paralleling tl431 - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: December 08, 2024, 09:22:59 pm
content: Hello my friends!I want to make a bipolar supply from unipolar, with quite high ground current, up to 250mA. I was thinking to use TL431 for that, from onsemi. Its voltage drop of 2.5V is perfect for the purpose of providing voltage headroom for opamps in my upcoming power supply (yep, yet another one).The problem is, tl431 cannot sink that much current alone. I could boost it with a BJT, but then I'll have to increase voltage, because there will be 2.5V drop on TL431 plus 0.7-0.8V Vbe voltage. So, minimum output would be like 3.2V or something.I could use a low-voltage version of tl431 that works down to 1.25V. That would do it, but those tend to be much more nosier. Like, four times noiser.So, my questions are:- can I just parallel three-four tl431 to get some 300mA total capacity? Also, perhaps, lower noise.PS I'm not sure if I really need low noise power rails. I'm trying to get the most from a 24bit ADC, trying to measure the voltage across the shunt. I'd like to get measurements below 100uV, if possible. Just for fun. But when calculating noise, I'm also not sure what's my frequency of interest. Is it 0.1-10Hz? Is it 1kHz? Not sure.

author: TimFox
author_rank: Super Contributor
timestamp: December 08, 2024, 09:31:59 pm
content: Maybe you can add a PNP bipolar by returning the third electrode ("Vref") to the emitter of the PNP rather than to the + terminal of the TL431, and get a shunt circuit with 2.5 V output.That's similar to using a voltage divider to drive Vref to obtain > 2.5 V.The + terminal of the TL431 would go to the base of the PNP (with a resistor in parallel with B-E) and the collector of the PNP to the - terminal of the TL431.I don't know if the TL431 would be happy with the Vref pin positive with respect to the + terminal; the data sheet doesn't mention that question.

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2024, 10:56:43 pm
content: Quote from: TimFox on December 08, 2024, 09:31:59 pmIt might not work because the supply voltage to the TL431 will only be 1.7V. Yes, I know the SPICE simulation looks good, but that doesn't necessarily reflect real life.It stands more of a chance with the TLV431. Refer to figure 21 on page 10 of the data sheet.https://www.onsemi.com/pdf/datasheet/tlv431a-d.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: December 08, 2024, 10:59:50 pm
content: If really needed one would use a NPN and PNP combination of 2 emitter followers in series to get an approximate compensation if the BE drops.

author: fzabkar
author_rank: Super Contributor
timestamp: December 09, 2024, 12:06:13 am
content: "Figure 1. Representative Block Diagram" on page 1 of the datasheet shows a forward biased diode from R to K. This would be in parallel with R1 and the B-E junction of the pass transistor (Q1).

author: magic
author_rank: Super Contributor
timestamp: December 09, 2024, 08:22:52 am
content: Exactly.https://www.eevblog.com/forum/projects/trap-for-young-players-tl431-reference-input-current-and-how-to-cook-your-tl431/Paralleling also won't work great, because one with the lowest reference voltage will turn on first and take all the current it can before others turn on.

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2024, 09:30:54 am
content: Good catch regarding the diode. I added one to the model and as exepcted, 80mA flows through the adj input.A Schottky diode can be used to increase the supply voltage to the TL431 so the internal diode doesn't conduct much.Note that the BC327 used in the examples I've posted isn't rated to 300mA 2.5V continuously. Use a higher power rating transistor, if this is to be used for more than short surges.

author: exe
author_rank: Unknown
timestamp: December 09, 2024, 09:39:42 am
content: Thanks everyone for responses!Yeah, I figured paralleling won't work well without balancing resistors. With 0.2 Ohms impedance and, say, 50mA per device, the voltage drop will be 10mV, which is much less than Vref spread between devices (according to datasheet).I another good option could be a discrete regulator from a voltage reference, opamp (sot23) and bjt. It's "just" three active devices, not that different from, say, four parallel TL431. Can be also potentially less noisy*, though I'm struggling to find a low-noise opamp in sot23 package (NJM8530 maybe? 10nV broadband noise density with up to 15nV at 1Hz).*Requires heavy filtering of reference voltage.

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2024, 12:36:00 pm
content: This sounds like an X Y problem.What are you trying to do?The TL431 isn't an especially low noise part.2.5V at 300mA implies it's for use as a power supply. Why not use an ordinary linear regulator (go LDO, if the input voltage is close to the output)?

author: MrAl
author_rank: Super Contributor
timestamp: December 09, 2024, 12:55:47 pm
content: Quote from: exe on December 08, 2024, 09:22:59 pmHi,Yeah putting devices in parallel often does not work very well, you have to take precautions. Lots of things can plague devices that are paralleled like thermal gradients which vary the parameters of each device differently. The balancing resistors have to take all that into account as well as with the initial setup.

author: xvr
author_rank: Frequent Contributor
timestamp: December 09, 2024, 01:18:14 pm
content: Quotetl431 is a shunt regulator. So load current and tl431 current is a two different things. You can sink 1A to load and 10mA to tl431 and it will be fine (as long as load current will stay at 1A). So main question is - what range of load current expected? Absolute value is not important.

author: exe
author_rank: Unknown
timestamp: December 09, 2024, 02:33:40 pm
content: Quote from: Zero999 on December 09, 2024, 12:36:00 pmI'm trying to make an asymmetrical rail splitter. I'd like ground to be some 2.5V above negative rail. I need to sink current into ground. Normal LDOs cannot sink current. I also tried to to to use AMS1117-3.3 in "shunt" topolgy, but that didn't work for some reason (https://www.edn.com/classic-3-leg-adjustable-regulators-have-a-shunt-mode-who-knew/).

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2024, 10:45:25 pm
content: Asymmetrical in what respect? Different voltages, sink and source current capacity?Start from the beginning. What voltages and currents do you require from the power supply?

author: exe
author_rank: Unknown
timestamp: December 10, 2024, 08:05:23 pm
content: Say, I want to convert +18V into +15.5V and -2.5V. Current is 250mA max. How would I do that?

author: magic
author_rank: Super Contributor
timestamp: December 10, 2024, 08:13:02 pm
content: For something ready to use (but still needing a capacitor for stability), maybe LM337 connected to a 3.75V reference.The "power transistor controlled by TL431" option could still work, but requires level shift - NPN emitter follower or such, plus some thermal and stability testing.

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2024, 11:04:23 pm
content: Quote from: exe on December 10, 2024, 08:05:23 pmIs that the ground current?The boosted zener circuit will only work if the the current from +V is higher than the current into -V.A potential divider + booster amplifier can be used to split 18V into +15.5V and -2.5V. The -2.5V is the tricky bit, since it involves finding an amplifier which can sink 250mA and saturate at under 2.5V with respect to its negative supply.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 11, 2024, 05:58:46 pm
content: TI datasheets show that the TL431 and the TLVH version have ~4uV p-p of noise, that's pretty quiet.Have you measured the noise in a real build, and how?Why cant the noise, such as it is, be bypassed?

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2024, 11:04:46 pm
content: I was thinking about something like this.

author: MrAl
author_rank: Super Contributor
timestamp: December 12, 2024, 11:50:21 am
content: Quote from: Terry Bites on December 11, 2024, 05:58:46 pmNoise can often be filtered out, but wideband noise is hard to filter because there's no set frequency it's all over the place, including low frequencies which are hard to filter out, and I don't think there is any limit to how low the frequency can go.

author: exe
author_rank: Unknown
timestamp: December 16, 2024, 12:19:55 pm
content: Quote from: Zero999 on December 10, 2024, 11:04:23 pmYes, ground current. I need -2.5V for opamps. Although I'm questioning myself, may be capacitor inverter would do the job too. Just worried about intrinsic noise.I like your circuit. Also the approach with shifting voltage with a bjt.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2024, 10:48:52 am
content: Quote from: exe on December 16, 2024, 12:19:55 pm250mA is massive for a charge pump.What does the amplifier circuit you're powering it with, do?