LMV431 acting strange at power on - Page 1
Forum section: electronics / projects

author: Mad ID
author_rank: Regular Contributor
timestamp: March 06, 2015, 08:17:09 am
content: Hi, I'm using an LMV431 (similar to TL431) shunt regulator as a precision switch and get something I cannot explain.Here is the circuit:And the waveformInput voltage is a step from 0 - 3.7V (shown in yellow). I'm interested in the blue voltage at the cathode.Why is the cathode voltage lagging the yellow voltage? I know that in DC state the LMV431 is consuming some current and there is 220mV difference, but I don't undestand why I have1.5V differenceduring the transision?That means the LMV431 is in ON state for a short time, but why? Turn on voltage is set to 4.5V via 27k & 10k resistors.EDIT: My only guess is that it's somehow related to the bandwidht of the signal, LMV431 is not fast enough...Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2015, 08:39:47 am
content: It is neither a switch nor a zener, it is an op-amp with finite gain-bandwidth and a large but precise input offset voltage. And an op-amp*, in turn, is best modeled as an integrator.*Specifically, the conventional, dominant-pole compensated, voltage type op-amp.So, it simply takes a long time for it to "make up its mind", the price you pay for having a (reasonably) accurate answer to the question "is input greater than 1.240V?", given the limitations of the device (size, cost, power consumption..).The exact nature of that bump or "hiccup" that you observe will depend on the frequency response and startup characteristics of the internal circuitry.By the way... uh... how in the hell are you getting a noninverting response? Because your circuit and your waveform are NOT consistent...Note, also, that because it is self-powered (either by the input or output terminals), using it for "switching" operation is generally dubious: saturation voltage is high and "off state" leakage is also high.Tim

author: Mad ID
author_rank: Regular Contributor
timestamp: March 06, 2015, 08:52:54 am
content: Quote from: T3sl4co1l on March 06, 2015, 08:39:47 amHi Tim,in my application a PFET is connected between the yellow and blue waveforms (gate at the blue chathode). Why would it invert? Let's say the LMV431 is working as I expected without the bump, the cathode voltage should follow the yellow voltage up to the point of reference equals 1.24V. At that point the cathode will go as you say to, as you say, high saturation voltage of approx 2V.Leakage is high but small enough not to turn on the PFET (gate source voltage).So basically the answer to my question is that LMV is not fast enough "to make a decision" when voltage is switching. I've tryied to put a 100n from reference to ground, it's somewhat smoother but still lagging.

author: eneuro
author_rank: Super Contributor
timestamp: March 06, 2015, 03:59:34 pm
content: Quote from: Mad ID on March 06, 2015, 08:17:09 amLook at page 6 figure 8cathode current vs cathode voltagein TL413I pdf below- it helps in many cases figure out many "issues" with this configurable Zener diodehttp://pdf.datasheetcatalog.com/datasheet/philips/TL431I.pdfBasic trap is thisImin(like in other Zener's too) marked with an arrow on figure 8 plot.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 06, 2015, 04:17:18 pm
content: Hmm .. I've always connected the R30 resistor between cathode and feedback. Connecting it to to positive supply??Furthermore, minimum current for a TL431 to operate is around 1mA. The R31 10k resistor seems a bit high value?

author: c4757p
author_rank: Super Contributor
timestamp: March 06, 2015, 04:37:44 pm
content: Feedback is connected wrong. Perhaps you're seeing a bit of phase inversion? TL431 can operate as a comparator but perhaps LMV431 cannot.Quote from: JohnnyBerg on March 06, 2015, 04:17:18 pmLMV431 is a micropower variant. 55µA min.

author: Mad ID
author_rank: Regular Contributor
timestamp: March 06, 2015, 04:59:11 pm
content: Quote from: c4757p on March 06, 2015, 04:37:44 pmHmm not so sure. As you say, I want it to work as a comparator, not a regulator.I have one application note where the LMV431 is working as a comparator. (AN-1533 Over Voltage ProtectionCircuit for Automotive Load Dump)The circuit works fine except at power up. If I drive a PFET between line voltage and cathode it will get turned on during this transition.

author: eneuro
author_rank: Super Contributor
timestamp: March 06, 2015, 06:43:26 pm
content: What input voltage level is in your circuit when this strange response happends?Made sometime ago battery overvoltage protection based on this circuit from this pagehttp://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htmLMV431 is slightly different, so will try it while it looks like it can be perfect for supercapacitors overvoltage protection where vmax is 2.5V and TL431 with typical 2.495V not too good, so LMV431 with 1.24V-30V looks very interesting and it might work very well while it has additionally much lowerImin50uA

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2015, 01:24:13 am
content: Can you please show your complete circuit, including all parts numbers / values?

author: Mad ID
author_rank: Regular Contributor
timestamp: March 07, 2015, 09:55:32 am
content: Quote from: T3sl4co1l on March 07, 2015, 01:24:13 amSure.http://tinypic.com/view.php?pic=2e32yqh&s=8#.VPrJ2PlzQeEI needed 1-Wire bus for communication and power, both bidirectional. So I can communicate with the other side, send power and charge my battery if the other side has power.+VCHGis connected to a buck regulator and then Li-Ion battery charger. The other side must provide 4.5V for the charging to start. That's way I have LMV431 as a comparator, I don't want the buck running from my own power when the bus is high.If you have a batter idea how to solve this please sayAs you can see, if my cathode voltage does not follow the bus voltage, the PFET will turn ON for a brief time on every rising edge during communication.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 07, 2015, 10:56:16 am
content: for clarity:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2015, 07:18:35 pm
content: I don't think much of that circuit is doing what you are intending of it...Can you describe the desired operation of the bus in detail?Tim

author: Mad ID
author_rank: Regular Contributor
timestamp: March 09, 2015, 04:07:20 pm
content: Quote from: T3sl4co1l on March 07, 2015, 07:18:35 pmIt must be able to:1) Communicate with another self-powered 1-Wire unit (this is not Dallas 1-wire!). Transistor Q4 is for Tx, Q1 is to disable pull-up if bus is shorted for a long time (to prevent battery drain). Q3 with MCU's internal pull-up is for level translation. This part works OK, not much things which can go wrong.2) It must be able to supply 0.5A of power to another unit. Overcurrent/short protection was needed so I have a current limiting power switch (FPF2124) which turn-off if fault occurs. This is controlled with software via 1-Wire-Power. Q4 will never turn ON when FPF is active.3) It must charge it's internal Li-Ion battery if the other unit applies 5-20V (this will first be communicated). Diode D1 will decouple the circuit in this case. +VCHG goes to the charger on another schematic. Now, here my problem starts. The reason I have Q5 is because I don't want my charger being connected during communication (very complex circuit, can take DC or alternator voltage, has a lot of capacitors) so that's way I want to turn-on Q5 only if bus voltage is higher than 4.5V. Please take note +VBAT is from Li-Ion battery with 4.2V maximum (Q5 should be off during communication)Unfortunately Q5 turn-on during every bus rise time (Q4 turning off) and that was my question. If you have any advice how you would implement this circuit differently I will appreciate.Cheers.

author: free_electron
author_rank: Super Contributor
timestamp: March 09, 2015, 04:41:24 pm
content: how will a logic zero (q4 ( in conduction )) travel through d1 ?

author: Mad ID
author_rank: Regular Contributor
timestamp: March 09, 2015, 04:52:52 pm
content: Quote from: free_electron on March 09, 2015, 04:41:24 pmThe other side has a weak pull-down. Communication is slow (baud 9600) and signals look good.I know this is all bad practice, but the requirement is tough...bidirectional power and communication over 1 line + ground..how how?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2015, 11:23:07 pm
content: Personally, I'd use an AC coupled duplexer (something like POTS telephone, but with voice modulated at RF instead of baseband), but that's just me. No, it won't be all that simple, though if you want to use crude DSP techniques to generate and detect the signal, it wouldn't be too bad on hardware.I think the hardest part about such a system would be isolating switching noise from the various converters necessary to use the power. But with judicious choice of operating frequencies and filter components, I don't think that would be too bad either.Tim