Why negative LDOs are more expensive than positive LDOs? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: November 17, 2023, 05:44:58 pm
content: I am looking for a fixed -2.5v LDO in small quantities. On Digikey, the least expensive is TPS72325DBVR at $2.99 while for +2.5V the least expensive is MIC5504-2.5YM5-TR at only $016. That's more than x10 more expensive. Why?

author: wraper
author_rank: Unknown
timestamp: November 17, 2023, 05:49:43 pm
content: Because demand and production volume is not nearly as high.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2023, 07:51:23 pm
content: Negative voltages are pretty rare in everyday electronics.Easily, sales of positive Vregs are 50x the negative ones.

author: TimFox
author_rank: Super Contributor
timestamp: November 17, 2023, 08:05:15 pm
content: A related question: before retirement, I needed a reference for a negative voltage (from -12 V power), otherwise similar to an ADR02-type "series" three-terminal positive reference regulator.(Voltage accuracy not critical, but good temperature co-efficient.)At that time, I could findabsolutely nonegative-output devices of that type, and had to use a two-terminal (shunt reference) that could be used with either polarity.(No, the circuit could not be changed to use a positive reference, unless I wanted to add a precision op amp and two really-good resistors to invert the voltage.)A quick check today at Mouser showed no negative three-terminal references.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 17, 2023, 08:09:58 pm
content: Quote from: zapta on November 17, 2023, 05:44:58 pmYou chose a combination of negativeand-2.5V, so have hit the 'rare' adder twice.lcsc shows many negative regulators, but the cheapest are 79xx series, fixed voltage, and Torex 3.3V parts do show at better prices.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: November 17, 2023, 09:43:07 pm
content: Depending on your design, just take a +5v and a +2.5v reg.Your GND is -2.5v, the +2.5v rail gets 0V and the 5V reg gives you +2.5v3.3V 5V and 12V give you several very cheap options.An adjustable Lm317 is very cheap as well...Another way is to introduce some "virtual GND" with some opamp.So no need for negative in a lot of cases...hence the negative once are expensive.73

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 17, 2023, 09:45:17 pm
content: It's all relative.

author: langwadt
author_rank: Super Contributor
timestamp: November 17, 2023, 10:01:37 pm
content: Quote from: wilhe_jo on November 17, 2023, 09:43:07 pmexcept now your "0V" can't sink any more current that the load on "-2.5V"

author: zapta
author_rank: Super Contributor
timestamp: November 17, 2023, 10:45:33 pm
content: I think that with positive +2.5 and +5V LDOs, the common rail will not be able to sink from the +2.5 rail, but something like this end of life IC would work TLE2426.My circuit needs to be floating from the +5V supply and have +/-2.5V rails so I am using this D050505T-1WR4 for +/-5V and then +/-2.5v regulators for the regulated rails.

author: langwadt
author_rank: Super Contributor
timestamp: November 17, 2023, 10:51:18 pm
content: how much current? a TL431 maybe?

author: zapta
author_rank: Super Contributor
timestamp: November 17, 2023, 11:48:43 pm
content: about 10-20ma. Yes the TL431 may work and is less expensive.I already placed the order at DigiKey. ;-)

author: Konkedout
author_rank: Frequent Contributor
timestamp: November 18, 2023, 07:31:28 am
content: I think you have problems if your +2V5 reg output (which is serving as ground) needs to handle ground return current from the +5 regulator. But I do like LM317s for what they are good at.....

author: Siwastaja
author_rank: Super Contributor
timestamp: November 18, 2023, 10:04:23 am
content: Usually the linear regulator is only small part of the total cost of having a negative supply. Where is it coming from? A center-tapped transformer, for example, tends to be more expensive, too.If lowest cost is important, you would usually try your best to modify the topology so that negative supply is not needed (e.g. by biasing all signals to halfway between GND and Vcc), even if that comes with some performance tradeoffs. When budget is not limited, then throwing a dual supply at all analog problems makes design easier.