Negative voltage charge pump in audio circuit: how to remove background noise ? - Page 1
Forum section: electronics / projects

author: microvia
author_rank: Regular Contributor
timestamp: December 16, 2024, 05:55:11 pm
content: Hi,I designed an audio circuit containing a charge pump generating up to -10V for driving an optional circuit which hardly draws over 1µA when properly used.The charge is a simple chain of BAT54 shottky diodes driven by a +3V square wave of variable frequency using the existing MPU of the initial design.The charge pump generates a low audible background noise, which I would like to suppress.Can ferrite beads do the trick, for example by inserting one between the main ground and all gound points of the charge pump, one at the charge pump input, one at its output ? If so, what types should be used to remove audible background noise ?The final pcb has 4 layers, with a few tiny tracks of the charge pump passing through internal layers (I minimized them already but can't suppress them all).If ferrite beads can't be used, what other methods are effective ?Are there low-noise ICs which can replace the charge pump and generate the required variable negative voltage without background noise ? I can't use anything expensive such as MAX, DACs and the like. However I can easily adapt the MPU software (for example if the IC is driven in I2C).Thanks in advance.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: December 16, 2024, 07:03:51 pm
content: Are the ICL7660 or MAX232 really out of your budget?Also, please share a bit more of your circuit (schematic and real life photos if possible) so we can help you better.

author: PGPG
author_rank: Frequent Contributor
timestamp: December 16, 2024, 07:05:49 pm
content: Quote from: microvia on December 16, 2024, 05:55:11 pmI don't design audio circuits.I have never written even a led blinking microcontroller program.If this frequency is high enough to be over audible range than may be what you hear are its variations.I think the key is to make this frequency being stable and not variable.If you have UART you can generate stable frequency by continuously sending 0x55 (I believe bytes are send low bit first).Interrupt telling you that the byte was moved into sending buffer gives you enough time to insert next byte allowing UART to send byte after byte without any interrupts.Quote from: microvia on December 16, 2024, 05:55:11 pmFerrite beds are used to suppress frequencies in the range 10MHz...1GHz and higher and not audible.

author: microvia
author_rank: Regular Contributor
timestamp: December 16, 2024, 08:10:21 pm
content: Quote from: pardo-bsso on December 16, 2024, 07:03:51 pmI don't see how the ICL7660 can easily produce a variable negative voltage from an MPU output.The schematic below shows the basic charge pump schematic.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 16, 2024, 09:02:09 pm
content: Quote from: microvia on December 16, 2024, 05:55:11 pmWhat is the exact load here ?If it is always low uA averaged, you could switch to a photo voltaic generator(s) like TLP3906 etc.Zero switching noise from those, as they are a string of photocells with a LED driver.

author: John B
author_rank: Frequent Contributor
timestamp: December 16, 2024, 09:03:28 pm
content: I've had success using RLC filters on both the input and output of a 7660, but they're designed to invert an input voltage so that the output is roughly -1*VinMaybe you'd have better luck with a different topology rather than a switched capacitor. Maybe some LLC switchmode coverters, like Cuk converters?

author: Smile2016
author_rank: Regular Contributor
timestamp: December 16, 2024, 09:21:09 pm
content: Try increasing the frequency to above 500kHz. Remove the last diode. Use PWM instead of "variable frequency". RLC filters on the output might help. ground connection is key...

author: tszaboo
author_rank: Super Contributor
timestamp: December 16, 2024, 10:32:09 pm
content: Quote from: John B on December 16, 2024, 09:03:28 pmWhat's the point of the 7660 if you place an inductor to filter it? Might as well use a regular inductor based DC-DC at that point.

author: moffy
author_rank: Super Contributor
timestamp: December 16, 2024, 11:49:27 pm
content: Slow down the edges of the square wave, slower edges less noise.

author: John B
author_rank: Frequent Contributor
timestamp: December 17, 2024, 02:09:26 am
content: Charge pumps are usually more efficient compared to buck boost topologies, which is useful when creating any battery powered audio device.As mentioned in this case, the charge pump won't create a regulated output without additional circuitry (there's examples in the app notes for the 7660), and it can't create a voltage greater in magnitude than -Vin. At least not without cascading multiple of them to make voltage doublers etc, which I think is a bit of a waste of space/parts at that point.

author: jonpaul
author_rank: Super Contributor
timestamp: December 17, 2024, 02:32:18 am
content: "charge pump" is a classic Crockofft -Walton multiplier originally used in HV particle accelerators back to 1930s.Sould not create audio noise, but add a lytic or mono cap on output.Check diode reverse recover timej

author: microvia
author_rank: Regular Contributor
timestamp: December 17, 2024, 07:50:36 am
content: Thanks for your suggestions.Increasing the frequency is a real trouble because even if I lower the capacitor values down to 1nF for example the frequency range doesn't change much if I want to have the same output levels.I will try replacing the square wave by a PWM and see if I can play with the duty cycle to get the same result without noise.I'm also wondering if the pcb layout is ok. I initially thought I could contour the charge pump to suppress residual noise, but it may have the opposite behavior. The 4 layers are Signal / Vcc / Signal / Ground.

author: moffy
author_rank: Super Contributor
timestamp: December 17, 2024, 08:35:18 am
content: You could try putting a resistor in series with the square wave, it will have have similar effect to slowing the edges, and adjust the value to reduce noise, a no cost experiment.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2024, 09:07:26 am
content: The charge pump chip like ICL7660 are notorious for EMI from switching pulese. So these chips would more make things worse.A series resistor at the drive side is a good idea. The layout can be a factor too, but the picture is missing a lot - no idea where the parts are and parts seem to be missing.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 17, 2024, 09:31:11 am
content: Things like ferrite beads, or resistors in series with your square wave, might well be effective at MHz frequencies but will do nothing at all at audio. Same goes for your PCB layout, which I'm sure is fine. What you're hearing really isn't the same problem at all.What's the frequency of your square wave? How does it vary? Why does it vary?If it's varying at a speed that's within the audio range, then there's your problem - after all, you've literally built a type of audio DAC in the form of a frequency-to-voltage converter.Try driving it from a fixed square wave, generated by a hardware timer and not modulated in any way. See if the noise is still there. If not, you know it's nothing to do with recovery times, layout, ferrite beads, or anything else that might influence *rf* noise.

author: hanakp
author_rank: Regular Contributor
timestamp: December 17, 2024, 09:36:18 am
content: I agree with @Kleinstein, most of these charge pump IC are unsuitable for audio, because they run at too low frequencies. Two years go, I've tried LT1054 in a headphone preamp, but no matter what I did (LC filters, shielding box), it injected noise into the audio band. LT1054 nominally runs at 25 kHz, but increasing it with an external cap didn't help much. And pumps that run at higher frequencies (like LTC1983) usually can't handle voltages over 5 V.@PCB.Wiz thanks for TLP3906 tip, that may come in handy sometime.

author: microvia
author_rank: Regular Contributor
timestamp: December 17, 2024, 12:03:21 pm
content: The square wave frequency determines the output voltage and varies between 200 and 20,000 Hz. It's generated by a timer of the MPU.This frequency is what I hear, even though it is a low background noise. There was no such background noise before adding this charge pump.

author: PGPG
author_rank: Frequent Contributor
timestamp: December 17, 2024, 12:34:34 pm
content: Quote from: microvia on December 17, 2024, 12:03:21 pmFrom your first post it was not clear that your frequency varies intentionally. It looked that you just can't get it being stable.It was also not clear that you use charge pump to get regulated voltage (up to -10V only informs that it is the value you get).Charge pump is not good choice to get other voltage ratio than x2, x3 and so on (subtracting diode voltage drop if they are used). When it works close to these ratios current pulses are small as only replenish the small charge that was lost between impulses. If you drive it with too low frequency than charged to much higher voltage capacitor is connected to discharged one and high current pulse is going between them generating noise. Output voltage also gets high ripple as average output voltage is get by charging capacitor to specific max voltage and waiting while it is discharged by load. If the charged voltage is for example 1V and expected average is 0.1V than capacitor is probably charged to 1V thane discharged to 0V and then stays at 0 for long time to get average of this pulse being 0.1V. I assume such pulses are not what you wanted to get.You can use charge pump (supplied with constant frequency) to get negative supply for circuits that then produce in analog way the voltage in a 0..-10V range.

author: tszaboo
author_rank: Super Contributor
timestamp: December 17, 2024, 01:35:04 pm
content: Quote from: John B on December 17, 2024, 02:09:26 amYou have it kind of backwards. Inductor based converters are very efficient across a wider frequency range, and wider current range. Charge pumps are only efficient when they are operated in the audio range, and that's not very good. Not only it's harder to filter them, with larger value components required, they are also more noisy in general.

author: microvia
author_rank: Regular Contributor
timestamp: December 17, 2024, 02:25:17 pm
content: Quote from: PGPG on December 17, 2024, 12:34:34 pmOnce set the frequency doesn't change.I only set it to obtain a specific voltage between -10V and 0V.It actually works very well with the target circuit, only the background noise is the issue.

author: mtwieg
author_rank: Frequent Contributor
timestamp: December 17, 2024, 02:27:13 pm
content: I think most of us missed the fact that the OP wants avariablenegative DC voltage. Hence why they are using variable frequency from the MPU to adjust the voltage (duty cycle isn't so effective in such a case).IMO if you need a predictable output voltage, the charge pump approach isn't viable (too dependent on diode characteristics not under control) without some sort of feedback loop.I would second PCB.Wiz's suggestion of using a photovoltaic isolator to generate the negative voltage, as it requires no switching circuits at all.One could use one or more PVI in series to generate a fixed negative supply which would then provide the supply rail for an inverting opamp circuit (with a very low Iq opamp), which would provide your variable negative vout based on a variable positive vin (from the MPU).Or you could get fancier and actually use the PVI(s) as part of the feedback loop (adjusting the LED current based on detected output voltage). Would likely be tricky to stabilize though.

author: microvia
author_rank: Regular Contributor
timestamp: December 17, 2024, 03:49:38 pm
content: Quote from: tszaboo on December 17, 2024, 01:35:04 pmI'd be happy to put a better circuit on the existing pcb as long as it doesn't cost significantly more, but space is quite limited. Unless something can fit in the layout I posted above.

author: tszaboo
author_rank: Super Contributor
timestamp: December 17, 2024, 04:11:45 pm
content: Quote from: mtwieg on December 17, 2024, 02:27:13 pmIf you have a noisy supply for your PV, you get a noisy supply for your isolated supply. So I'm somewhat perplexed what it would actually solve. Plus the efficiency would be like 1% or something close to that. Negative supply is a solved engineering problem. You use a switched capacitor for low effort. Negative output DC-DC or a buck converter with grounded output for higher currents. If you need low noise, then you post regulate, with a regulator that has meaningful PSRR in the switching frequency and the required harmonics.Quote from: microvia on December 17, 2024, 03:49:38 pmAs I understand your current circuit doesn't fulfill the design requirements. I don't advocate throwing out entire circuits, but if it doesn't work well, there is only so many things you can do. I would try to hack in a negative LDO if you really don't want to change too much.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2024, 04:18:19 pm
content: A series resistor could make the charge pump a bit more sensitive to the PWM ratio and not just the frequency. It would already reduce the current peaks too.

author: mtwieg
author_rank: Frequent Contributor
timestamp: December 17, 2024, 05:18:29 pm
content: Quote from: tszaboo on December 17, 2024, 04:11:45 pmI'm operating under the assumption that the charge pump itself is the source of the audible noise (not sure this has been verified though). In that case, removing the switching circuitry eliminates the OP's problem.QuoteAFAIK efficiency is not a concern of the OP.QuoteAgreed, and there are many different solutions. It's likely that the charge pump could be modified to resolve the issue, but it's not clear how the noise is contaminating the audio circuit (if at all). There's no guarantee that adding an LDO would help either. One way around this is to not use a switching DCDC at all.

author: Ian.M
author_rank: Super Contributor
timestamp: December 17, 2024, 05:39:36 pm
content: Quote from: microvia on December 16, 2024, 05:55:11 pmI think we need more info about the other tasks that MCU performs. The charge pump is a large capacitive load (10 nF) on its GPIO pin, which as it switches will draw large current spikes which will inevitably pollute the MCU's internal Vcc and Gnd rails, which will in turn pollute the logic levels on other output pins with pairs of positive and negative going spikes at the switching frequency. If any of those other pins directly couple into the audio path, the resulting noise will be very difficult to suppress.IMHO the first thing to try would be to drive the charge pump from a well decoupled single gate buffer, with a resistor (try 10 ohms) feeding it from logic Vcc. It probably should also have a 1K resistor between it and the GPIO driving it, as I expect a fair bit of ground bounce.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 18, 2024, 10:11:57 am
content: If possible use less stages from higher input voltage rather than straight from MCU.Switch above audio frequency.Limit current peaks as previously mentioned.Pay special attention to layout, any ground shared between charge pump and audio will be a problem. This also applies if connecting your option that requires -10V creates a ground loop. (Switching current peaks will cause voltage variations on shared ground lines, consider star grounding.)

author: mtwieg
author_rank: Frequent Contributor
timestamp: December 18, 2024, 02:22:36 pm
content: QuoteFine suggestion. I would suggest doing some simpler tests first:1. Disable the charge pump entirely and provide the negative voltage from a low noise DC bench supply. If the noise issue persists, then the charge pump isn't the issue. Otherwise goto 2...2. Enable the charge pump, but with its output still disconnected from the rest of the circuitry (but give the charge pump a dummy load). If the noise does not come back, then it was likely coupled through conduction on the output, and you could resolve it with a simple RC or LC filter on the output. If the noise does come back, then goto 3...3. It must be coupling either through conduction in a supply rail (or gnd), or via EM fields (not likely at audio frequencies). You should experiment with other methods of driving the charge pump like what Ian suggested above.

author: microvia
author_rank: Regular Contributor
timestamp: December 18, 2024, 07:52:21 pm
content: Thanks to all of you for your interesting replies, that's much appreciated and will surely help soon or later.Quote from: mtwieg on December 18, 2024, 02:22:36 pmI have the design both in breadboard prototype and assembled pcb. I didn't notice the background noise with the former, but will test again tomorrow.However, the noise is definitely the charge pump frequency.The real problem is that such circuits behave differently between their breadboard form and the assembled pcb form, and I am a total novice in term of complex noise reduction / cancellation, especially with 0402 smd components and a pcb that had to keep its initial size.Quote from: Ian.M on December 17, 2024, 05:39:36 pmExcellent suggestion, thanks ! I happen to have a schematic of a positive charge pump made around two schmitt trigger inverters, but I didn't figure out what the first gate did...Due to size restrictions, I wonder if an equivalent negative charge pump can be made out of inverters rather than all those diodes. I don't have time to experiment this at the moment, but will try to find online examples to save some time.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 18, 2024, 08:27:16 pm
content: Quote from: microvia on December 18, 2024, 07:52:21 pmWhat frequency does the MCU generate ?You could also soften the edges before they enter the charge pump.eg A 74LVC1GU04 with input R and feedback C will let you define a slew rate.Quote from: microvia on December 18, 2024, 07:52:21 pmYou need some switching elements in a charge pump.What current do you actually need ?

author: microvia
author_rank: Regular Contributor
timestamp: December 18, 2024, 08:45:05 pm
content: Quote from: PCB.Wiz on December 18, 2024, 08:27:16 pmFrequency: 200Hz to 20,000Hz roughly.Quote from: PCB.Wiz on December 18, 2024, 08:27:16 pmCurrent needed is very low, typically 1µA if the target circuit is used correctly.The problem is, I can hardly increase the frequency without making the output voltage increase drastically (it must be within -10V and 0V).

author: mr ed
author_rank: Regular Contributor
timestamp: December 18, 2024, 09:21:10 pm
content: Use a battery. Trickle charge it in standby. Nothing quieter. High performance low level amps, audio and instrumentation used batteries.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 18, 2024, 09:31:28 pm
content: Quote from: microvia on December 18, 2024, 08:45:05 pmThen the simplest solution is a Photovoltaic generator as already suggested.Quote from: microvia on December 18, 2024, 08:45:05 pmI'm unclear what that last comment means ?You can add a zener to lightly clamp the charge pump, and push the MCU drive source ultrasonic, so you cannot hear the crosstalk.If you have excessive clamping, you might be able to reduce the number of stages at higher frequency.If you need to vary the output voltage, you can vary the supply of the 74LVC1GU04 buffer.The LVC1GU04 has• Overvoltage tolerant inputs to 5.5 V• Wide supply voltage range from 1.65 V to 5.5 VSo you could vary the supply up to 5.5V if you have that available, to reduce the pump chain stages.If you have more voltage available, you could use a SOT23 gate driver like x27517, that has 3V in and up to 20V Vcc, so a single stage can get you -10V, with ~ 11V in.

author: microvia
author_rank: Regular Contributor
timestamp: December 19, 2024, 08:44:05 am
content: Quote from: PCB.Wiz on December 18, 2024, 09:31:28 pmI use a dual LDO delivering 3.3V and 5V. The former is to power a bluetooth device, so I cant afford to send any noise there.I tried using 1nF capacitors for testing at higher frequencies, but the maximum output voltage was significantly reduced, and the frequency range didn't change much. Increasing the first two caps to 10nF increased the max output voltage of course, bit this brings back to square one.The positive charge pump schematic I mentioned earlier uses two 74LVC1G14. I have a pair of 74LVC1GX04 mounted on DIL8 adaptors. The 74LVC1GX04 includes one 74LVC1GU04 (usable separately) and 3 x 74LVC1G04 in series. I'll try adding the GU04 section as buffer with the 10 ohms at Vcc and 1K at the output.

author: psydaddy
author_rank: Contributor
timestamp: December 19, 2024, 09:45:41 am
content: You can use a linear regulator after the charge pump, this will decrease the noise (ripple rejection) but it'll waste more energy while being used.Can you divide the supply in half with 2 capacitors to create a negative voltage?

author: microvia
author_rank: Regular Contributor
timestamp: December 19, 2024, 10:06:01 am
content: Quote from: psydaddy on December 19, 2024, 09:45:41 amThe output voltage is variable, so no regulator at the output.I can't divide the power supply unfortunately.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 19, 2024, 04:39:23 pm
content: Quote from: microvia on December 19, 2024, 10:06:01 amOver what variable range, to what precision and what is the exact load ?What is this concoction actually doing ?The currents in charge pumps are very spiked, and that’s why it is important to keep this ultrasonic to avoid audible injection, as you have found.If you have two MCU pins, you may be better to have one as a fixed f pump and a second one as pwm dac that controls a micro power opamp post regulator.An alternative is an inductor fly back design, which is better suited to variable out.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 19, 2024, 04:42:45 pm
content: Quote from: microvia on December 19, 2024, 08:44:05 amFrom what main supply voltage ?

author: tom66
author_rank: Super Contributor
timestamp: December 19, 2024, 05:27:06 pm
content: Quote from: tszaboo on December 17, 2024, 01:35:04 pmADI has some DC-DC converters based on charge pumps that operate in the MHz+ range with 98%+ efficiency (example). Charge pumps are universally more efficient than inductor based converters for similar load currents. Their major limitation is relatively low output currents because your switch impedance is the only thing limiting the current flow in/out of the fly capacitor. But there are applications I've designed where a charge pump converter is absolutely the most appropriate solution and achieves excellent efficiency. Inductor based converters always have magnetic losses to contend with, ideal efficiency will always be better with a charge pump but it's hard to get without extremely careful layout due to the very high peak currents involved.

author: mtwieg
author_rank: Frequent Contributor
timestamp: December 19, 2024, 06:40:36 pm
content: Quote from: microvia on December 19, 2024, 08:44:05 amOdd, you should be able to get the same output by just increasing the drive frequency by a factor of ten (BAT54 should be fast enough for ~1MHz). Maybe there's parasitic leakage and capacitance on various nodes of the charge pump bogging it down.Quote....huh? Isn't switching frequency something you control directly?Did you try and of the tests I proposed (where the charge pump is disabled and/or its output disconnected)?Could you explain what the negative bias is used for? Does it directly interface to the audio circuitry in some way?

author: Someone
author_rank: Super Contributor
timestamp: December 19, 2024, 09:27:39 pm
content: Quote from: microvia on December 19, 2024, 10:06:01 amThen you've disregarded one of the quickest and cheapest ways to solve the problem of "I want a low noise 0 to -10V variable output and dont have a negative supply"Right now the design on my desk is relatively similar, requiring a low noise adjustable negative voltage at relatively trivial DC current. Switching regulator + opamp as a linear regulator.

author: tszaboo
author_rank: Super Contributor
timestamp: December 19, 2024, 11:28:13 pm
content: Quote from: tom66 on December 19, 2024, 05:27:06 pmThis is not a voltage regulator. This is a converter, that halves the input voltage, and probably generates so much ripple on the input and output, that maxim didn't dare to place it into any documentation. Let's just try to compare apples to apples.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 19, 2024, 11:56:35 pm
content: Add a small negative LDO. If it hasn't been suggested before.

author: tom66
author_rank: Super Contributor
timestamp: December 19, 2024, 11:59:01 pm
content: Quote from: tszaboo on December 19, 2024, 11:28:13 pmOk, but the point is it's possible to make an efficient charge pump converter. Though, I suppose unlike an inductor-based converter, you cannot make an infinitely variable converter with excellent efficiency. You can only make some pre-defined fraction efficient. The most efficient converters switch between e.g. 1.33x, 1.5x and 2x output and then pulse modulate between those modes to obtain high efficiency. But switching at audio frequency is definitely not needed.There is no need for a converter like that to produce any more ripple than an SMPS, as long as the layout is tight with the input and output capacitors where they should be, it won't be too bad. Samsung's Galaxy A23 smartphones use a charge pump voltage halving circuit to charge the 3.7V battery from a 7.4V charger input at 25W, which saves substantial board space over an inductor solution. IIRC there was also a Chinese vendor selling a phone capable of~100W charging using the same technology but I can't find it right now. Point is they're absolutely commercially viable at high powers and high switching frequencies, they are not restricted to audio.

author: Someone
author_rank: Super Contributor
timestamp: December 20, 2024, 01:12:28 am
content: Quote from: tom66 on December 19, 2024, 11:59:01 pmPossible, but getting off into things the OP isn't interested in such as the LTC7820.Efficiency of a uA load at 10V is pretty much irrelevant given the rest of the system the OP describes. Heads towards the ultra low activity/frequency fixed on time converters that get used for battery powered radiation tubes for example.

author: microvia
author_rank: Regular Contributor
timestamp: December 20, 2024, 08:18:16 am
content: Quote from: mtwieg on December 19, 2024, 06:40:36 pmSorry I didn't express myself correctly. I meant that the frequency range required to obtain -10V to 0V using 1nF capacitors didn't really change and still covered a good part of the audio spectrum.I was a bit busy yesterday but got time to test the 74LVC1GU04 buffer with a 10R at its Vcc and 1K at its output on the breadboard prototype. This board doesn't have the background noise issue because the charge pump isn't as close to the audio circuit as on the target pcb, and doen't use tiny smd components. As it can only improve things, I added the buffer to the pcb, which is really tight now.I also thought about some possible crosstalk with the Vcc layer underneath and added a ground area under the charge pump. Can only help as well.My biggest problem is that I can't modify the target pcb while testing.Quote from: mtwieg on December 19, 2024, 06:40:36 pmIt's to drive a 6FG6 magic eye vaccuum tube as a percentage level indicator (so not a VU). The charge pump goes to the grid, hence the very low current draw.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 20, 2024, 08:48:56 am
content: Quote from: microvia on December 20, 2024, 08:18:16 amYou can also place a series R at input and a cap feedback.That gives a user defined slew rate that helps current peaks in the charge pump.You could also test varying the supply of the 1GU04.For a uA ballpark micro power ldo / opamp there are surprisingly few low uAand> 10V options out there.Rare demand.There is a NJM2825 in SOT23 that is 0.7uA shunt regulator, high side reference voltage.Discrete regulators need more pcb space, but can do uA regulation.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 20, 2024, 08:52:51 am
content: Quote from: microvia on December 20, 2024, 08:18:16 amThat means you probably do not care too much about ripple on the supply.How critical is it to get to 0 V ? ie what is the control range of the 6fg6 ?

author: Ian.M
author_rank: Super Contributor
timestamp: December 20, 2024, 09:12:47 am
content: As others have explained above, a charge pump is a poor choice for applications that need a variable output voltage, while keeping the switching frequency well above audio frequencies, and minimising supply rail disturbance.Assuming you can implement the required voltage control loop in your MCU, and have a PWM capable GPIO you can drive at a high enough frequency with sufficient duty cycle resolution, its worth investigating an inverting buck-boost topology.The high side switch is controlled by a fixed frequency PWM. The PWM off time must always be sufficient for the current through the inductor to reach zero*, and its maximum on time must be limited to avoid the inductor saturating. As cycle skipping could cause undesirable subharmonics in the audio band, the switch on time should always be greater than zero. The feedback divider resistance should be chosen to provide sufficient load to allow this even at minimum (least negative) output voltage.A high gain PNP BJT may be preferable to a P-MOSFET if you don't buffer the MCU PWM output, as the noise on the MCU supply rails from its base current may be less objectionable than that due to the current spikes charging and discharging a MOSFET gate.If you need a smoother negative voltage, as the load current is negligible, add several stages of RC filtering. Feedback should be taken from before the filter.* i.e. The circuit must operate in discontinuous mode, as it is impractical to operate in continuous mode (inductor current does not fall to zero each cycle) without a current sensor + hardware support for terminating the switch's on time if the increasing inductor current indicates its close to saturation.

author: mtwieg
author_rank: Frequent Contributor
timestamp: December 20, 2024, 12:56:46 pm
content: Quote from: microvia on December 20, 2024, 08:18:16 amWait, now I'm even more confused. You said that with 1nF the voltage range decreased greatly, but now you say you were still able to get -10V without changing the drive frequency significantly... those two things can't both be true...QuoteHard to give feedback on this without some pictures of the breadboard/pcb setups...QuoteI'm not a tube expert, but I'm assuming ripple on the grid will cause ripple on the plate/cathode current, which might be the path for this to contaminate the rest of the system. If you disable the tube by disconnecting the plate bias, does the audio noise go away?

author: microvia
author_rank: Regular Contributor
timestamp: December 20, 2024, 05:08:07 pm
content: Quote from: mtwieg on December 18, 2024, 02:22:36 pmFurther tests show that the noise is still there even with the charge pump output disconnected. I had a 2M ground resistor at its end.I tried putting a 47K at the end, which reduced the noise a bit, but the output voltage was affected, so not a solution.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 20, 2024, 07:57:56 pm
content: Quote from: microvia on December 20, 2024, 08:18:16 amI found some info, these are cute devices !https://mauser.pt/catalog/product_info.php?products_id=086-0216https://www.radiomuseum.org/tubes/tube_em84.htmlThey give varying indications of drive voltage.Can you measure the actual voltages and currents uA you need ?What MCU are you using?Spice indicates you can get wide voltage control from a damped ringing fly back inductor, with uA loads, but the pulse widths get down to sub us for a fixed 30us period, which means jellybean MCU may be a bit coarse.A sawtooth + fast comparator can give smooth control.Or, a MCU could give a fixed -ve voltage you then regulate with a small linear circuit.

author: microvia
author_rank: Regular Contributor
timestamp: December 21, 2024, 03:50:41 pm
content: Quote from: PCB.Wiz on December 20, 2024, 07:57:56 pmPIC16F (up to 32MHz).

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 21, 2024, 10:10:52 pm
content: Quote from: microvia on December 21, 2024, 03:50:41 pmDoes that model have a 8MHz or 32MHz PWM clock ?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 21, 2024, 10:51:05 pm
content: Here is a quick example of a MCU pulse driven simply flyback converter.There is no regulation, the load resistor determines the voltage per impulse. You could measure the Vo on a ADC pin, if you wanted this more precise.I took a 125ns step and a 1210 sized 1mH ballpark inductor - spice says this can give 30-40 steps to cover a 10V grid sweep.That may be good enough for an indication meter ?Frequency is always ultrasonic and fixed, and you can increase R8 grid resistor for softer edges, at the expense of more loss.( at 3v3 drive, you would choose a lower threshold PFET, I picked a very modest FET as this is uA loads)

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 22, 2024, 01:20:43 am
content: I'd still suggest you get some of these - they are low cost (39c 10+) and claim 12V 8uAhttps://www.lcsc.com/product-detail/Transistor-Photovoltaic-Output-Optoisolators_SUPSiC-GAQV1123S_C22385005.htmland experiment with linear or PWM drive into RC filter load for gate.Addit: I find the Vishay Photovoltaic models include nice curves for Io/Vo and applications circuitshttps://www.vishay.com/en/product/84639/

author: microvia
author_rank: Regular Contributor
timestamp: December 22, 2024, 08:29:51 am
content: Quote from: PCB.Wiz on December 20, 2024, 08:52:51 amThe 6FG6 has a grid voltage range of -22V to 0V. For now -10V is perfectly sufficient as I don't need that it be fully off. However, 0V corresponds to full deflection (maximum indication) which is most important.I will eventually use a similar tube with a lower grid range limit of -12V or -8V.

author: microvia
author_rank: Regular Contributor
timestamp: December 22, 2024, 08:50:36 am
content: Quote from: PCB.Wiz on December 22, 2024, 01:20:43 amIt's a great suggestion. The only limitation is that the part isn't available from usual providers (Mouser, Digikey etc.).Quote from: PCB.Wiz on December 22, 2024, 01:20:43 amNot easy. The breadboard prototype doesn't behave like the 4-layer target pcb. The charge pump works almost pereftly on the breadboard with no audible background noise, whereas the target pcb has the noise issue (mostly because the components are close to each other).Quote from: PCB.Wiz on December 22, 2024, 01:20:43 amToo expensive unfortunately: $3-$4 as opposed to the 3 BAT54 arrays which cost $0.05 each. Not that I can't afford the part itself, but production cost must stay reasonable.Other problem: its 'If' is 50mA. Even if I added a buffer, this would increase the current draw too much as to what the dual 3.3V+5V LDO can provide (80mA for both outputs and 50mA max per output when both are used).Edit: Panasonic makes two candidates: APV1121S (8.7V, 14µA short-circuit current) and APD2121S (8.2V, 8µA short-circuit current). Both draw only 3mA. Datasheets are vailablehere.Thanks for your suggestions. I'll try to test that.

author: sarge
author_rank: Contributor
timestamp: December 22, 2024, 04:45:31 pm
content: Someone in the IRC channel spotted this thread, and asked kindly if someone could forward information; via the user Kuschelpony:Quote

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 22, 2024, 06:17:22 pm
content: Quote from: microvia on December 22, 2024, 08:50:36 amYou do nothaveto feed to the data sheet max, if you look at the 8uA curves, it is a more modest drive.I linked the (new) Vishay parts because they are the only ones I find that have full curves, a great help to understand how these niche photovoltaic parts work.

author: microvia
author_rank: Regular Contributor
timestamp: December 22, 2024, 06:54:03 pm
content: Quote from: PCB.Wiz on December 22, 2024, 06:17:22 pmYou mean the slope difference between the VO1263 (p. 3, Fig. 2, orange curve) and APV1121S (p. 10, first graphic) ?Indeed the APV seems to raise much faster, which could be a problem.I ordered a few pieces for the sake of testing. I plan to use two in series, which allows up to -16.4V and a direct LED connection.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 22, 2024, 07:39:33 pm
content: Quote from: microvia on December 22, 2024, 06:54:03 pmNot really, the Vishay parts have Io/Vo load lines, which few others give. The APV is Vo/IfQuote from: microvia on December 22, 2024, 06:54:03 pmToshiba have Spice models, if you want to experiment.The Photovoltaic parts with a box called [control circuit] have an additional active turn off (a modest current sink) to discharge FET gates.The Toshiba parts with [control circuit] simulate slightly better with a series diode, when used as PWM to DC generators with output RC loads.If you read back the gate voltage on the MCU via a resistive divider, a second clamp diode to stop positive valve gate would be needed.If you can rewire the valve as suggested in #58, you then only need to sink cathode current with ~ 20V compliance.You could even current drive the cathode.

author: microvia
author_rank: Regular Contributor
timestamp: December 23, 2024, 09:23:13 am
content: Quote from: PCB.Wiz on December 22, 2024, 07:39:33 pmI must confess that I hardly do Spice simulations. I once tried under KiCad but had troubles.Groundig the grid is an option, but this method isn't necessary since the photovoltaic device is isolated.Also, most other magic eye grids require -8V to 0V. The 6FG6 requires a higher range, but at the end I may opt for another type to make it easier.I'll try the APV1121S to see how it performs, and have ordered a few SUPSiC GAQV1123S as well. Both part numbers are amazingly similar...

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 23, 2024, 12:43:25 pm
content: Have you considered just using a filtered 0 to +3V PWM output from the MCU and doing the necessary level shifting on the magic eye / supply board to keep the grid in the -15V to 0V range relative to cathode ?

author: microvia
author_rank: Regular Contributor
timestamp: December 25, 2024, 11:15:54 am
content: A quick update after testing a pair of APV1121S in series: driving the magic eye works, but the output rapidly raises from 0 to 3V or so when powering the LED between 1.03V and 1.07V (just above the 1V required to light it), causing the magic eye to deflect too quickly. Same behavior with a pulsed output, where the frequency range giving the same effect seems too narrow (I need to experiment a bit further on this).I'm happy to see that such components could be used, the only problem being to find one with an appropriate curve. I will try the Vishay you recommended.I also ordered a few pcb samples for testing the buffered charge pump which, if no longer causing background noise, will be the default design until a photovoltaic device gives satisfactory results.Tthanks a lot for your suggestions. Will keep you informed when further tests are done.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 25, 2024, 07:07:03 pm
content: Quote from: microvia on December 25, 2024, 11:15:54 amYou should current drive the led, not voltage, and experiment with load resistors.The Vishay part was linked because it has very good curves, but they should all have similar actions, just at different current levels.There will be some temperature variations, so you could feedback to a MCU ADC for high precision. (or use an opamp, as shown below)

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 25, 2024, 08:15:45 pm
content: Here are the Toshiba TLP3905 / TLP3906 Photovoltaic generators spice curves, to match the Vishay test condition ones, you can see the similarities.Essentially constant current until the voltage max clamp action occurs.The second plot sweeps the current to 10mA and plots the voltage across a low load, adjusted to give appx 4.7V max in both cases.The effect of that active clamp shows up a bit more here.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 25, 2024, 09:25:50 pm
content: and here are more complete voltage generation alternatives.One uses PWM drive and output side filtering to give DC voltages.The second one uses a simple OpAmp with feedback to eliminate In-Iout transfer gain / non linearities. It is 100% linear, no switching at all*, and very tolerant of OPTO device selection.If you look at I(R3) I(R5) you can see the bending effects of non linear drive current to give a linear V output.This needs a DC 0~2.17V DC* drive range for appx full Photovoltaic output on these parts. It is slightly over driven in spice to check the limits.*You may use PWM to generate the DC reference control voltage, but that can be any frequency and very local/low energy.

author: microvia
author_rank: Regular Contributor
timestamp: December 25, 2024, 10:40:24 pm
content: Thanks a lot for taking the time to share these detailed examples. The op-amp circuit looks really interesting even though I won't have enough pcb space to fit everything on.Precise linearity isn't a problem as the frequencies can be adapted in software. I'll try to see how the circuit performs without op-amps first.Also, I would like to obtain "maximum" -17V or so using two photovoltaic devices in series, which I am currently testing using 1121S.I found that a rather correct deflection could be obtained with a pulse ranging between 8KHz and 10KHz. I didn't put a resistor before the LED and will test further tomorrow.I also ordered a two pairs of Vishay. They were pretty inexpensive at RS even in very low amount.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 25, 2024, 11:51:57 pm
content: Quote from: microvia on December 25, 2024, 10:40:24 pmYou can get opamps in SOT353 packages, which may help (eg LMV321) ?Quote from: microvia on December 25, 2024, 10:40:24 pmDo you vary the duty cycle ?

author: microvia
author_rank: Regular Contributor
timestamp: December 27, 2024, 07:56:05 am
content: I did further tests yesterday. After fixing a minor bug the basic circuit works well using the 16KHz-24KHz range. I used a pair of APV1121S in series directly connected between the MPU and the magic eye grid. I will make the same tests with the other devices when they arrive. Next step will be to use the MPU's PWM to replace the software-generated square wave which slows the program a bit, which requires a different pin configuration.

author: Datman
author_rank: Regular Contributor
timestamp: December 28, 2024, 11:45:45 am
content: It could be a wrong ground path. It would be useful to know better what are you doing...

author: microvia
author_rank: Regular Contributor
timestamp: January 05, 2025, 05:30:45 pm
content: Ground is currently a full copper layer located on the bottom side.I'll try to change this to a star configuration, which I hope won't require moving everything.

author: microvia
author_rank: Regular Contributor
timestamp: January 05, 2025, 05:33:00 pm
content: I also made quick trivial tests using the SUPSiC GAQV1123S and Toshiba devices. None of them work as good as the APV1121S. I think this is due to the raising time of the LED and the lack of feedback control.I will try to test usint the Op-Amp design to see how this changes things.

author: microvia
author_rank: Regular Contributor
timestamp: January 05, 2025, 06:13:48 pm
content: Quote from: PCB.Wiz on December 25, 2024, 11:51:57 pmI'll try to make a test board using this design. However I would really like to do up to -20V to drive EM84 / EM87 tubes. I think is possible using a pair of SUPSiC GAQV1123S. However, I'm a bit worried with the LED turn-on time of 0.23ms. Would your circuit still work by putting two 1123s in series with a 100 ohm resistor ? Will your circuit work with other classic OPAs ?Quote from: PCB.Wiz on December 25, 2024, 11:51:57 pmNo, so far I only vary the frequency because (as far as I recall as of writing) the pin allowing using the PWM block is already in use and can't be swapped with another.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 05, 2025, 07:52:18 pm
content: Quote from: microvia on January 05, 2025, 06:13:48 pmYes, it should work in series, just scale the feedback resistor.The GAQV1123S shows the active turnoff, which is more nuisance than asset in this use case, but the OpAmp makes operation more tolerant and linear.If you have a signal generator, feed it with a triangle wave 10-25Hz and check the waveform.

author: microvia
author_rank: Regular Contributor
timestamp: January 05, 2025, 08:43:11 pm
content: Quote from: PCB.Wiz on January 05, 2025, 07:52:18 pmI added a typical schematic in my previous message.I wish the VO1263 wasn't that expensive compared to the SUPSiC 1123S. The former can deliver up to -14.7V or-29.4V depending on where you pick the output voltage. The latter, which has the active turnoff, can deliver -12V only (-24V with a pair in series as in my previous schematic).Still, this allows two magic eye grid outputs: one at the first sensor, the other at the second, hence two voltage ranges using the same duty cycle.I don't have any AD8601 handy, only classic types used in Hi-Fi.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 06, 2025, 12:26:31 am
content: Quote from: microvia on January 05, 2025, 08:43:11 pmIf you double the voltage, you would also double the feedback resistor (220k -> 470k ) as you want the same current operating points.

author: microvia
author_rank: Regular Contributor
timestamp: January 22, 2025, 06:29:49 pm
content: Quote from: Ian.M on December 17, 2024, 05:39:36 pmI modified the pcb and added a 74LVC1GU04 single gate buffer powered at 3.3V. I used a 680 ohms resistor as I didn't have any 1K fitting the very small space available.The background noise is much reduced, but sometimes still audible if you pay attention.The resistor values need being increased, but I can't determine their values using the final pcb where the crosstalk problem happens.Edit:I added the same buffer on the breadboard version and put two trimmers to monitor the output.Although this won't tell me from which values the crosstalk issue disapepars (the breadboard doesn't suffer from this issue), this will show up to which values the circuit works correctly.After several tests, I found that the resistor before the buffer's Vcc pin can be considerably increased: even at a few Ks I only see a minor voltage drop near the maximum negative voltage, and it's only a few hundred millivolts. The 1K resistor before the buffer's input isn't cricital at all. I saw no noticeable changes even at 220K.All I need to do is to pick two 0402 resistors that I can put on the final pcb to see what happens. Hopefully, the crossalk will disappear.In any case, a big thank you to all of you who proposed various solutions and took the time to simulate their circuit. That's much appreciated.