LDO Linear Regulator with automatic bypass?  - Page 1
Forum section: electronics / projects

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: April 17, 2018, 08:04:46 am
content: Just wondering if such a thing exists? A single chip regulator that will automatically bypass itself if the Voltage drops below some threshold?I'm designing an application where I would like to give users the option of supplying 5V (perhaps higher) and 3.3V to my device. Problem with LDO's or even non LDO's is that they will just stop working at some point if the voltage goes below the dropout point.So If the user wants to provide 3.3V what I really need to do is bypass the regulator but that's a bunch of extra external components, and have not yet thought how to do it.I've looked at all the components and the highest minimum supply for any chip is 3.13 Volts... so I started thinking of an LDO with a 0.17V drop. The XC6221 @ AU$0.82 is the cheapest I could find. BUT that idea I feel is asking for trouble because it really leaves the users power supply no wriggle room. I just want to bypass the LDO if the whole thing browns out no harm no foul and I only need 100mA.I wanted Linear specifically to avoid any switching noise, cheapness and stability and that efficiency is not a big deal in this application.Thanks

author: BrianHG
author_rank: Super Contributor
timestamp: April 17, 2018, 08:44:51 am
content: You can test a KF33BDT LDO. Or, try building your own, you can try breadboarding my design below in this thread:https://www.eevblog.com/forum/beginners/ldo-to-supply-close-to-5v-from-5v-or-12v-input/msg1458929/#msg1458929QuoteChange the zener diode for a lower voltage. You may need to add a cap between Gate and Drain to prevent overshoot on powerup and oscillation.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 17, 2018, 10:13:29 am
content: Quite a few LDOs will "track" the input voltage when it's below the minimum required for regulation.The NCP500 for instance probably fits your needs?http://www.onsemi.com/PowerSolutions/product.do?id=NCP500

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2018, 10:36:07 am
content: Quote from: SiliconWizard on April 17, 2018, 10:13:29 amYes, that's the default behaviour for most low drop-out regulators, down to a certain voltage of course. Unfortunately, the minimum voltage often isn't specified or is just assumed to be Vout+ dropout.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 17, 2018, 10:45:13 am
content: The output vs input behavior of the NCP500 is perfectly documented in its datasheet. That's why I mentioned it.

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2018, 12:26:54 pm
content: Quote from: SiliconWizard on April 17, 2018, 10:45:13 amI didn't mean to suggest otherwise. The NCP500 is unusual for mentioning the input vs output characteristic on the data sheet.With regards to the XC6221, mentioned by the original poster, the data sheet specifies a minimum operating voltage of 1.6V, so it should work down to that. As with any low drop-out regulator, with a P-channel output, the drop-out increases, as the input voltage is reduced, as the gate has less drive. The maximum drop-out voltage is specified as 140mV and it will stay around that figure, until the input drops below about 3V.https://www.torexsemi.com/file/xc6221/XC6221.pdf

author: mdijkens
author_rank: Regular Contributor
timestamp: April 17, 2018, 01:24:39 pm
content: Maybe a somewhat similar situation:For powering my ESP8266 projects I use the HT7333 LDO which has a drop of only 90mVWith that I can power my ESP's from 12V down to 3V (2.9V actually)

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 17, 2018, 01:54:31 pm
content: Quote from: Hero999 on April 17, 2018, 12:26:54 pmGranted it's one of the few that document this as well. Actually a lot of the LDOs in the NCPxxx line are similar in that respect. I've used them on occasion and they work according to the specs. The nice thing is that the drop-out voltage when Vin is below Vreg is very low, so for battery operated stuff, that's nice.Of course, there are other situations in which this behavior is, on the opposite, not wanted. When your design is not guaranteed to work below a specified voltage, you'll want the regulator (or another supervisory circuit) to shutdown your circuit instead of powering it at unsafe voltages.

author: David Hess
author_rank: Super Contributor
timestamp: April 17, 2018, 05:28:32 pm
content: Integrated bipolar LDOs usually have anti-saturation built in which increases the dropout voltage but keeps quiescent current low at dropout. CMOS based LDOs do not need this so should follow the input voltage within the limits of their Rds(on) and I am surprised you have not found one which is suitable.If you cannot find an integrated regulator which meets your specifications, then one which has a low input voltage detection circuit can be configured so that at low input voltages, the detection circuit turns on an external pass transistor to bypass the regulator.

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: April 18, 2018, 07:58:41 am
content: Thanks for the ideas and info all.I think I have found some winners, while Vin is limited to 5.5 in both cases I'm going to live with that for the convenience of having 1.2V and 3.3V integrated in to the same package. Sorry I forgot to mention I needed those two rails (FPGA related).TLV7111233 AU$0.78 in singles @ 200mA - 6-WSON (1.5x1.5)MIC5355-S4YMME AU$0.70 in singles @ 500mA 8-MSOP 3mm (absolutely massive!!)