Adjustable LDO - a few questions

[Saimoun]

Hi

I have a USB 5V supply, and I want ideally a voltage as high as possible but free of noise.

So I chose an LDO with a high PSRR ratio: I put a 3.3V LDO and that worked beautifully.

Problem is I am using this for audio output and 3.3V peak to peak is not very loud audio. So I have been looking for a 4.5V high PSRR LDO which are hard to find (or expensive).

The best solution I found so far is this LDO which has an ADJ version: https://datasheet.lcsc.com/lcsc/1806130337_BL-Shanghai-Belling-BL9193-ADBAPRN_C216210.pdf (costs under 7 cents)

I am thinking using a ratio of 4.7k and 12k resistors to get about 4.4V output.

My questions:
1) Am I going the right way?
2) Regarding this LDO, the feedback pin is normally a bypass pin where you should put a 22nF cap to GND. But when it's a feedback pin they mention to add a 22pF cap between the pin and the output. Where does this value come from?

[ledtester]

There are audio amplifier chips designed to run at 5V that also have a very high PSRR. The MAX4298 comes up a lot in google searches. What kind of load do you want to drive?

[Terry Bites]

Thats a sensible ratio.
I'd use 24k and 62k to get the nearest ratio using E24 parts.
Your divider has to pot down Vo to Rref.
If you ever need a reistor ratio quickly use an on line resistive divider tool, enter your Vout=Vh and enter the Vl= Vref
http://mustcalculate.com/electronics/automaticresistivevoltagedivider.php?vin=4.4&vout=1.23&eser=E24&rmin=1k&rmax=100k

The 22n gives stabilty to the feedback path with R2=30k It probably wont need to be changed for slightly lower or higher values in the FB.

[wraper]

Voltage over USB wire under load can easily drop to something like 4.5V, so using USB as power source does not guarantee 4.5V output after LDO

[Saimoun]

Thanks for the replies everybody.


@ledtester: actually that's a good point, my opamp has already quite a high PSRR so maybe I could try running it directly off the 5V.

@Terry: Thanks for the link I do not really care about the actual voltage, just the higher the better. 12k and 4.7k are E12, also more current gives less noise according to the datasheet. Unless there is another reason for using different resistors?
I am still confused about the capacitor, the datasheet mentions a 22pF (pF and not nF) between FB and OUT pins?
How do I calculate the value for another R2 (f.x. 12k)?

@wraper: yes very good point - the whole device does not use much, something like 70mA I think so hopefully that will not happen.

[Saimoun]

@ledtester: I tried simply droping the LDO and powering the audio circuit directly from the USB 5V. I added a little RC filter:

VBUS -> 10 ohm res -> 4.7uF cap to GND.

But that did not work (too much noise) which actually makes sense since I have an analog switch and the DAC powered there which do not specify a high PSRR (like the opamp does).

So I think the 4.5V LDO seens to be the way? Alternatively what do you guys think of adding a boost converter to maybe 7V or so, and then a 5V LDO with high PSRR? I do not have much experience with SMPS though but I can try.

[magic]

Run the DAC from 3.3V and add a bit of gain at the opamp to bring it back to 5Vpp?

BTW, 3.3V vs 5V is ~3dB difference.

[Saimoun]

You're right the DAC is not the issue, but the audio circuit after has an analog switch which I believe would add noise anyways? I can try

I've attached the audio circuit. Input on the left is the DAC (with a coupling cap), and output on the right is a pair of headphones (with coupling caps as well).

And yes also correct about the +3dB, I have been debating whether to change my VDDA from 3.3V for a while now 

But some poeple have been trying the product and thought the audio output was not very high.

Problem is, changing it to something higher (or maybe adding a negative power supply) would mean redoing the whole circuit since the opamp and the audio switch are made for 0 to 5.5V.

[T3sl4co1l]

Filter and FB are two different things.  The filter node is an internal VREF node, with a modest impedance and noise level that can therefore benefit from a relatively bulky external capacitor.  AFAIK, it is always ahead of the error amp, so plays no role in compensation, but acts to reduce the noise level at medium to high frequencies, and improve PSRR, to the extent the ref's PSRR is poor at least.

The small cap from output to FB, is a feed-forward, "speed up" or "zero" capacitor.  It acts to increase negative feedback at high frequencies, improving transient response.  The value is determined by error amp and pass device dynamics, which aren't ever published (for any regulator I've seen), so you're best using the recommended value, and the recommended Thevenin value of the resistor divider with which it acts.  (Since the important part is actually the RC time constant of those three components together.)  A smaller value (including zero) is likely fine, just with poorer response; a larger value may cause too much loop gain and thus oscillation.

I suppose it's also possible that the FB pin has some conductance itself, and the resistors/cap act against that as well; in this case, it wouldn't be strictly the external RC time constant, but also their value in relation to this value.  (Imagine an example like, the FB pin happens to be the emitter input of a common-base amplifier.)  I don't know that this is ever used either, just putting it out there as a possibility, for which such a connection would be important.


As for loudness, if it's not just gain but it's clipping at maximum and still not loud enough, yeah that's definitely a supply problem.  It may be fruitful to just drop in a +/-5V or higher switching supply (might as well go bipolar for ease of use, eh?), and use it as an exercise in good filtering/shielding.  Assuming you have the space to do so, of course; no idea how big this is, but it sounds like the kind of thing that could be quite small, and this would likely be a significant change in size.

Tim

[Saimoun]

Thanks Tim that helps a lot 

I guess by "filter" you mean what they call the bypass pin in the datasheet, correct?

I understand the idea about the small cap, I was just wondering if there was a typo or like and it should actually be 22nF (instead of 22pF).
Just wondering though, if the idea is simply to match the RC time constant, logically if I choose the resistors of 4.7k and 12k which are about 5-6 times smaller than their suggested values, then the capacitor should be about 5-6 times bigger, i.e. about 120pF?

PS: the PSRR of the adjustable LDO I mentioned is not super high that is true - but similar to the 3.3V LDO I currently use, which I know is enough for the circuit (audio output is dead silent).

[Saimoun]

It may be fruitful to just drop in a +/-5V or higher switching supply (might as well go bipolar for ease of use, eh?), and use it as an exercise in good filtering/shielding.  Assuming you have the space to do so, of course; no idea how big this is, but it sounds like the kind of thing that could be quite small, and this would likely be a significant change in size.

Yea that would be ideal The size is not a huge problem, the board is not that busy (or at least I have seen a lot busier), but it's more the price, as well as my own very limited experience with SMPS and filtering its output. But true it will be great learning for me 

And right now I am using that analog switch to change the (analog) volume between -22dB and 8dB (volume levels in between are done digitally), which will not work with a negative power supply.

The original problem is:
* noisy 5V supply
* headphone audio output that is not quite loud enough with 3.3V peak to peak

I find it easy to get lost - there are so many different ways to do this

[T3sl4co1l]

Well duh, there's plenty of analog switches out there.   TC4W53FU,LF for instance, or DG4xx, or good olde {74HC|CD}4066.  You don't need the R_on particularly low, it's just a feedback signal.

BTW, better to put the switch on the -in side, with feedback resistor from switch to output.  This way the common mode voltage changes very little (it's a virtual ground), and thus R_on changes little (it's dependent on common mode voltage, most datasheets show a curve, getting more severe at low Vcc; which, the BL1551 apparently does not, so there you go).

Also regarding LDOs, mind that they're usually rated for PSRR at modest voltage drop; expect it to degrade as the drop goes down.  In particular, CMOS / Pch types will degrade due to a combination of rising junction capacitance and operation in the triode region (where both transconductance and drain resistance are low).  Eventually at Vout < Vo(nominal) it's fully saturated and PSRR is zero, more or less.  It's a continuous transition between these cases, but not one that is normally well documented (if ever?).

So, for this case, you may find higher noise levels from a computer with low Vout to begin with, and dirty contacts, and a long or cheap (thin wire) cable, etc.

All the more reason to have good PSRR in the op-amps.  Which they usually do pretty well at, so, just a matter of getting the product of both down to the desired level.

Tim

[Doctorandus_P]

I'm guessing that bridged amplifies do not work well for headphones because L+R share a common GND wire.

I would probably go for an SMPS solution.  First step up the voltage to +12V or so, or even a symmetric voltage, then add a lineair power supply with some proper filtering. If you have a few volts of headroom for power supply regulation, it's a lot easier to build a power supply.

If you're too lasy to design a SMPS circuit, then you can simply use isolated modules such as B0512 or S0512.
These do need extra buffer capacitors, and come in many (similar) variants from different manufacturers.

[KT88]

If 5V are sufficient, I would use only proper bypass filtering with a good low ESR electrolytic together with a high value ceramic multilayer cap. Reason for that is that the PSRR of an opamp is highly frequency dependant.
A quick (LT-) Spice simulation would help to estimate the required cap-sizes... But mind the voltage coefficient of the ceramic cap.

Cheers

Andreas

[Saimoun]

Thank you all for the input. Very good point Tim, you're right the 4.4V LDO will work fine until the case where the USB voltage will be 4.5V and then the PSRR will drop.

I am thinking still staying away from boost converters but using a charge pump instead to double the voltage, followed by a 5V High-PSRR LDO which will be very easy to find.

[perieanuo]

Thanks for the replies everybody.


@ledtester: actually that's a good point, my opamp has already quite a high PSRR so maybe I could try running it directly off the 5V.

@Terry: Thanks for the link I do not really care about the actual voltage, just the higher the better. 12k and 4.7k are E12, also more current gives less noise according to the datasheet. Unless there is another reason for using different resistors?
I am still confused about the capacitor, the datasheet mentions a 22pF (pF and not nF) between FB and OUT pins?
How do I calculate the value for another R2 (f.x. 12k)?

@wraper: yes very good point - the whole device does not use much, something like 70mA I think so hopefully that will not happen.

you miss the point, usb isn't 5V, it's between 2 levels, wraper pointed the right approach
if tomorrow your usb dies and you change it, maybe the new one will output 4.4V
this is how it gets done, by taking limit values into account, not what your specific device does. the forum is about learning also the correct reflexes in this electronic world

[Saimoun]

@perieanuo: alright I see. Then I guess my options are either just letting it be at 3.3V like it is now or do the option raise voltage + 5V LDO?
I'll try option 2 with a charge pump

[langwadt]

do you actually need a regulated voltage or would a capacitance multiplier do?