Voltage regulator, LDO + PSRR (10+ kHz), does they exist? - Page 1
Forum section: electronics / projects

author: MasterT
author_rank: Frequent Contributor
timestamp: May 19, 2019, 05:53:25 pm
content: For a project I'm working on, I need a regulator +7.5V that satisfy two simple requirements LDO & PSRR.I see, there are lm317 and all 78xx series, that have good PSRR above 10 kHz (-70 dB or so up to 20+ kHz) but not LDO.On the other side modern mosfet p-channel LDO's that have miserable low PSRR, about -10 dB, some companies even don't publish any data except 120 Hz. And its strange for me, unlike 40 years ago nobody should care much about 120 Hz rejection since almost all power supplies switchable, and PSRR in the sub-MHz range should be primary goal for good properly designed voltage regulator. And I don't envision any obstacle to build fast circuits , since technology step far away from lm317 manufacturing process.What, inductors makers rule the market?

author: Boscoe
author_rank: Frequent Contributor
timestamp: May 19, 2019, 09:54:30 pm
content: http://www.ti.com/lit/ds/symlink/lp38798.pdfI found this using TI's parametric search in a minute or so, finding parts is dead easy when you master these things. I use a lot of TI parts as their website and search is particularly good.

author: ogden
author_rank: Super Contributor
timestamp: May 19, 2019, 10:55:42 pm
content: Champion of LDO's (today): LT3042.RMS Noise: 0.8µVRMS (10Hz to 100kHz)PSRR: 79dB at 1MHz

author: ConKbot
author_rank: Super Contributor
timestamp: May 19, 2019, 11:23:03 pm
content: Make sure you check the datasheet curves for PSRR as you approach dropout, and at your intended running voltages. A regulator may specify a 60+ db PSRR, but that may be at Vin-Vout = 1V, not when it's at 0.2V or lower that the dropout voltage is specified at.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 20, 2019, 12:05:15 am
content: Quote from: Boscoe on May 19, 2019, 09:54:30 pmThanks for the tip, TI search engine is much better than at mouser. Though seeding through hobby friendly package groups, I was able to locate non TO-92, and just 1 part in soic-8 adjustable that goes above 5.0V, with good PSRR data. It's LM2931. There are two more parts LP2951 & LP4951 that have unreliable PSRR over mA current range. Will order 2931, to see how it goes.

author: ogden
author_rank: Super Contributor
timestamp: May 20, 2019, 08:22:51 am
content: Quote from: blueskull on May 20, 2019, 12:01:00 amSure. It's the best LDO one can get (today).QuoteLOL. With same success one can say that LM1117 can do 99% the thing LP5907 can do.LT3042 PSRR: 79dB at 1MHzLP5907 PSRR: ~39dB or so at 1MHzDifference is ~40dB which is100 times worse

author: ogden
author_rank: Super Contributor
timestamp: May 20, 2019, 08:34:50 am
content: Quote from: blueskull on May 20, 2019, 08:28:08 amGoalpost shifting into LC filtering? OK. Let's assume that main subject of conversation, PSRR is not hat important anymore. Let's compare RMS noise of both.LT3042 : 0.8µVRMS (10Hz to 100kHz)LP5907 : 6.5µVRMS (10Hz to 100kHz)Still LT3042 is better by HUGE margin, thus price is fair.

author: ogden
author_rank: Super Contributor
timestamp: May 20, 2019, 08:56:34 am
content: Quote from: blueskull on May 20, 2019, 08:48:49 amThe cost is worse PSRR performance. All this argument finding nonsense just to disagree that LT3042 is best LDO around and it's price is fair?

author: ogden
author_rank: Super Contributor
timestamp: May 20, 2019, 10:24:41 am
content: Quote from: blueskull on May 20, 2019, 09:09:43 amRight. Instead you build LDO out of opamp, pair of transistors andno voltage referenceQuoteSure. You build sub-uV RMS moise LDOs just like that. I get your point and we are way too off-topic now.

author: tszaboo
author_rank: Super Contributor
timestamp: May 20, 2019, 03:29:16 pm
content: Place an RC filter in front of the LDO, and magically now you have good PSRR for high frequencies. 1 Ohm 100uF gives you about -20dB @ 10KHz. With minimal loss in voltage.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 20, 2019, 04:10:33 pm
content: Quote from: NANDBlog on May 20, 2019, 03:29:16 pmCould be an option, for 20 mA I can push resistor up to 10 Ohm and get -40dB. Still too low, 18-bits ADC 1.33 msps (EVAL-AD7984-Pxxx) is very sensitive.My first try to get clean -2.5V / +7.5V power using dc-dc isolated +5/ +-12V than downregulate with lm317l /lm337l turns out a disaster on EMI interference side.Second build using TC7660 charge pump converter get 6-10 times lower EMI, but I get into the limits of lm317l, loaded converter (with 2x schottky multiplier) supply only 8.8V, and regulator switched itself into bypass - mode.I put -70db as a base line, and was wondering why there are no lm78xx / lm317 generation-2, or G-3 etc. Some TO-92 easy to use upgrades of the common and very popular ICs. Though, at least I found one (lm2931) with the help of this forum.