Need low-current adjustable LDO in TO-92 package, can't find, can I use 1.2V? - Page 1
Forum section: electronics / projects

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 04, 2025, 02:20:30 am
content: I have a need for a low-current (<= 100 mA) low voltage adjustable (1-5V) LDO regulator in a TO-92 package (there's no room on the PCB for a surface-mount part, so TO-92 with body folded over will fit mechanically.)The supply voltage is 5V (so expected maximum output from the regulator would be 5V minus the input-output drop-out. The reference voltage will be supplied by a DAC (0-5V.)I started with the LM317L (TO-92 low-current version of LM317), but then realized its specified input-output drop-out voltage is 2.5V, so it's not a good fit for my 5V supply and ~1.2-4.0V output requirement.After some searching, I realized that there is evidently no such (TO-92 adjustable LDO) device (and very few adjustable LDO regulators at all).I remembered that a 3-terminal regulator can be configured with a diode in the common (normally ground) path to ground to elevate the output voltage by the same amount.This takes me to the Microchip MCP1700-1202 (or MCP1702-1202) which are available in TO-92 package, have suitable current capability (250 mA, more than is needed), suitable input-output drop-out voltage (625 mV, although that spec pertains to the 2.8V version), and is available in a 1.2V type. This means my minimum output would be 1.2V (common pin grounded) and maximum output would be about 4.3V (common pin lifted by 3.1V).My question is this - am I missing something that would impair/preclude this approach (DAC controlling common pin to make a fixed LDO provide an adjustable LDO solution)?Thanks (and of course, if I've simply failed to locate a TO-92 adjustable LDO, I'd like to hear about that!)Dave

author: prosper
author_rank: Regular Contributor
timestamp: August 04, 2025, 02:36:02 am
content: all linear regulators are adjustable. they provide their rated voltage plus whatever voltage is present on their 'ground' pin.This also usually means you can adjust them down to zero volts out by supplying a negative voltage.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 04, 2025, 02:54:32 am
content: Thanks, that is very helpful!Dave

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 04, 2025, 05:28:44 am
content: Quote from: wb0gaz on August 04, 2025, 02:20:30 amYou need to check the DAC impedance and check the regulator IQ and GND current.The MCP1700 looks to have a low IQ, but the GND current varies with load up to ~ 50uA so the DAC needs a low output impedance.You will also need to check stability, as your use case is not their default.

author: aeg
author_rank: Regular Contributor
timestamp: August 04, 2025, 11:51:26 am
content: Any chance you can use an emitter follower instead of an LDO?

author: Psi
author_rank: Super Contributor
timestamp: August 04, 2025, 12:03:49 pm
content: Quote from: wb0gaz on August 04, 2025, 02:20:30 amLooks at TO92 part size.Looks at smallest SMT regulator package available.I know you do probably have a reason you cant use SMT. it just made me laugh how small of a regulator you can get in SMT vs TO92 size

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: August 04, 2025, 02:10:28 pm
content: I agree with Psi;Thru hole parts are becoming extinct faster than the dinosaurs after the Chicxulub extinction event.Can you design a small SMT board? With the small size of available SMT components you will be able to fit all the required components well within a TO92 footprint.

author: Zoli
author_rank: Frequent Contributor
timestamp: August 04, 2025, 03:35:40 pm
content: Quote from: UnijunctionTransistor on August 04, 2025, 02:10:28 pm+1; make your own regulator board. As suggestion, do it on flexible PCB, since it is easy to cut down with a scissor to the optimal size, and it's easier to handle and build on bigger size.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 04, 2025, 07:53:17 pm
content: OK - very much thanks for the new suggestions/ideas, and of course my post lacked some informationThe DAC will be Microchip MCP4726, which can source (at short circuit condition) 15 mA typical. Therefore, I suspect Zout of the output amplifier is low enough to survive this use case (reference current supply to the regulator.)Stability? Yes, it is not a documented use case! As time response is not critical, I'll leave provision for a bypass cap at the interface between the DAC and the regulator "ground" pin.Small PCB, etc.? As this is just build of a few devices, I'm seeking to avoid a secondary PCB, however, with similar constraints and a production scale build, that would make much sense.Emitter follower - hadn't thought of that option (never considered that function in a DC-type application like this) - thanks for the ideaThanks again for the very helpful discussion!Dave