Voltage regulator adjustment resistor recommended values? - Page 1
Forum section: electronics / projects

author: ftransform
author_rank: Frequent Contributor
timestamp: January 26, 2013, 07:26:44 pm
content: How low of a resistor value can I get away with using in a resistor adjustable voltage regulator? I know that lower value resistors will produce less noise.I am using a LT1965 and the datasheet only says to use less then 12.1k.I know that with OPAMPS choosing uber low value resistors may cause you to run into problems due to drive limitations but I am not sure how this works with a linear regulator, which I guess is really just an opamp and some transistors.Does anyone have some practical advice? What kind of absurd extremes can this be taken to?

author: Christe4nM
author_rank: Unknown
timestamp: January 26, 2013, 07:51:38 pm
content: As far as I can see on page 12 of thedatasheetthe adjustable setting is done by using a voltage divider from the output of the LT1956 to ground. The ADJ pin takes the point between the two resistors as voltage reference to set the output.When creating voltage divider there is the term "stiff", indicating how much the voltage at the point between the two resistors changes when current drawn from that point. The less the voltage changes, the stiffer the voltage divider. Remember V=I*R. Say we have two identical resistors forming the divider, and ADJ is connected precisely in between. 100 mA goes into the top resistor, and for example then only 50 mA is going through the second, and the other 50 mA is drawn by the ADJ pin of the LT1956. (This is very exaggerated) The voltage across the second resistor is no longer half of the output voltage, but only 1/3.Say the ADJ pin draws only 1 mA. If there is at least 100 mA flowing though the divider, the voltage at the ADJ pin will be off by only 1%. Make 1A flow, and only 0.1% fault remains. See why you need a relatively low resistance?In other cases you could overcome that by using an opamp as buffer between the voltage divider and the 'load'. The opamp, having a really high input impedance will draw almost no current. So the resistors can be quite high of value, and the voltage divider doesn't draw too much current.Now finally on to your question: how low can I go. Well, since the current must come from the LT1956's output, you can only draw as much current as it is capable of supplying. Keep in mind though, that the current flowing into the voltage divider will not flow into the to-be-powered circuit. So it's advisable to keep the resistance as high as possible.

author: madires
author_rank: Super Contributor
timestamp: January 26, 2013, 08:07:17 pm
content: You need a voltage divider for setting the output voltage, not just a single resistor. The design goal is to find the largest possible resistance for the voltage divider to minimize losses (voltage divider is a load) while keeping the regulation stable (required bias current). If you decrease R1 (between ADJ and ground) with a fixed R2, you increase the output voltage. At some point the LT1965 will just follow the input voltage, no regulation at all. Another problem will be the required bias current for the ADJ pin. Since the datasheet doesn't tell us what's inside, I could only guess :-) With a too low bias current the regulation will become unstable and maybe you'll see oscillation or some weird reaction.

author: CarlG
author_rank: Regular Contributor
timestamp: January 26, 2013, 08:19:03 pm
content: The absolute min FB resistance is of course determined by how much current the regulator can source, but that's obvious so I guess that's not the answer you want?I don't think there's any general "best practice" of min FB resistance related to the noise aspect. I usually pick something that give a reasonable power loss in relation to the application, while not choosing too high resistor values (to risk noise pickup or voltage error due to FB pin bias current).I admit I've never done an estimate of the noise generated by the FB divider, but I would be more worried about using toohighresistance, partly due to the FB bias current, but more to the fact that the FB input generally is high impedance and thus can pick up noise easily if the FB node is not minimized and the FB divider is also high impedance.I couldn't see the LT1965 DS mentioning anything about noise vs FB impedance. In the section "output voltag enoise" they rather point at layout issues, decoupling capacitors, input voltage noise etc.

author: ftransform
author_rank: Frequent Contributor
timestamp: January 26, 2013, 08:20:33 pm
content: Quote from: madires on January 26, 2013, 08:07:17 pmI understand that you need a ratio of resistors I don't know why you think I did not understand?So the reason why old regulators like the 317 tend to use lower resistor values is because they need a larger bias current? If you look at say the lt3080 or such the feedback resistors they use are like 100k+, while on a 317 they use 240 ohms, though the lm3080 is a single resistor feedback so I'm guessing it has a buffer or something, I did not look at the datasheet.I'm guessing that since its not really mentioned in the datasheet I will probably be ok if I use a 10kohm and a 110 kohm for a ~15V divider then, which was my first choice, but then I wondered about using lm317 size resistors, like 100 ohms and 1000ohms.And I just came to the realization that loading the device down would cause it to heat up and that severely ruins its ripple rejection properties, making the choice obvious, especially when its meant to power op-amps which will draw little current.This device has a very curious ripple rejection ratio (not linear like some other devices) which would benefit from cooling lol. Dry ice! Liquid nitrogen!

author: CarlG
author_rank: Regular Contributor
timestamp: January 26, 2013, 08:40:21 pm
content: Quote from: ftransform on January 26, 2013, 08:20:33 pmBasically, yes. Though the LM317 DS mentions 1.2k as maximum programming resistor, giving 1 mA, which is reasonable error budget with IFB= 50uA.QuoteI would also go for R1 = 10k.QuoteInteresting, hadn't noticed that! Never seen a PSRR vs temperature plot beforeDo you have any example of any other device with that spec (where it's linear as you say)?

author: ftransform
author_rank: Frequent Contributor
timestamp: January 26, 2013, 08:57:18 pm
content: Quote from: CarlG on January 26, 2013, 08:40:21 pmWell I swear I saw a linear one somewhere, but take thehttp://cds.linear.com/docs/en/datasheet/3082f.pdfThis one has a parabolic curve (gets worse at lower temperatures).Well, consider the LM3080 curve. It looks parabolic too, but if you look at the scales, its pretty damn linearhttp://cds.linear.com/docs/en/datasheet/3080fc.pdf76 to 74 db across the range, I'd consider this fairly linear.While the LM1965 has 95 to 75 across the range. That's a bit of difference. I wonder how it would perform even lower!or the LM3015http://cds.linear.com/docs/en/datasheet/3015fb.pdfeven more linear then the 3080!

author: madires
author_rank: Super Contributor
timestamp: January 26, 2013, 09:04:50 pm
content: Quote from: ftransform on January 26, 2013, 08:20:33 pmNo offense meant :-)QuoteIt's based on the regulation circuit inside. Modern stuff is more sophisticated and optimized for low power.Let's see:- LT1965 needs 1.3µA (max 4.5µA)- LM317 needs 50µA (max 100µA)That's a factor of around 40 and with 240 ohms * 40 = 9600 ohms we're not off too far :-)QuoteAnother good point!

author: CarlG
author_rank: Regular Contributor
timestamp: January 26, 2013, 10:06:20 pm
content: Quote from: ftransform on January 26, 2013, 08:57:18 pmThanks a lot for the links! Really interesting, adding a mental note to check that in future designs! BTW, aren't LT's datasheets the best

author: ftransform
author_rank: Frequent Contributor
timestamp: January 27, 2013, 04:59:09 am
content: I'm fairly new to looking at datasheets but I like them because you can find pretty much the same parameters in every one. I don't know how good other companies are about it but sometimes I run into some piss poor data sheets.

author: madires
author_rank: Super Contributor
timestamp: January 27, 2013, 03:53:32 pm
content: Quote from: ftransform on January 27, 2013, 04:59:09 amEspecially if an unknown company just copies an electronic part from a known company.