Looking for low quiescent discrete LDO - Page 1
Forum section: electronics / projects

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2025, 08:54:50 pm
content: Looking for schematic ideas for a uA-range quiescent current LDO, but made with discrete components, without integrated circuits like opamps or ready made LDOs. Any known schematics or tricks to DIY a low quiescent LDO?More specific, the input is a Li-ion cell between 4.2V...3.5V or so, and the output should be 3.3V, with short 200-300mA spikes of top current. Most of the time the load will draw some tens of uA (an MCU in deep sleep with rare spikes of 1-2ms of high current activity).

author: langwadt
author_rank: Super Contributor
timestamp: October 10, 2025, 08:59:30 pm
content: but why?

author: Someone
author_rank: Super Contributor
timestamp: October 10, 2025, 09:09:03 pm
content: Quote from: RoGeorge on October 10, 2025, 08:54:50 pmThe trick is a silicon designer can arbitrarily size each FET/transistor to be exactly the characteristics required. It's like asking how to get a good copy of the Mona Lisa by only using bumperstickers without cutting them into smaller pieces. You might get lucky and find some parts which do the necessary, but more than likely it'll just end up a mess.

author: thm_w
author_rank: Super Contributor
timestamp: October 10, 2025, 10:10:35 pm
content: If your MCU and peripherals support it, I go with 3.0V instead of 3.3V. Much easier to find (or design?) LDO with that extra headroom.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2025, 10:47:26 pm
content: Should work just as well at 3V, but I didn't try. What bothers the most is not the low voltage drop, but the low quiescent current. 1uA idle or so doesn't look easy to achieve. At a brief search, couldn't find online any schematic, only integrated circuits theory, though it wouldn't be practical to try to copy that.Since I need 2-3 such low quiescent LDOs this weekend, and don't have any at hand, I guess I'll just go and scavenge some LDOs from the EE scrap boxes, hope to find some in former GSM phones.

author: thm_w
author_rank: Super Contributor
timestamp: October 10, 2025, 11:14:45 pm
content: Will you no longer have access to the device? Can't you order some parts and swap them out later.For future use,6Vin 500nA for 2c each:https://www.lcsc.com/product-detail/C48678125.html20Vin 2uA for 3c each:https://www.lcsc.com/product-detail/C486069.html

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 10, 2025, 11:25:19 pm
content: For sure implementing such an LDO with only discrete parts, and parts that you'd happen to have in your drawers, is like the challenge of the decade.Regarding those ultra-low quiescent current LDOs, you mentioned having short but high current spikes: keep in mind that these LDOs will usually have worse load transient response. So use a liberal amount of output capacitance to make up for it.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 10, 2025, 11:40:10 pm
content: Quote from: RoGeorge on October 10, 2025, 08:54:50 pmA strange request, as you will need some voltage reference here, and those are integrated circuits, not discrete components.LDO's are made in very large volumes, so will be cheaper and better performance than anything 'discrete'.Quote from: RoGeorge on October 10, 2025, 10:47:26 pmIf you only need them 'for the weekend' then low quiescent current is not going to bother you. Just use what is closest, until you can source the correct part.

author: coppercone2
author_rank: Super Contributor
timestamp: October 11, 2025, 01:24:28 am
content: I like being able to finish projects. its a headache to have to go back to it laterUnfortunately, I think this circuit would be a head turner and I doubt anyone has one.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2025, 02:12:08 am
content: Quote from: coppercone2 on October 11, 2025, 01:24:28 amIt does not exist for a reason.eg you can get a 'regulator' with modest performance using 1 PFET and 1 LED and 5 transistors (2 matched pairs) plus 4-5 resistors, plus caps, but the load regulation is below average.It's ok with 5-300mA but at very low currents, Vo climbs. Voltage tolerance will be as variable as your LED + BJT combination.At very low currents, you have less scope for temperature compensation.That's a lot of parts and manual checking, to replace a single 1~10c part!

author: MariuszD
author_rank: Regular Contributor
timestamp: October 11, 2025, 07:19:58 am
content: The most important components for building an LDO are an amplifier and a reference voltage source. I've never seen a discrete amplifier with decent specifications drawing microamps, nor have I seen a discrete voltage reference drawing such a small current. I've never seen a discrete amplifier with decent specifications powered by 5V or lower.Low current causes transistor circuits to become slow and have low gain. Integrated circuit designers can create transistors that will perform better under such conditions, and they can also use complex multi-stage designs.Building circuits powered by low voltage is an additional challenge, requiring more complex circuits.