Stability of TLV1117 with TL431 - Page 1
Forum section: electronics / beginners

author: satyamfifa
author_rank: Regular Contributor
timestamp: December 25, 2021, 08:59:13 pm
content: Hey All,I have a design in which I have decided to use TLV1117 along with TL431, I want to use TLV1117 because it's a LDO and TL431 for limiting the drift over time as this will supply the ADCs. I also want to use the jellybean components. The design requires bringing down the input voltage which can be between 12 to 48 volts to 6.5 volts (for which I have decicided to use L4971 switching convertor) and then further down to 5 volts using the TLV1117 with TL431. However I am little unsure regarding the stability on TLV1117 along with TL431, also I would like to have the maximum output current of 500 mA although realistically the maximum output could be as low as 40 mA. Here is the schematic I have so far:Any comments, design example, imrpovements etc will be well appreciated.Kind Regards,Sparsh

author: satyamfifa
author_rank: Regular Contributor
timestamp: December 25, 2021, 09:57:35 pm
content: https://www.ti.com/lit/ds/symlink/tl431.pdf?ts=1640428663361

author: YurkshireLad
author_rank: Frequent Contributor
timestamp: December 25, 2021, 10:42:47 pm
content: Quote from: Benta on December 25, 2021, 10:32:09 pmCan you explain for those of us that don't understand?

author: xavier60
author_rank: Super Contributor
timestamp: December 25, 2021, 11:34:01 pm
content: R2(560Ω) could be place between the Out and Adj pins to keep the TL431's load current constant.The circuit should be checked for high frequency stability.C1 should be removed.

author: xavier60
author_rank: Super Contributor
timestamp: December 25, 2021, 11:50:08 pm
content: A minimum operating current is specified for the TL431, 1mA from memory, but actually can be much less.Make it 1mA to be safe.Edit: the 2 mA supplied by the relocated R2 would be fine anyway.

author: satyamfifa
author_rank: Regular Contributor
timestamp: December 25, 2021, 11:56:09 pm
content: Quote from: xavier60 on December 25, 2021, 11:50:08 pmWell currently it is around 5mA to be safe, since voltage drop accross R2 is estimated to be around 2.75V, unless you are suggesting that I should lower it

author: xavier60
author_rank: Super Contributor
timestamp: December 26, 2021, 12:08:37 am
content: Quote from: satyamfifa on December 25, 2021, 11:56:09 pmRelocating R2 to the output side will make the TL431 current about 2mA regardless of input voltage.Can you breadboard the circuit to check for unforeseen problems?

author: satyamfifa
author_rank: Regular Contributor
timestamp: December 26, 2021, 12:29:24 am
content: Quote from: xavier60 on December 26, 2021, 12:08:37 amWell the input of this stage is suppose to be a constant 6.5V anyways, so I wouldn't worry too much weather it's connected to input or output, I would love to test it on breaboard, infact I was thinkinng of making a small prototyping board, but the truth is that I am little short on the test equipments, that's why I posted it here to be safe....

author: xavier60
author_rank: Super Contributor
timestamp: December 26, 2021, 12:40:51 am
content: One unknown is the possibility of high frequency instability or oscillation. Make provision for up to a 10nF capacitor between the K and REF pins on the TL431.Symptoms will be inaccurate and poor reregulation.BTW, the TL431's reference voltage will likely be a few mV less than 2.5V.

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2021, 08:38:44 am
content: Why are you deriving the reference for the ADC from the power supply? Does the MCU not have the option to use a different reference voltage for the ADC? If not, why not get one which does? Then you can skip the LDO, power the MCU directly off 5V and use a standard TL431 circuit for the ADC reference.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 26, 2021, 08:44:25 am
content: Quote from: satyamfifa on December 25, 2021, 08:59:13 pmshort answer: there is absolutely no need to add the TL431long answer:the stated max input voltage (48V) exceeds TL431 AMR (36V)I had missed it is preregulated at 6.5VC1 is like a finger in the eye (the 130Hz TC kills transient response)time drift of TL431 is not even specified so I expect it to be worse than LM1117

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 26, 2021, 09:04:27 am
content: Quote from: xavier60 on December 25, 2021, 11:34:01 pmsince input is pre-regulated the current is already constant: (6.5-3.75)/560 ≈ 4.9mAas the input comes from a switching module it might be better to provide current via a lowpass filter, but, in any case,there is no need for the TL431 as TLV1117 is more stable than TL431 (temperature and time)

author: satyamfifa
author_rank: Regular Contributor
timestamp: December 27, 2021, 02:44:36 am
content: Quote from: not1xor1 on December 26, 2021, 09:04:27 amThanks to you I went deep diving into datasheets, as it turns temperature stability of a TL431LIBC (which I intend on using) turns out to be 63 PPM / deg C (MAX) (11mV deviation over 70 deg C operating temperature range), and for TL431C part it is 92 PPM / deg C (MAX) (16mV deviation over 70 deg C operating temperature range) as opposed to poorly specified temperature stability of TLV1117 or LM1117 part which is 0.5% which can be as high as 5000 PPM / deg C. This data is in keeping with Dave's video where he said that a jellybean Voltage Reference can expected to be an order of magnitude better than a jellybean voltage regulator. So I think I have to conclude that TLV1117 is definitelyNOTbetter than TL431 at temperature stability. It is indeed true that long-term stability for TL431 is not specified, however I have no reason to think that it will be worse than TLV1117 since TL431 is a voltage reference with hundreds of application.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2021, 04:16:42 am
content: Using a TL431 to stabilize an LM1117 will work but C1 needs to be removed or adjusted to provide the proper frequency compensation for good low noise performance.The TL431 operates as the error amplifier and reference in this circuit. Separating them from the power output stage will significantly improve line and load regulation by removing thermal feedback.

author: satyamfifa
author_rank: Regular Contributor
timestamp: December 27, 2021, 04:25:23 am
content: Quote from: David Hess on December 27, 2021, 04:16:42 amYes indeed C1 needs to be adjusted or removed, however I didn't get that last sentence, would please mind explaning a bit what do you mean by "Separating them from the power output stage will significantly improve line and load regulation by removing thermal feedback"

author: asdf336
author_rank: Contributor
timestamp: December 27, 2021, 04:26:31 am
content: The Tl431 is not known for precision. It’s known for cost and versatility.I pulled up the regulator data sheet quickly and saw 1% over temp. That’s the ballpark of the best 431 variants over temp.The circuit can work but I also immediately wondered what benefit the 431 was.

author: xavier60
author_rank: Super Contributor
timestamp: December 27, 2021, 05:58:45 am
content: TI must think it to be worth while. Though they should have thought to connect the TL431's load resistor to the output side of the LM317, giving the TL431 a clean constant current load in case of a noisy unregulated supply and improved line regulation.https://www.ti.com/lit/ds/symlink/tl431.pdf?ts=1640428663361

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 27, 2021, 08:54:31 am
content: Quote from: satyamfifa on December 27, 2021, 02:44:36 amTLV1117 temperature stability is not 5000ppm/K as you stated but 5000ppm over the full operating temperature rangei.e. 40ppm/K and if you check figure 5 at page 8 you can notice that within the 300-380K (about 25-100°C) temperature range the typical variation is about 10-20 ppm/K.If you want a stable reference for an ADC you have to separate the power supply circuit and the reference circuit using something better than TL431... e.g. LT1009 (or similar) that is still cheap but much better than TL431and if you still want to improve the TLV1117 stability you can just clone the LT1009 datasheet circuit at page 5 (Low Temperature Coefficient Power Regulator)that will work much betterhttps://www.analog.com/media/en/technical-documentation/data-sheets/lt1009.pdfIt would still be affected by the variation of the LM317 reference... if I'm not wrong those are reduced just because the total reference becomes 2.5+1.25V and the 2.5 value is more stable...

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2021, 08:58:57 am
content: Quote from: satyamfifa on December 27, 2021, 04:25:23 amThe separate TL431 will not get very warm from the load current. A single regulator will see the power loss and thus extra temperature variations when the load changes. With a small constant load the effect my not be large, but it can be a big factor with a higher load.So the TLV1117 could see quite some extra temperature variations, not just the ones from the environment.The graphs in the DS are typical and thus often way better than the actual specs.Just for the feedback, there is no need to use so low resistors - it may be needed for a minimum load current, but I would expect less current needed. With the current for the TL431 from the output, this current can be part of the minimum load.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2021, 03:46:33 pm
content: Quote from: satyamfifa on December 27, 2021, 04:25:23 amWhen the error amplifier and reference are part of the same chip as the power output stage, then changes in load current or input voltage cause changes in heating which changes the temperature of the error amplifier and reference, which affect the regulation. This thermal feedback actually dominates the other error sources on an integrated regulator.By using the TL431 to control the 1117, this thermal feedback path is broken and load and line regulation is improved.The circuit shown below does the same thing but to a much greater level of precision. I liked using the LM329 6.9 volt buried zener as a reference but it is only suitable at higher voltages.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 27, 2021, 05:00:59 pm
content: Quote from: David Hess on December 27, 2021, 03:46:33 pmof course you're right, but I've not understood what is the real purpose of the circuit. If it is just to provide a stable power supply for an ADC using an internal reference, then IMO TL431 is still redundant, if the purpose is to provide both power and reference then it would still be better to separate that in 2 different sub-circuits

author: satyamfifa
author_rank: Regular Contributor
timestamp: December 27, 2021, 06:19:32 pm
content: Quote from: not1xor1 on December 27, 2021, 08:54:31 amYes you are indeed right about that temperature stability of TLV1117 is not 5000 ppm / deg C but 5000 ppm over whole range so it's about 40 ppm / deg C. I stand corrected on that one, thanks for pointing that out

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2021, 08:24:45 pm
content: Quote from: not1xor1 on December 27, 2021, 05:00:59 pmI thought the idea was to have a fault protected reference with greater output current capability. For better precision, a circuit more like my example would be needed.