Basic part question - Shunt regulator diode - Page 1
Forum section: electronics / beginners

author: pigtwo
author_rank: Regular Contributor
timestamp: May 31, 2016, 10:37:18 pm
content: Hello everyone,I have a pretty basic question about a shunt regulator diode. I'm looking at this part:http://www.diodes.com/_files/datasheets/TLV431.pdfWhat I'm confused about is what is the purpose of the reference pin. At first I thought this was like an adjustable zener diode(where reference would be the break down voltage) but that seems to be wrong. The data sheet says that ref sets the output voltage but the output voltage of what? None of the diagrams label anything as the output.I guess the simplest way to describe what I am missing is what does changing the voltage on the ref pin actually change?Thank you!

author: ebclr
author_rank: Super Contributor
timestamp: May 31, 2016, 10:48:48 pm
content: If part ia very comum part on switched power supply, it;s like a adjustable zener 1.24 and 18 volts by selection of two external dividerresistors

author: Seekonk
author_rank: Super Contributor
timestamp: June 01, 2016, 02:11:58 pm
content: It is called an amplified zener, a voltage reference with an op amp. The op amp drives a shunt transistor as a ratio of the input voltage divider. When driving a LED or relay these make great voltage monitors.

author: amspire
author_rank: Super Contributor
timestamp: June 01, 2016, 02:33:27 pm
content: Quote from: pigtwo on May 31, 2016, 10:37:18 pmThe reference pin is a low current (0.15uA) input that controls how much current the device conducts from the anode to ground. The way it works is this. If the voltage on the reference pin to ground is above 1.25V, it will turn the anode on. If it is below 1.25V, it will turn the anode off. In regulation mode, then reference input to ground is exactly 1.25V and this is achieved by controlling the anode to ground current to produce the 1.25V on the reference input.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 01, 2016, 02:37:40 pm
content: It's NOT a zener, or a diode, or even a voltage source.A much better way to think of these devices is:An op-amp, self powered, with open-collector output, and a large, precise input offset voltage.The REF pin is actually -in, and it's offset by 1.24V or so (2.5V for the regular TL431).VCC and output are the "cathode" pin. Unlike an op-amp which can source or sink current, this can only sink current.VEE and +in are the "anode" (common) pin.Another way to look at it is, a transistor with precision Vbe, very high beta (>> 1000), and low fT* (~1MHz).*Not precisely "fT", because of the high beta. Voltage gain, or gain-bandwidth product (GBW), would be more accurate.Now, as an op-amp, you do have to worry about feedback effects. Fortunately, it is unity-gain stable, so you don't have to worry too much in typical applications. Example: REF strapped to K gives a 1.24V reference. Because of the limited gain (especially at high frequencies), this reference will have a constant voltage characteristic at low frequencies, becoming inductive at higher frequencies (where the gain isn't high enough to stabilize the voltage against changes). As an inductance, it can resonate with a capacitance, so you mustn't connect an unstable amount of capacitance across the device. Which is familiar from regular op-amps: they don't usually appreciate capacitive loads.If you want a higher voltage, no problem, just connect a voltage divider from K to REF to A. This is precisely the same as a non-inverting amplifier circuit. The output impedance and noise level both rise proportionally, of course.If you use the device in a feedback loop, then you probably need to connect compensation from K to REF. Usually, an R+C is connected here. Then REF is connected to a voltage divider which senses the output, and K connects to the rest of the control circuit. TL(V)431s are frequently used in switching supplies for this reason. It's an error amplifier AND reference, all in one!Tim

author: pigtwo
author_rank: Regular Contributor
timestamp: June 01, 2016, 04:33:33 pm
content: Oh ok that makes more sense. Thank you for the responses and T3sl4co1l for the detailed response.I'm trying to understand a circuit with this component in it and I don't completely get it yet but I'm much closer now.Thank you.

author: danadak
author_rank: Super Contributor
timestamp: June 01, 2016, 07:18:45 pm
content: The datasheet, and attached snapshot, shows the load C stabilityconditions.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: June 01, 2016, 07:21:32 pm
content: A decent discussion of the part -http://www.homemade-circuits.com/2012/12/explaining-programmable-shunt-regulator.htmlRegards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 01, 2016, 11:44:14 pm
content: For stabilizing against heavy C-loads, compare the traditional methods for op-amps:It's the same thing, hence talking about TL431s as op-amps, which they are.For both cases, R3*C3 ~= R2*C1 for best results.Tim

author: danadak
author_rank: Super Contributor
timestamp: June 02, 2016, 09:46:06 am
content: A discussion of methods for comp and their effects -http://www.analog.com/library/analogDialogue/archives/31-2/appleng.html?doc=ADA4627-1_4637-1.pdfhttp://www.analog.com/library/analogdialogue/archives/38-06/capacitive_loading.html?doc=CN0343.pdfhttp://www.st.com/content/ccc/resource/technical/document/application_note/1b/e5/06/83/93/4c/4a/6c/CD00176008.pdf/files/CD00176008.pdf/jcr:content/translations/en.CD00176008.pdfhttp://electronicdesign.com/datasheet/whats-all-capacitive-loading-stuff-anyhow-pdf-downloadhttp://www.ti.com/lit/ug/tidu032c/tidu032c.pdfhttp://web.mit.edu/klund/www/papers/ACC04_opcomp.pdfhttp://cds.linear.com/docs/en/application-note/an148fa.pdfhttps://www.intersil.com/content/dam/Intersil/documents/an10/an1092.pdfNote these discuss some of the limitations of adding Rout for compensation, likeclosed loop BW, and inability to optimally comp with varying Cload.Regards, Dana.