Hi in my latest power supply video, in my Revc schematic I showed how I used what's called a tracking pre-regulator uh to power my regulator my Uh Lt380 regulator up here so that it didn't dissipate as much power I.E the input side of it here I wanted to keep uh only a couple of volts above the output voltage so that it doesn't uh drop out, but it keeps the dissipation in the Lt380 as low as possible while still getting the advantage of uh, the lower noise um, inherent in the in a linear regulator for the output as opposed to a switching regulator. So I used a tracking pre-regulator down here and it used an e uh, pot, one of these adjustable 5K um Iqu C interface potentiometers that simply adjusted the lower feedback resistor in a typical boost converter like this Mikel 2253 and uh, quite a few people asked um, uh, maybe I should consider a tracking pre-regulator that doesn't require software, which is one of the disadvantages of this one. Um, is that? Well, one of the advantages too, but a disadvantage in that it requires the software to measure the output voltage and then compensate. continually do that and then continually compensate uh, the tracking pre-regulator voltage to match the output.

So it requires software overhead, but it allows some Uh flexibility in other respects. But people were asking why didn't I use A or consider using a track? a fully analog tracking pre regulator which simply took the output voltage of my LT 38 regulator and fed it back analog without any software and adjusted this DC Todc converter output voltage on the Fly and it's a good question. uh I use the Es Uh pot because I've done this before before it worked I wanted to use another I Squ C device it was I didn't want to muck around with a Um analog uh solution for that so um, but I thought I'd uh consider it and as it turns out, the Lt380 data sheet itself Um actually has here it is. it's an An example circuit you doing exactly that.

It's got a step down Uh converter here on the input. but and it's got this: uh TP Uh 0610 uh P Channel Mosfet here working as the Uh pre-regulator element to set the input voltage in this case, approximately 1.4 Vols above the Uh output voltage of the LT 380. So that's actually uh, rather novel that because it uses the Uh threshold voltage of the P channel uh Mosfet to actually uh, set the voltage above um the Lt3 3080 output voltage. It's very simple and uh, elegant.

I rather like it so I thought I'd Get the Uh LT Spice circuit simulator out and uh uh, see how it worked with the generic Um Step Up uh DC Todc converter So let's give it a go. All right. So here we are inside Uh LT Spice, which is a free circuit simulator. It's very good I Highly recommend you get it if you want to play around with a circuit simulator.

and because It's from linear technology, it has all the linear technology parts and pretty much only linear technology. Parts But uh, you can add other parts but uh, it has got the Lt380 Fantastic which we use. That's great. So I've uh, put the Lt380 over here and I'm using an Lt19 Uh 35 which is a uh, fairly, uh generic uh Step Up DC Todc converter.

It works just like the micro ones. They all basically work pretty much identically. Only very minor differences is in there they have the same reference voltage and they work in exactly the same way. So I Have no doubt that if uh, it works for the Lt9 34, uh 35, it'll work for my Micr part as well.

So I've just uh for the mosfet here. I've just chosen a generic one from the list. It's got a whole list of Uh mosfets down here. I Just chose the first Fairchild one.

I Don't really care? um I've got a 1meg uh upper feedback resistor and a 10K lower feedback res resistor. so the upper resistor R2 here. that's just going to set the um upper uh voltage? um, threat upper Uh? Limit that Uh, this DC Todc converter can actually go to I've set it to, you know, very high up to one uh Meg and then the Uh mosfet will actually uh control the output voltage of the DC to DC converter. but you can actually lower R2 to give you an absolute like a safety feature so that your DC to DC converter doesn't go over a certain voltage.

So let's give this a go, shall we? I'm Um, got R1 down here. Um on the set pin of the Lt380, it sets the output voltage. as you know, there's a 10 microamp current uh from the set pin of the Lt380. So 10 microamps, uh time 100K is 1 volt.

So uh, we should be aiming for a 1V uh output voltage on our voltage regulator. so we'll run this: I'm um, running a simulation uh command here, a transient uh response stop Time 1 second. Um, you know time to start saving data a millisecond. that sort of stuff.

so fairly generic. We'll give that a go. let's run it and uh and see what happens. That's look at it.

Probe our output voltage here and it's set to 3.52 volts and our output voltage here is spot on one volt. so that's working a treat. And if you watch the values uh, ramp up here you'll notice that the voltage differential um, after it's settled down is higher here. This differential voltage here is uh, higher than the differential voltage when it's ramping up, but it eventually settles down to a um steady state.

and then what you see in there is the Ripple And let's take a look at the Uh differential voltage where um got uh 10 volts output here and it looks like we're about H 12, 11 .85 volts or there. Abouts, let's you know, say 11.8 Vols Um so we're 1.8 Vols uh differential above the output voltage that's not high high enough for us. it's high enough for the Lt380, But we've got an additional if you remember the schematic for the power supply, an additional Uh 1 Ohm Uh current shunt resistor in there, which at 1 amp can drop up to a volt. So we're looking for about a 3vt voltage differential and that differential voltage will of course depend almost entirely upon the Uh typee of mosfet which you actually, uh, use Here, it has, um, some dependence upon the Uh current as well.

uh, going through here via your resistor values, but not a huge amount. So uh, really. um, you have to get the specific type of Um of uh mosfet in here and we've actually got the Uh. If we have a look at here, we've got the FDC Uh 5614 p and we're getting about 1.8 volts on the Uh simulator differential.

So if we go in here and actually have a look at the data sheet for that, let's have a look at the gate threshold voltage Vgs There it is. it's about 1.6 volts at uh, an ID current of uh uh, 250 microamp. so that's not uh, too far off at all. So, but it can actually uh, vary in the range from Uh 1 to 3 Vols But we're getting uh, fairly consistent.

We're getting close to the uh typical valume measured there. So um, to get a higher voltage, we're going to have to choose a different mosfet and I've chosen the second one on the list. It's a Philips BSS 84 and we're getting once again and it's not too far off the other one. It's around about uh 2 volts offset or 12 volts uh input voltage.

And if we have a look at the data sheet for that device, well, it, uh, here it is here. the gate threshold voltage at 1 milliamp ID is um, from anywhere from8 to 2 Vol So really, there is no typical figure and we're actually getting uh, that 2 Vols So it says uh, go have a look at figure eight down here and that's the gate Source uh threshold voltage as a function of Junction temperature. So really, you know we don't care because our Junction temperatures uh, pretty much only going to be uh, ambient here. but it's um, showing a gate um, threshold voltage of around about uh 1 volt at Uh 25 which is what the simulation would be running at.

So uh, that is a whole Vault out. So this is, you know, tricky business. You've got to, uh, choose the right device and you may have to actually practically measure it as well. But I know what you're thinking, that's not a milliamp uh flowing through um ID there because I'm now that red waveform there is actually the current flowing through Um R3 here.

which is, you know, in the order of like 50 microamps or thereabout. So uh, really, we need to uh, drop that to a K and run that again and see what we get. Well, there it is. It's up to around about a milliamp or so just over and uh, we're getting.

um, let's have a look here. we're getting. uh, still our 2v uh voltage differential slightly over it. Well, looks like we've run into a fail here.

I've um I've changed the device to a uh, si uh 343 mosfet and look, what's happening to the output voltage? It's a fixed like 3.5 volts and the output uh voltage of course, uh isn't working because we're trying to set it uh, to a vault down here and I don't know what's going on there. What? Fail The simulation obviously doesn't like this. SI 343 Device: Very curious and there you go. The same thing has happened again with an Irf 744 Mosfet.

Exactly the same thing it's It's really weird. It's is it that model is not compatible with this circuit. Somehow there's something some little glitch in there, but this is not uncommon with Uh simulators. You'll find that you know there are various Uh configurations of things that calls them to play up and you know, just not play ball.

Uh, generally with what you're trying to do, so it looks like, um, the particular type of mosfet which we're choosing there makes a difference. Let's go back to the BSS uh 84 here and let's uh, close that and let's run that again and uh, bingo. Now we're fine. No problems whatsoever.

So yeah, that is weird. Now the first thing I'm going to suspect here is um, the Lt380 because the gate here of the mosfet is trying to read back from the output of the Lt380 and that output is dependent upon the input voltage which is set by the mosfet which then generates the 10 microamps to generate the Set current through here which generates the output voltage so that somehow maybe I'm thinking that extra Loop in there is confusing it somehow. um, to do with this particular uh Moss fit of course. So uh, really, uh, there's only one way to prove that and that I think is to take the Uh gate voltage from the set pin down here and drive our set pin directly like we're doing inside our, um, the actual Uh PSU circuit.

Let's delete this here. and well, we can actually delete the resistor. We're not going to need it now. What I'm going to do is I'm going to, um, actually, uh, insert a voltage source here on our set pin like this and we will.

That's exactly what we're doing inside our power supply circuit. So we'll do that and we'll set it to say 10 Vols again and I changed the mosfet back to the one that was giving the trouble I've got the Irf Um 747 in there and let's take that gate voltage directly from down here and let's uh, simulate this sucker again and see what happens. and Bingo we are getting. Ah, there you go.

There you go. It's working. It's working. A treat.

Again, not a problem. So there you go. That was the mosfet that was giv us trouble before if we measured our gate voltage from the output there. So obviously there's a there's a trap there in the model of how the LT either the Lt380 model or how the mosfet or the interaction of the two.

You have to go in and look at the Uh Spice models themselves and know how the Spice engine works and does all the simulation and things like that. and uh, maybe um, someone who's uh, more knowledgeable on, uh, that sort of stuff might be able to, uh, um, figure out exactly what it's actually uh doing there. But what I'm going to do is uh, I'm now going to, um, take the gate voltage here and sense it from the output and see if it still works. Let's give that one a go and here we go.

What? No, There we go. It's a problem. So clearly, um, there's an issue there when it senses it from the output based on that mosfet. So it's not really the Um set pin that's actually uh, it's not really the voltage down here that's doing it.

Um, it's the it looks like it's the sensing from the output. So let's actually, uh, do that again and take that gate voltage from down here. but don't have the voltage so forced on there. So just rely on the Uh one Meg to set the output voltage to 10 volt.

So let's run that again and see what happens. Bingo There you go. So we don't need to force that voltage. So it's not not the So.

it's not the voltage being forced on there, it's it's actually the act of taking the output from there. So I it's got something to do with that extra uh loop, a calculation or something that's got to do in there to uh to do that. So maybe there is, um, some sort of uh, something we can, a few knobs we can tweak in the simulator to actually, uh, get around that I don't know I'm not too, uh, fussy. I'm not going to look into it any further.

If any anyone knows exactly why. uh, please, let us all. no, But there you go. That's not an uncommon trap in these circuit simulators to actually find that, uh, you know something.

There's some little Oddball thing between the Uh two components or with the simulator that doesn't work and gives you unexpected results. Now if we actually, um, oh, hang on, something's going on here. Look at this. our uh, blue voltage.

Our output voltage is dipped back down. it's overshot. It doesn't. It's gone up to uh 12 Vols it's gone way above our, um, yeah, that doesn't work.

Okay, that's not, that's not good. So we need to actually, uh, set that with the V Set uh with the voltage set there clearly so that's not going to work. But anyway, these, um, it's a common thing. these circuit simulators.

You can get little traps like this and imagine if we got that Flatline that we had before. um, first up, when we first started running the simulator. If the first thing you see is a a silly Flatline um coming from this thing, then you're going to sit there scratching your damn head going. what the hell's going on I You know why isn't my thing working I'm using the right models and it's following everything and everything's hunky ding.

Imagine if you got that straight off the bat, you'd be be absolutely uh, you know, racking your brain trying to figure out what was going on there and that can be a real track with circuit cator sometimes you got to play around with them to get them to play ball. Now here's a mosfet the Fqb uh1 P6 uh that actually has a 4volt uh Vgs? uh which is what it's showing here, but unfortunately that's only available in like a power type package. It's a little bit expensive. not really what I want I don't need a power mosfet here I want a little signal? uh, mosfet.

So the parametric searches in like uh Digi key and stuff like that might actually be hard to uh, come by because they do actually list the uh Vgs here. Um, but it's you know it's only a maximum value. They don't list the typical what it's going to be, so there's the you know there it is so it's really hard to sort of. um, might be a bit hard to find a suitable device using uh, parametric uh search like this.

Well, there's one way to actually, uh, deal with this. If you've got a particular, uh, low Vgs device which you like, it's low cost, it's in the right package you want, and it's only got the 2vt offset, but you need say three volts or something like that. Well, you can just whack in a couple of diodes in here and that's exactly what I've done here I've uh, put in a couple of 1n 914s which depending on the current are going to drop about 0.6 volts each. so you'd expect if this was if this BSS 84 was given us a 2vt offset before, it'd expect it now to be about 3.2 and Bingo! That's exactly what we get here over on the simulation and there it is.

10 volts output voltage, the blue line, and about 13.2 Volts for the Um DC to DC converter voltage. So that's a little trick that allows you to boost the Vgs of your chosen Uh mosfet just by adding a couple of diodes in there because gate and Source You're just um increasing the value by putting a couple of diodes in there, but that's not very good. If you use those diodes somewhere else in your design, that might be okay, but you got two of them. What's a better way? Well, let's try something else and one neat way to do it, of course, is to use an LED.

So that's exactly what I've got here: I've got a Uh Qtp, Uh 690c LED It's a typical Uh surface mount red um LED and basically you expect about a 1.8v uh drop at a very uh, low current for this Led. So if we zoom in here and have a look, we're getting Bingo If we're getting normally without, without uh, the Dioe without the LED there at all, we'd be getting about a 2vt offset. We're getting there. it is 13.8 or 3.8 volts 2 plus 1.8 volt drop in our diode.

And as a bonus, uh, you might be able to use that diode as a power indicator or something like that as well. I mean there's not much uh current flowing through this thing. if we take a look, there's only 1.2 milliamps flowing through this, which is set by the lower the lower value of the resistor down here, but um, you know you can, uh, change that value to actually, um, give you a usable indication. but a good uh, modern high efficiency LED will um still allow you to get you know a bit of brightness out at 1.2 milliamps.

but there you go. So um, really, that's a way that we can uh, tweak this um to choose a mosfet of our choosing that has the right package and the right voltage. but just, uh, boost it up a bit more so that we do have. Um.

just adequate margin on our Lt380 here cuz we need Um depends on the output Uh Current I think it's 1.6v maximum is the absolute maximum uh Dropout Voltage It can be lower because our V control doesn't have to be tied to our input pin can be Tri tied to the other side of our Um offset our current uh offset pin if we take a look at the schematic back here and we take a look at the LT 380 again, once again, the V the Um VC pin which Powers internal circuitry doesn't have to be powered from the input pin, we can actually take that. In fact, that's what I'm doing in this design I'm taking it from the other side of uh, the Um current uh, shunt resistor. So the Dropout voltage effectively Um is much smaller than if we just tied these two pins together. But depending on the package you get for the LT 380, um, some small smaller pin count packages actually have those two pins tied internally so you would have to take into account the voltage drop.

But there you go. Um, that's not a bad solution. I Rather like that I think I might probably implement this in my power supply design. so I hope you enjoyed that and I'll catch you next time.