Discrete zero-dropout voltage regulator / limiter - Page 1
Forum section: electronics / projects

author: elektrinis
author_rank: Regular Contributor
timestamp: November 30, 2011, 09:40:55 pm
content: Hello. I am designing a protection device for precise 1.8V circuitry. Requirements:- Input voltage: 1.8...10V- Output voltage: 1.8V ± 0,1V- Output current: 0,05...10mA- No-load consumption: max 50µA- Noise immunity: anything (<1MHz) between 1,8V and 10V- Dropout: 0,15V max- Protections: not required- BOM list as short as possible- BOM price no more 1$ @10kpcsSo far I managed to do this:When not loaded, it is working fine. But when loaded with 75 Ohm resistor, it oscillates badly. The higher the load, the bigger oscillation.I've added 1µF input and 10µF output ceramics - helped a bit, but not by much. Tried connecting random caps to  random places - can't find good solution.Any thoughts?BTW, I have also evaluated MIC5235: working good, when noise is under 2Vpp. If over, output voltage drops to below 1V. Schematic just like in datasheet, with caps, etc. How to prevent this behavior?

author: amspire
author_rank: Super Contributor
timestamp: November 30, 2011, 11:59:40 pm
content: I like your effort at the FET circuit. Stability is always the no.1 problem with any regulator and you have the gain of their MOSFETs in series. Because they all have a fixed drain capacitance, but the drain currents vary widely in all your MOSFETS, you are going to have a huge variety of differing phase shifts around your feedback loop with different loads and different input voltages. It will not be easy to stabilize.Ultimately though, I cannot see the turn-on voltage of your regulator MOSFET being an accurate enough reference for the 1.8V, so even if you could stabilize it, it will not be good enough.The MIC5235 should be a good solution. When you say it fails if the noise is over 2Vpp, what are you actually applying on the input? Can you give us the DC and AC volts into to the MIC5235 when it does go to 1V out? Are you applying 9Vdc + 2vpp noise, or 1.8VDC + 2Vpp noise? It makes a very big difference to our understanding of the problem.Also in your noise immunity spec, what type of noise do you need immunity from? Is it a lot of half-wave rectified noise plus a little bit of higher frequencies?Richard

author: elektrinis
author_rank: Regular Contributor
timestamp: December 01, 2011, 07:41:18 am
content: Wow, thanks for your answer. Didn't expect itIt looks like a lot of work with discrete version, so let's not give up on MIC5235 just yet. What I am feeding it currently, is:Power supply is 10Vdc. If noise goes over 2Vpp, output voltage sags. I've been reading datasheet and some regulator theory last night and output cap rises some concerns. Will try better one.And here is what it will get in real world situation:The regulator will work directly on lithium cell, that has a huge noise from power electronics. The problem is that power electronics comes from other various vendors and I can't control what garbage it is putting to supply line. Also, the end product is assembled by customer (usually with no or very little electronics knowledge) from products of various vendors (including mine), so line filters are out of question (but I could add a filter on my product's supply, but it has to be no taller than 1.3mm and as cheap as possible). This is why my products must be as rigid as possible. MIC5235 was chosen due to high voltage protection and reverse-battery protection.

author: amspire
author_rank: Super Contributor
timestamp: December 01, 2011, 07:52:53 am
content: The regulator IC could be shutting down when the input voltage drops down below 1.8V, and dalaying restarting. Try adding a 10 ohm resistor and a 47uF or 100uF to ground before the regulator.Richard

author: elektrinis
author_rank: Regular Contributor
timestamp: December 01, 2011, 08:17:28 am
content: Yes, this is clear and will be approached, but now I have a problem with input voltage not dropping below 8V.Anyway will try bigger output cap and share the results.

author: amspire
author_rank: Super Contributor
timestamp: December 01, 2011, 11:14:06 am
content: Is the enable pin directly shorten to the input pin? If you have it connected elsewhere, it may be picking up a transient from the input test signal.Richard

author: elektrinis
author_rank: Regular Contributor
timestamp: December 01, 2011, 04:06:51 pm
content: EN pin is tied to input. I have also tried to filter it with RC 22k/100n - no effect.Tried increasing feedback resistor values 10x - no effect.Tried increasing input/output caps - very little improvement.Tried connecting quality film caps - no improvement over ceramics.Instability is very noise-dependant in terms of amplitude and frequency. There are certain resonant frequencies and certais noise amplitudes that make it crazy. Noise low level is nowhere near minimum operating voltage. Layout looks correct, with "start grounding", etc. Any suggestions?

author: ejeffrey
author_rank: Super Contributor
timestamp: December 02, 2011, 12:02:26 pm
content: Quote from: elektrinis link=topic=5764.msg75341#msg75341I would try a bypass capacitor on the adjust pin. I don't see that recommended anywhere on the data sheet, so you may have to experiment to find a value that helps and doesn't cause oscillation of its own.Edit: fixed quote tag

author: elektrinis
author_rank: Regular Contributor
timestamp: December 02, 2011, 12:48:39 pm
content: Tried that, no improvement..

author: blackdog
author_rank: Frequent Contributor
timestamp: December 02, 2011, 08:43:30 pm
content: Hi elektrinis :-)Sorry no discrete schematic....Bus my schematic dues not generate unwanted signals :-)Its really low noise!!!www.bramcam.nl/1,8V-LM10.JPGJust my 2 centsKind regarts,Bram

author: Zero999
author_rank: Super Contributor
timestamp: December 02, 2011, 09:11:56 pm
content: Quote from: blackdog on December 02, 2011, 08:43:30 pmNice try but it doesn't meet the 50µA quiescent current requirement.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 03, 2011, 04:35:53 am
content: Have you tried a LDO? The MAX1725 meets all those specs, including the dropout voltage as long as your load current is 10mA or less.

author: Zero999
author_rank: Super Contributor
timestamp: December 03, 2011, 10:10:25 am
content: The MAX1725 is only specified down to 2.5V but he needs it to work down to 1.8V.I was thinking he could use some kind of micropower boost converter to get 12V which could power a micropower op-amp and reference driving an N-MOSFET. The trouble with that is I couldn't find anything which will work from 1.8V to 10V.

author: eliocor
author_rank: Unknown
timestamp: December 03, 2011, 11:59:58 pm
content: Not sure for the price, butTPS62120can satisfy almost any of your requirements (except for 2.0V minimum input voltage).I used on several projects and it works flawlessly.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: December 04, 2011, 03:35:20 pm
content: You could try a local peak rectifier in front of your regulator. A low-Vf schottky and a decent capacitor.This prevents the other circuitry from discharging the capacitor, so your circuit receives a fairly constant 2.5 volts or so (3V-Vf).