Auto-Standby circuit doesnt work as expected  - Page 1
Forum section: electronics / projects

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 02, 2020, 02:11:00 pm
content: So this is an Auto - stby circuit for a 2.1 Ch Amp with 3x TPA3116. I copied this circuit from a H/K Sub so it works (yea somehow) So I assembled the pcb (without the TPAs) and tested the circuit, everything fine the Mosfet is switching off after ~15 mins. But now everything is assembled it doesnt switch off anymore.. the voltage at pin6 on IC9b is 7.7V @24V input and on Pin 5 it rises. So when the voltage at p5 is >7.7 it stays for like 0.1sec over 7.7 and gets back to like 7.6 and it starts over and over and i dont know what it is, the input Voltage is stable and im running out of ideas, HELP! (Audio signal would Come in on the 2 47k resistors, the voltage on the output at Pin 7 ic9 would stay low if signal applied)

author: OM222O
author_rank: Frequent Contributor
timestamp: August 02, 2020, 02:13:35 pm
content: maybe start by posting a schematic of the actual circuit?

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 02, 2020, 02:16:27 pm
content: I was just adding it

author: tooki
author_rank: Super Contributor
timestamp: August 02, 2020, 09:24:19 pm
content: Post the whole schematic, not just part of it — at least not without identifying the inputs, outputs, etc.(Not sure why you’d even bother posting a screenshot of a part of the schematic here, when in another thread you posted the entire schematic as a PDF!)

author: tooki
author_rank: Super Contributor
timestamp: August 02, 2020, 09:26:15 pm
content: You could also just use the TPA3126, which is pin compatible but has much lower quiescent power consumption, then dispense with the whole auto-power circuit.

author: tooki
author_rank: Super Contributor
timestamp: August 02, 2020, 09:30:30 pm
content: Also, on the full schematic, I see that you’ve got the shutdown pins permanently pulled high to VCC, and then you use the auto-standby circuit to disconnect VCC via a mosfet. But you should be sequencing power to mute and shutdown before/after VCC, so as to prevent popping. If you’re not actually using the auto-standby to disable the power supply (which you aren’t), it’d be smarter to leave VCC connected and instead control the shutdown pin. You’ll get pretty much the same power savings, but without the popping. Even without sequencing the mute pin it’d still be superior to just turning off VCC.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 02, 2020, 09:40:02 pm
content: I wanted to keep the focus on this section of the schematic thats whyOh I have no turn off pop, thats the op amp for on the mute Pin, and I wanted to stop the op's running too so thats why I did it that way. But yea it wont turn off as I wrote, the voltage on Pin 5 is just ~0.1sec high er than on Pin 6. Still have no clue why

author: tooki
author_rank: Super Contributor
timestamp: August 02, 2020, 09:45:58 pm
content: I figured that’s why you posted a section, but it’s missing critical information that make it more difficult to analyze, as one has to guess about key things like the signal inputs...

author: tooki
author_rank: Super Contributor
timestamp: August 02, 2020, 09:57:11 pm
content: FYI, I don’t know if it has anything to do with the auto standby problem, but I see you’ve got the mute pins connected to one of the TPA’s GVCC pins via a big 10μF capacitor — the same GVCC pin the data sheet expressly says to not connect ANYTHING to except for the 1μF bypass cap to ground, and the resistor dividers for configuration. (Datasheet page 4: “Not to be used as a supply or connected to any component other than a 1 μF X7R ceramic decoupling capacitor and the PLIMIT and GAIN/SLV resistor dividers.”)

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 06:11:24 am
content: It has nothing to Do with my problem, its a well known anti turn on pop methode for the 3116, there isnt much current flow so its ok but yea, I could connect the cap to VCC as well. U doesnt seem to have a clue why auto stby dont work either right?

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2020, 07:09:00 am
content: The timing network appears to be R71 2Meg charging C78 220uF. Its time constant is approximately 7 minutes. Its decreased a bit by the effective load of Q3 base, which can be approximated as a 30Meg resistor to ground (emitter_load*hFE) so it will only charge to 94% of the supply, less approx 0.7V Additionally you are comparing it with half the supply voltage so it *should* only delay for approx 5 3/4 minutes.The fact you observed a 15 minute delay means either the leakage current of C78 is a large proportion of its charging current, or the reset transistor Q2 is leaky or is getting partially turned on intermittently.The whole approach of an extremely large time constant RC network with an electrolytic cap is irretrievably flawed due to the unstable and temperature dependant leakage current. IMHO you need to redesign it with a digital timer, either an oscillator and divider chain, or a MCU.Consider using a CD4060 configured for a low frequency RC oscillator, with a diode from the desired output to the RS input to inhibit its oscillator at the end of the delay. Drive its MR (master reset) with your activity signal.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 08:18:56 am
content: But the circuit works?It just doesnt work when I soldered the ICs on the PCB cause on Pin 5 the voltage drobs as soon as its higher than on Pin 6 on IC9b. I know they r smarter ways to do it but I want to solve the problem not re design it. So the time delay thing is strange yea, but anyway it works (almost)What I found out so far:-when the voltage goes up and down (7.7-7.6V at Pin5 IC9B (measured to virtual ground) the voltage on the Base of Q3 doesnt change, so that means c78 is full charged, so it has something to do with the comparator itself?-when I remove R78 it works-when I remove D16/17 it worksIm sure im close but still cant see where the problem is

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2020, 08:34:34 am
content: What do you expect to happen when IC9B acting as a comparator causes the MOSFET Q4 to switch off the Vdd supply to the timing circuit feeding its own inputs? The circuit is fundamentally flawed and if it operates at all its by pure luck.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 09:14:53 am
content: Oh my fault! I forgot to switch the Power Symbol, the circuit is always powered ofc.

author: tooki
author_rank: Super Contributor
timestamp: August 03, 2020, 09:27:27 am
content: VDD is the unswitched always-on power, and VCC is the switched power, correct?(And no, I don’t know why it’s not working, sorry! But I’d listen to Ian.M.)

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 09:39:28 am
content: Correct. And i did put the Symbol on the drain of Q4 so I dont know what Ian.M means, the Timing circuit is always powered ofc.

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2020, 10:03:11 am
content: Quote from: Michaelaudio on August 03, 2020, 09:14:53 amQuote from: tooki on August 03, 2020, 09:27:27 amQuote from: Michaelaudio on August 03, 2020, 09:39:28 amThat's not what your schematic shows. If VDD was always on, theIFR5505 P-MOSFETwould be ineffective for switching VCC as its body diode would conduct when there was no gate drive.Ergo you haven't built it as drawn, don't understand it and our attempts to help you are as futile as pissing up a rope.As a reminder and to prevent O.P. being tempted towards revisionism, here's the relevant section of the schematic(annotated with my understanding of its connections).The only other points worthy of note is that the rail splitter providing GNDA is also fed from VDD.

author: tooki
author_rank: Super Contributor
timestamp: August 03, 2020, 10:08:10 am
content: As I gather from the full schematic from his other thread, Con7 is where the 24V is fed into the circuit, so it’s definitely not switched. But yes, you’re right, the way that mosfet is used, it won’t ever turn off VCC!

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 10:55:28 am
content: Okay yep! Q4 is wrong on the schematic.. S and D need to be switched, so on the pcb the Mosfet is connected right (S to power supply D to the Diodes) guess I should update it and post the right schematic.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 07:13:07 pm
content: Okay so I figured out, that the voltage drop after the voltage goes above 7.7V on Pin 5 is the Voltage of the hysteresis. If I replace R76 with a 100K the voltage drop is 1.2V with 1M its 0.1 so its obviously the hysteresis voltage.

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2020, 07:28:32 pm
content: What's the state of the pin 7 output after you observe this voltage drop? If the OPAMP is correctly wired as a comparator with hysteresis, you should get a step change of the pin 5 voltageupwards, as the rising input voltage crosses the reference level on pin 6.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 07:40:25 pm
content: So the Voltage at Pin 6 is 7.7V when the voltage at Pin5 is at lets say 7.71 Pin 7 is at ~23V (the LED blinks for like half a sec) after that the output is ~0.7V and the voltage on Pin 5 is 7.6V. When i take R78 out (Gate resistor) The voltage rises just fine and the output is constant. I tried a higher value for R78 but changed nothing. Really weird..

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2020, 08:04:09 pm
content: What happens if you disconnect R69 (base drive to the timing reset transistor Q2)?Where is IC9 getting its supply from?

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 03, 2020, 08:24:57 pm
content: Havent tried that yet.Thats getting the Power from VDD (directly from power supply)

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 04, 2020, 04:41:46 pm
content: Okay verry interesting things going on here. When I disconnect R69 while circuit powered up, Q4 opens! When I then connect it again, Q4 is closing (low on Pin 7 IC9b). When I turn psu off, wait 10 seconds (resistor disconnected) turn it on again Q4 still open. Verry interestingBtw I replaced R71 with a 100K to Charge te cap faster, shouldnt be a matter

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 05, 2020, 05:10:44 pm
content: So I found out that when I short R69 the Voltage on Pin 5 ic9 goes back to zero when its been over7.7V , so Q2 fully closed then. Same thing when I remove R70. When I remove R69 and look at Pin 1 ic9 with the scope when Pin 7 gets a high, Pin 1 is also getting also high for also short time(its pulsing a little bit)

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 06, 2020, 06:48:50 pm
content: So it seems no one can help my but anyway, when I remove R64 and R65 it works. Now I havent any more ideas (C1 and c15 arent the problem either)

author: Ian.M
author_rank: Super Contributor
timestamp: August 06, 2020, 07:39:29 pm
content: Am I correct that R64, R65 carry the signals going to the activity detector circuit round IC9A?If so, it would seem that cutting power to VCC causes a glitch affecting those signals, which causes a false activity detection so turns the power back on.You need to modify it to either lock out the activity detector for a few seconds after the timer cuts power, or to slow down the speed with which Q2 can reset the timer so it needs sustained activity, not just a btief glitch. Inserting a resistor in series with Q2 collector may do the job - I'd start by trying 1K.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 06, 2020, 08:08:52 pm
content: Yep.It actually doesnt turn off at all, the voltage of Pin 7 IC9 switsches so quickly that it doesnt turn off Q4. Looked at VCC while Pin 7 is pulsing - nothing to see, looked at the inputs (r64/R65 going straight to the RCA connector) same thing - nothing. And I dont see why a 1k to the collector would change something, there is already a 2M in series isnt it?

author: Renate
author_rank: Super Contributor
timestamp: August 06, 2020, 08:15:26 pm
content: I'm late to the party, but 2 M ohm and an electrolytic?You like living dangerously.A long analog RC delay is so Flintstones.Even back in the day, they would use an RC oscillator and count 16384 or more pulses for long delays.Quote from: Ian.M on August 03, 2020, 07:09:00 amA quick glance at the TPA3116 shows that there is no disable pin (or did I miss some multi-pin usage?)Use a chip that has both mute and disable.Intercepting power to the chip is a poor approach.(I just retrofitted my guitar amp. It has both mute and disable. I have an ATTiny85 running power sequencing.)

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 06, 2020, 08:42:55 pm
content: I know I know. But I already have the pcb and its working fine except for this problem, so I dont wanna make a new board and new schematic, i wanna fix the actual problem (spent way too many hours on this project to give up)

author: tooki
author_rank: Super Contributor
timestamp: August 06, 2020, 11:28:35 pm
content: Quote from: Renate on August 06, 2020, 08:15:26 pmYou missed it. There’s a mute pin and a shutdown (SDZ) pin. One of the op-amps in the full schematic (which he inexplicably refuses to post in this thread, despite posting it in another thread already) is part of another circuit (drawing power from an internal power rail despite the datasheet explicitly saying not to do this) to control the mute pin.

author: Renate
author_rank: Super Contributor
timestamp: August 07, 2020, 01:43:49 am
content: Quote from: tooki on August 06, 2020, 11:28:35 pmI did!If you look at the block diagram (which is what I did) the innocuous "SDZ" in the upper left corner doesn't really seem to go anywhere.Unrelated: Looking back at my notes, my guitar amp uses a TDA7296, 60 W, class AB.Interestingly it uses max 65 mA quiescent current, compared to the TDA3116 at 50 mA (which is class D).

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 07, 2020, 06:19:48 am
content: @tooki i wouldnt do it again but the cap on gvdd is charged with 70uA for like 3sec. So shouldnt be a problem. Everything works just fine except for the problem im trying to get help with.

author: Renate
author_rank: Super Contributor
timestamp: August 07, 2020, 12:39:27 pm
content: Why don't you remove the big P MOSFET, bypass it, invert the sense of your gate drive circuit and make it an open collector into the FAULTZ & SDZ junction.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 07, 2020, 01:01:49 pm
content: Bc I want to cut the Power for everything not just the tpa's

author: Renate
author_rank: Super Contributor
timestamp: August 07, 2020, 01:34:25 pm
content: Quote from: Michaelaudio on August 07, 2020, 01:01:49 pmAw, c'mon, how much current is the other stuff taking?What other stuff? If you power it down how can you tell when the audio goes live again?

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 08, 2020, 08:45:09 pm
content: As soon as a signal is applied to the inputs

author: Renate
author_rank: Super Contributor
timestamp: August 08, 2020, 09:16:59 pm
content: Trivia: This thing has a Bluetooth module, but you can't use it unless you feed the line inputs to deactivate your muting circuit.The TDA's you can turn off with the SDZ.Your detect circuit has to stay powered all the time.You are only saving the power consumption on those middle opamps if you switch power.That's only 1.4 mA/op amp.So you really want to brute force switch the power to that middle section?Without using a reliable shutdown/mute timing?

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 09, 2020, 12:19:24 pm
content: I realized that too, so I will make that it doesnt turn off when bt is deleted.I want it to work yes, I already changed connector nsfw on the pcb, spent hours of work just to get rid of some Problems (and 3 weeks to make schematic and layout) and now that everything works beside the stby circuit i want it to work no matter what, so yea I dont wanna redesign it or make it work another way than expected. I guess thats understandable.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: August 15, 2020, 04:15:44 pm
content: I found it out. It was the switching ic! (SN3257-Q1/TSSOP)Does anyone know if a pin compatible switching ic exist?

author: tooki
author_rank: Super Contributor
timestamp: August 15, 2020, 09:34:41 pm
content: Quote from: Michaelaudio on August 09, 2020, 12:19:24 pmmmmm.... no. If I discover my approach was wrong, I’d rather scrap it and start again with the right approach that will work well. Anything else is putting lipstick on a pig.

author: Ian.M
author_rank: Super Contributor
timestamp: August 15, 2020, 09:58:21 pm
content: Quote from: tooki on August 15, 2020, 09:34:41 pmThere's a rather more earthy phrase that comes to mind: 'gilding a turd'. Mind you, if you are willing to spend enough money that can literally be done. First freeze dry it, next resin impregnate it and fully cure, then plate it with electroless copper to build up a conductive layer, and follow on by electrolytic copper to the desired thickness. Finally send it out for the ENIG plating process we are all familiar with from PCB production. At the end of the day, however gilded, its still a turd, useless for anything except a scatological paperweight or amodern art exhibit.