are drop in replacments for 7805's that are switching.. and they have similar part numbers...

author: wraper
author_rank: Unknown
timestamp: December 11, 2016, 10:16:21 am
content: Quote from: SL4P on December 11, 2016, 06:48:29 amWat? How can it be sort of switching if there is no inductor around. LDO is just a low drop kind of linear regulator. Different topology of output stage often makes it less stable. Switching regulator is something completely different from both.QuoteIf you looked at it, then I dunno how you could figure out it's a switcher.

author: Psi
author_rank: Super Contributor
timestamp: December 11, 2016, 11:31:22 am
content: Quote from: wraper on December 11, 2016, 10:16:21 amThe drop in 78xx switching regs are in fact modules,they include the fet/drive/inductor/diode/bypass caps all potted in one package.

author: wraper
author_rank: Unknown
timestamp: December 11, 2016, 11:38:28 am
content: Quote from: Psi on December 11, 2016, 11:31:22 amThey aremodulesas you said, and don't look like IC. Moreover he looked onto internal diagram.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 11, 2016, 11:47:26 am
content: Quote from: mrpackethead on December 11, 2016, 08:38:08 amhttps://www.dimensionengineering.com/products/de-sw050

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 11, 2016, 07:00:17 pm
content: These are switched modules I have, 78SR105HC from power trends. Hard to find them now.

author: SL4P
author_rank: Super Contributor
timestamp: December 11, 2016, 08:28:24 pm
content: I'll admit to not researching this from the start - but was curious what the 1KHz internal oscillator does in the S05.I realise they don't have any inductors, and are not true switchers...What's the trick? Are they just some sort of 'chopper' to reduce the nominal output before it gets smoothed?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 11, 2016, 08:41:41 pm
content: Quote from: SL4P on December 11, 2016, 08:28:24 pmURL would be helpful. I can't find any datasheet showing a 1kHz oscillator, and I looked at a few from different manufacturers.QuoteThey are not untrue switchers either; they are not any kind of switchers, or fake switchers for that matter.Just like saying "oranges are not true apples" makes no sense.QuoteI can't understand this sentence. Reduce nominal output what?My opinion is that there is no "trick" whatsoever. Just a basic linear regulator.

author: SL4P
author_rank: Super Contributor
timestamp: December 11, 2016, 09:30:52 pm
content: I've already proven myself an idiot in this thread.Let's just say, i had two datasheets on the desk in front of me, and took the characteristics from one, and the internal block from the other.Mea culpa! Admitted.