Voltage regulators - Exact meaning of min/reg/max output voltage in data sheet - Page 1
Forum section: electronics / beginners

author: 9voltbrain
author_rank: Contributor
timestamp: March 27, 2023, 03:23:18 am
content: I need a fixed voltage reference for my circuit. Doesn't matter what it is, as long as it's the same all the time, so I can design my circuit around it.I'm looking at a cheap voltage regulator that says the output voltage is anywhere between 4.8 and 5.2 volts. My question is, is it going to fluctuate between those values depending on circuit conditions, or will it always be a fixed specific value somewhere in that range, that's just different between units?

author: ataradov
author_rank: Super Contributor
timestamp: March 27, 2023, 03:55:28 am
content: It will be mostly fixed in that range. There will be some dependency on the load and input voltage too, but that is usually specified separately and may or may not be included in that range, it depends on the part.

author: srb1954
author_rank: Super Contributor
timestamp: March 27, 2023, 04:33:00 am
content: Quote from: 9voltbrain on March 27, 2023, 03:23:18 amIf you check the datasheet you will probably find that the specified range of 4.8 to 5.2V applies for a range of operating conditions:Input voltage ranging from minimum usable operating voltage to maximum specified operating voltageLoad current from minimum specified load current to maximum permissible load currentJunction operating temperature less than maximum allowableThe manufacturer typically specifies the performance in this way to give the design engineer the required information for performing worst case design analysis.However, for a given fixed set of operating conditions, the regulator will stay moderately constant in its output voltage in normal operation. You can determine any deviations by looking at the following spec figures:Load regulation will show the variation of O/P voltage as the load current is variedLine regulation will show the variation of O/P voltage as the input voltage is variedTemperature coefficient will show the variation O/P voltage as a function of junction temperatureAlso affecting the O/P voltage are ripple rejection, noise and long term drift. You need to take into account all of these figures before you can determine whether your regulator will provide a sufficiently stable O/P voltage for your purposes.

author: mariush
author_rank: Super Contributor
timestamp: March 27, 2023, 05:36:53 am
content: Regulators have a voltage reference in them, common values for such references are 1.25v and 0.6v - the regulator uses the output of the voltage reference to adjust the output voltage and keep it as fixed as possible.These voltage references are factory calibrated up to some limit so you get some variation from chip to chip but usually very smallThese references are also affected by temperature so as the regulator heats up the output of the reference could drift a tiny bit - fancier regulators will have internal compensation for this, cheaper ones won't.So it's normal for a datasheet to specify a minimum , typical , maximum even on a fixed output regulator.

author: sleemanj
author_rank: Super Contributor
timestamp: March 27, 2023, 05:48:30 am
content: If you need a voltage reference, use a voltage reference. TL431 is the classic jellybean reference.

author: 9voltbrain
author_rank: Contributor
timestamp: March 27, 2023, 06:01:46 am
content: Quote from: sleemanj on March 27, 2023, 05:48:30 amActually, if I just need a voltage reference to stay at the same voltage over time, and don't care much what that voltage actually is, can't I just pick any diode for the job, and then design my circuit around its exact voltage drop?

author: magic
author_rank: Super Contributor
timestamp: March 27, 2023, 07:11:51 am
content: Diode forward voltage drop decreases 2mV per °C, so they are more often used as thermometers than voltage references.TL431 is good enough for nondemanding applications, as is any voltage regulator (mind that they often have minimum load current for correct operation).

author: 9voltbrain
author_rank: Contributor
timestamp: March 27, 2023, 07:18:13 am
content: Quote from: magic on March 27, 2023, 07:11:51 amMy application is just to feed the voltage into the + input of an OP-AMP, so I don't really need any constant current. Do all regulators/references have minimum load current, or just the diode-based ones?

author: Zero999
author_rank: Super Contributor
timestamp: March 27, 2023, 07:28:52 am
content: Quote from: 9voltbrain on March 27, 2023, 07:18:13 amThe TL431 is a shunt regulator, which requires 1mA to work properly.https://www.ti.com/lit/ds/symlink/tl431.pdfNo not all regulaors and references require a minimum load current. Read the data sheets for more information.

author: 9voltbrain
author_rank: Contributor
timestamp: April 02, 2023, 01:55:37 am
content: So I bought this LM285-1.2 shunt reference.https://www.ti.com/lit/ds/symlink/lm285-1.2-n.pdfI made this circuit to test it, and I'm not able to get it to stay at the same output voltage when the input voltage changes. A 2 volt Vin difference results in something like 20mV difference in Vout. Is it supposed to be like that? Is there a specific ratio between the resistor and the load resistance I'm supposed to use?

author: srb1954
author_rank: Super Contributor
timestamp: April 02, 2023, 03:00:58 am
content: Quote from: 9voltbrain on April 02, 2023, 01:55:37 amThe change you are seeing is a little bit excessive. Are you sure it is a genuine part and not counterfeit?You will get better performance with a little bit more reverse current through the LM285. I would recommend you should aim for at least 100uA current through it. Slightly better performance is obtained with up to 1mA reverse current but there lesser gains above that figure.So, allowing for the load current with Rload= 300k, you need to reduce R1 to:(6V - 1.23V)/(100uA +1.23V/300k) = 45.8k, say 47k.

author: 9voltbrain
author_rank: Contributor
timestamp: April 02, 2023, 07:11:18 pm
content: That made it a little better, but not much. 10mV change per 1v Vin change. It also seems to have a mind of its own, where if I slightly touch or move a cable back and fourth, during the course of the next 10 seconds it's going to gradually take a trip up to 1.24v, dance around between 1.24 and 1.23, and if I touch a cable again it (maybe) comes to its senses and goes back to 1.225.Meanwhile the LM317 (voltage regulator) isn't even budging 1mV when I do the same change in input voltage, and it's not even supposed to be accurate. Another question regarding that, if I just want the 1.25 reference voltage from it, can I just ignore the resistors and connect the adjust pin to ground?

author: shapirus
author_rank: Super Contributor
timestamp: April 02, 2023, 08:48:01 pm
content: This is interesting. I have some LM385-1.2 which is essentialy the same IC as LM285 but rated for a narrower temperature range. I'll test and see how it works tomorrow.

author: shapirus
author_rank: Super Contributor
timestamp: April 03, 2023, 03:35:14 pm
content: Quote from: shapirus on April 02, 2023, 08:48:01 pmOk, so yes, it seems like there's something wrong with your LM285 or, maybe, the way you perform measurements.I built the same circuit with LM385Z-1.2, R1=100k, Load=300k and measured the drop across the reference with Brymen BM869s in the 500k counts mode while changing V1 from 3.0V to 15.0V:3.0V -> 1.22525V15.0V -> 1.22527VLast digit is fluctuating within a range of approx. +/- 2, so I specified what seems to be the middle point. Either way, it is obvious that it is very stable across a wide range of input voltage, or, to put it correctly, the current flowing through the reference.p.s. any chance you mixed up the anode and cathode? Make sure that you identify the pins correctly,onsemi datasheetshows a front view pinout that's hard to misunderstand, unlike the TI's bottom view one.

author: 9voltbrain
author_rank: Contributor
timestamp: April 03, 2023, 06:26:43 pm
content: Yep, I goofed. Completely missed the "bottom view" text, and connected it in reverse. Never considered that I had connected it wrong because I figured I shouldn't have gotten anywhere close to the correct voltage if it was the other way around.Now it's stable at2.222v1.222v with any input voltage over 2-3v.

author: shapirus
author_rank: Super Contributor
timestamp: April 03, 2023, 06:34:32 pm
content: Quote from: 9voltbrain on April 03, 2023, 06:26:43 pmSomething's wrong there again. Where does that extra volt come from?LM285-1.2 has a nominal Vbr voltage of 1.235 V, that's what you should be seeing on a voltmeter connected between its cathode and anode.

author: 9voltbrain
author_rank: Contributor
timestamp: April 03, 2023, 06:51:38 pm
content: Quote from: shapirus on April 03, 2023, 06:34:32 pmThe extra volt came from me mistyping.