TL431 and ceramic caps - Page 1
Forum section: electronics / beginners

author: ledtester
author_rank: Super Contributor
timestamp: September 27, 2020, 08:02:52 pm
content: I'm curious why I'm seeing this behavior.I built up a simple 2.5V supply using a AS431 (a TL431 clone) like in the datasheet:Here R1 is 0R and R2 is infinite (not present). Rsup is 440 ohms. Vsup is 8V.When I try using a 100nF ceramic for the load capacitor I see around a 40mV drop on the output (from 2.49V to 2.45V). I don't see that drop with a 0.22uF electrolytic or a 47nF disc capacitor.I'm wondering what's causing the drop and if it's just my caps or if it is something to be expected.Pics of the "bad" ceramics:

author: exe
author_rank: Unknown
timestamp: September 27, 2020, 08:24:51 pm
content: Did you readhttps://www.ti.com/lit/an/slva482a/slva482a.pdf"Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet"?Also, in my experience, in such cases an oscilloscope is much more useful than a dmm. What appears as drop can well be an oscillation on output. Sometimes measuring voltage in AC can help catching that, but not all dmms have enough bandwidth to do that.edit: formatting

author: floobydust
author_rank: Super Contributor
timestamp: September 27, 2020, 09:24:01 pm
content: Also be aware the stability region varies a lot between manufacturers of the "TL431".

author: krzysssztof
author_rank: Contributor
timestamp: September 27, 2020, 09:58:26 pm
content: TL431 is very nice and cheap device, but you also need to be careful when using it.As mentioned above, take special care about stability conditions - verify it with the datasheet from specific manafucturer and additionally verify in your circuit.

author: exe
author_rank: Unknown
timestamp: September 28, 2020, 07:44:55 am
content: Thanks for plots! It's very interesting to see how the devices becomes more and more stable as we apply less and less feedback. The least stable configuration is "unity gain". It gets more stable with increased output current. I think that's because output impedance gets lower as output transistors are "more open". Such characteristics are typical to opamp circuits with negative feedback.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 03, 2020, 10:26:37 pm
content: The minimum operating current TL431 is 1mA. You are very close to this minimum. Reduce the Rsup.

author: newbrain
author_rank: Super Contributor
timestamp: October 04, 2020, 08:02:56 am
content: Quote from: S. Petrukhin on October 03, 2020, 10:26:37 pmYou might have lost a 0 somewhere...With 8 V supply and 440 Ω series resistor we have:I = (8 - 2.5) / 440 = 5.5 / 440 = 12.5 mAWhich should be plenty.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 04, 2020, 08:18:28 am
content: Quote from: newbrain on October 04, 2020, 08:02:56 amSorry, I made a mistake. The calculation in my mind has failed, I need to use a calculator.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 05, 2020, 01:53:44 am
content: A ridiculously (or deliciously) detailed work on SMPS stability, and TL431 (and other stuff).There is a lot of focus on loop stability with the very common combinatiion of TL431 and feedback through an optocoupler.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2020, 02:00:06 am
content: This is the usual way to stabilize it:Note that, at high frequencies, REF and K are tied, so it has the dynamics of unity gain (2.5V setting) with no bypass. The series resistors act in parallel to it, so as long as the resistors are high enough (10s of ohms is enough for most types?), this allows any C load. The exact values of R and C depend on the stability of the device, and the load C; typically the load RC time constant should match the feedback RC time constant.Tim