Can someone explain the pinout of this LDO? - Page 1
Forum section: electronics / beginners

author: Makar
author_rank: Newbie
timestamp: October 31, 2023, 05:33:52 pm
content: I am trying to implement the AZ1117IH-5.0TRG1 LDO into my PCB, but noticed that in the schematic it shows using input, output, and GND/ADJ, although there is a pad for VOUT (Doesn't show a connection for it on the datasheet). Attached are images of the footprint and typical application schematic. If someone could explain what the VOUT pin does that would be greatly appreciated. Datasheet:https://www.diodes.com/assets/Datasheets/AZ1117I.pdf

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 31, 2023, 05:43:52 pm
content: Vout and output are the same thing.This is SOT89 package but shows the idea:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2023, 06:17:29 pm
content: Though SOT-223 does not show the metal inbetween (as above), it's almost guaranteed the middle pin is an extension of the tab.This is actually true across a great many packages. I have seen some exceptions in either protected switches, or some LDOs, in SOT-223, where a side pin was common with tab instead; there are also 4+-pin variants where the tab is separate. Most (all?) E-B-C TO-92s are also made with a sideways arrangement, so the collector pin can also be the internal tab the chip is mounted on (which kinda explains the BCE and ECB pinouts being the most common: they get to use the more conventional middle-tab arrangement).Even TO-220, TO-247 and the like, with their thick tabs, have the middle lead either coined down, or welded on, with few exceptions!In any case, it's merely a typo; no pin is defined as "VOUT". As for the part itself, I wouldn't use it: it is a bipolar type LDO, which does not define ground pin current vs. input voltage (including in dropout), and does not provide a stability plot vs. output C and ESR. It's also an HDO type anyway.So... there used to be "LDO"s (low dropout regulators)in distinction tothe traditional types like the 78xx family, with >1.5V dropout, the internal structure being a Darlington emitter follower, so costing two diode drops minimum, plus extra overhead; this one is a compromise using Sziklai arrangement instead, which improves dropout a bit, but it's notarbitrarily low, it saves only one diode drop. "True" LDOs used a common-emitter (or -source) arrangement that allows arbitrarily low (< 100mV) dropout, at least at light loads.Nowadays, LDOs are the predominant market share, and distributors have taken to labeling the product category as "LDO". The term has lost meaning, because now "HDOs" are sold under the same label.So, just beware these terms have been rendered meaningless; it's classic redefinition right before your eyes.Anyways, I would get a proper LDO, of newer design (anything -1117 is quite old, 80s era; 78xx are 70s era), that specifies stable load capacitance, particularly that claims stability into low-ESR or ceramic capacitors (which are also the dominant type now; back then, electrolytic or tantalum would be most common in uF+ values).GND pin current is a concern with bipolar types, which can crank up internal base current in a futile attempt to maintain regulation; good ones limit this under dropout, so that usually the current rises a little bit but remains a small fraction of the total (a few extra mA is fine, but 10s or 100s, get that out of my face).CMOS regulators are also an option, particularly at low voltages (say dropping 5 to 3.3, etc.). These might not document their GND pin current, but given the PMOS pass device, it strains the imagination how they would draw excessive current this way, in any condition (dropout or otherwise).Bipolar regulators remain dominant in higher voltage ratings; there are even automotive types with no damage from backflow (whereas classic regs like 1117 and 78xx need an antiparallel diode to prevent backflow through the chip), and indeed the output can remain biased while the input gets reversed to like -40V.Or for more specialty applications, there are even a few regulatorcontrollers, add external MOSFET (or whatever), so the ratings can be much higher than for integrated types. You should probably consider an SMPS by that point; but it's interesting that they exist, and useful in the rare case you can't use an SMPS, or have to burn off the power anyway for other reasons.Tim

author: magic
author_rank: Super Contributor
timestamp: October 31, 2023, 08:11:01 pm
content: Quote from: T3sl4co1l on October 31, 2023, 06:17:29 pm1117 is a floating regulator; ground current is the current of the internal feedback divider (fixed versions) plus some 100μA or similar bandgap cell bias which is also present in the adjustable. I don't think it will have any problems with dropout or low input voltage.Unlike "true" LDOs with PNP. If something has less than 1V dropout and isn't CMOS, run...

author: liaifat85
author_rank: Regular Contributor
timestamp: November 01, 2023, 07:09:29 am
content: Vout is the output voltage. You will find it in the first table on page #4 of the datasheet.