[retired] [WIP] 20mm single sided & 17mm double-sided ?-amp linear driver - surprisingly good!

Just posted my build with this driver. Link

Was delighted to see the 7136s chips finally get here today, so I built up a test board and did a bit of testing.

Build report:

A self inflicted nightmare of epic proportions. I won’t get into too many details but it was pure frustration, nothing at all to do with the design of the board really. Just…I really miss having a hot air gun. :stuck_out_tongue:

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Then I forgot that this board actually has a trace for Turbo bypass and needs a firmware tweak to actually turn on regulation. Running stock STAR Offtime the thing is constantly in some kind of direct drive! So be warned!
Of course that didn’t click with me until after I had melted a couple sense resistors and blown one of my long suffering test XMLs to smithereens :_(
I remember Wight saying something about putting the Bypass pin into High impedance mode or something, but at that stage I couldn’t be bothered to figure out how to change it in firmware so I just cut the Trace and continued along my merry way. :stuck_out_tongue:

Regulation was then working fine until the light was on high for more than a few seconds…then it just turned off. Some components get quite hot so I figured something went poof, but no all was fine. MCU pwm looked normal, sense resistor still there…all looked fine except the 7136 was no longer outputting anything. Damn must have killed it, maybe my crappy frustrated soldering?

I ended up replacing the 7136 3 times thinking I’d heat damaged it and it was dying prematurely on me. A few seconds on high and all would go dark.

Turned out it was a patch of unmelted solder paste underneath the mosfet that would partially short out the gate pin once the thing got up to temp…urgh!
This is why I like my mosfets nice and big! Haha :wink:

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Alrighty, so after all that mess here goes the first batch of testing.

I wanted to get to the bottom of why frequency has such a drastic affect on the pwm performance of the driver so I hooked up my Rigol and went to town.

I probed the MCU pwm output (Channel 1), the 7136 output/gate signal (Channel 2) and the drain output of the mosfet (Channel 3).
Note: Ch2 and 3 are AC coupled so they jump around a bit, should have locked them down with DC but hey, I’m new at this scoping business :stuck_out_tongue:
In any case it’s the shape of the curves that matter not the absolute measurements here I think.



Here is what things look like at 1khz: i.e Phase correct / 4.8Mhz / 8x divider


PWM: 30 - No light, with warm leds very very dim moon mode

PWM: 60 - Low output

PWM: 100

PWM: 180

PWM: 255 (Just for good measure)

So as expected things work ok at 1khz. And it’s also clear to me what the issue is here.

The 7136 shows a very clear ramp up behaviour on it’s output, it’s pretty slow and even at 1khz it only manages to fully turn on it’s output during a cycle when it’s set to 100 pwm or higher. Anything below that it’s going to start ramping up but then get’s cut off. That has a knock on effect on the mosfet, which also takes some time to get up to speed. At 30pwm you can see the 7136 start ramping up a fair way but it’s not enough to make any real impact on the mosfet so the leds stay dark.

Usable Range: 30-255



2.4 khz i.e Fast PWM / 4.8Mhz / 8x divider


PWM: 30 - No light

PWM: 60 - Barely on Moon mode, very very dim

PWM: 100

PWM: 180

PWM: 255 (Just for good measure)

It’s clear to see the PWM range compression taking place as the frequency ramps up, a moon mode at ~60pwm also matches up well with what pilotdog68 reported seeing. This is at 2.4khz and I think still a good usable range, unfortunately PWM flicker is still apparent at this frequency. Better than 1khz for sure and you have to look for it but it’s definitely still there. Sweeping your eyes back and forth quickly reveals the evil flicker :frowning:

Usable Range: 60-255



Hoping for a still usable range with less visible flicker I reran the test at 4.7khz i.e : Fast PWM / 9.6Mhz / 8x divider
———————————————————————————————————————————————————————-

PWM: 30 - No light

PWM: 60 - No light

PWM: 100 - No light

PWM: 180 - Yay a few stray photons - A dim but usable moon mode

Ahem, well that’s not great is it. Moon mode at 180pwm! :stuck_out_tongue:
That said I need to test a bit more and see if this is enough of a PWM range to get some usable spacing on the modes. I suspect it may be workable with some specific mode picking. Also by looking at that one moon mode, I can’t detect any flicker at this speed. Maybe it will still be obvious with higher output but I’m optimistic.

“Usable?” range 180-155

Edit: Nope not a usable range at all. It’s all moon modes until you hit 255. Even 253 is very dim, like dim enough to look directly at the led… dim :frowning:
Also some really funky stuff happens to the mosfet waveform at this stage.

Yuck!


Finally I wanted to know exactly what the components do on a 0-100% turn on and how long everything takes to settle into place. Fairly tricky to capture actually, since I was using a clicky and if simply left to bang on it leaves some godawful noise on the waveforms. Gently does it! :slight_smile:

Looks to me like the 7136 takes almost exactly 300uS to ramp up to fully on and the mosfet has a delay of another 150uS after the start of the ramp before it does anything.
I’m still not sure how threshold voltage, gate charge, gate resistance or timing characteristics play into this behaviour but this looks like a very good method for finding out! :slight_smile:
Anyone want to send me a spare DNT30N02 to compare? :bigsmile:

In the meantime I think I’ll just slap a couple of random mosfets on there and see how they respond, then cross check to see what specs may be contributing to those differences.
Fun times :wink:

Great post LinusHofmann!

I was unable to get multiple channels doing what I wanted like you did when I scoped the thing. I’ll have to take another crack at it. I may end up having to ask for some specific advice when I do.

In the text I quoted above you pick out the 7136 as the culprit for the slow rise on the DRV / gate trace. Remember that there are two ends to that trace, DRV on the 7136 and gate on the MOSFET. Both of those devices have an effect on that behavior.

Of course you’re right, nothing is operating on it’s own and I’m far from the right person to make the call on what’s actually going on here. I just saw that very consistent and mostly linear ramp up behaviour on the 7136 trace and thought it looked like a deliberately controlled thing. Maybe to help ease into regulation or something like that? Could be way off target of course.

Is there a way to scope the 7136 output without a mosfet gate attached to it?

Sure, just disconnect the gate. If you remove the MOSFET entirely the 7136 will attempt to fully open the gate as quickly as possible. This should not harm anything.

Cool will try that, didn’t think it would function properly without something attached.

‘properly’ is relative. With the sense voltage at a steady 0v we will not get ‘normal’ operation. All this test will really show is whether the slow rise is a 100% arbitrary characteristic or not. It’s worth checking IMO.

Well without the fet, output from the 7136 shoots up to 7.62v instantly then falls off slowly. Certainly no ramping but not sure what to make of that behaviour…
(Setup for this driver is input voltage @ 7.96v and a 4.3v zener with the 200ohm resistor

Image removed

-You know I’m wondering if I shouldn’t just stick the 5v LDO on this board and see if it works any better, don’t trust that zener much anymore . :wink:

Where are you getting GND for Channel #2? The 7136 should not be able to develop 7.62v, it’s supplied by the MCU.

Yeah I just checked, I’m an idiot. :stuck_out_tongue: Forgot to reattach the ground wire/battery negative to the driver after desoldering the mosfet…it’s getting late even for my sleep schedule…heh

Alright, so here’s what the output actually looks like without the mosfet.

And here at the same horizontal timing as the graphs from the first test.

It’s still ramping up, but much quicker without the mosfet load. It only takes about 40uS now.
Interesting, maybe this is another situation where an intermediary transistor or small fet could help turn the big power fet on quicker?

Maybe, but I’m not really interested in that solution. The 7136 is a very large component already and is intended to drive an FET directly.

There are other ways to achieve this which do not involve the 7136. I’ve always intended to work on using an op-amp based linear driver but got side-tracked by what appeared to be a ready made solution in the 7136.

On the first page of this thread I determined that we needed a better FET, one similar to the DTU30N02. With an FET like the DTU30N02 this would be a decent driver. Without an FET liket hat the driver is not so great and I do not intend to pursue further development of it.

Well I just checked the datasheet again and it does mention a “soft start” as a feature of the QX7136.
Maybe that’s what we’re seeing here and the DTU30n02 just manages to start that bit faster to help hide the effects of the weak ramping up/soft start gate signal.
Certainly doesn’t seem to play nice with any kind of fast PWM input because of that “feature”.

I’m just a little bummed that 4.7khz didn’t work at all, as long as I don’t have visible PWM I’d be happy to run this driver in place of 7135 options. Shame

I’m just not well enough versed in MOSFETs to determine for certain what properties of the DTU30N02 make it work so well here.

Been testing a few random power mosfets with the 7136 today and while it’s been interesting to see how each behave in the ramp up period I didn’t come across a really good candidate at 4.7khz.

Or indeed a much better idea of what specs are contributing most towards it working better or worse with the 7136 signal.

One particular standout comparison comes with two ON Semi power mosfets, the NTD60N02RG and the T70N03G
Both of these fets came on the old East092 DD drivers, the 70N first and then the 60N on the later models.

These two fets going by the datasheet appear to be very very similar, certainly I can’t really find any major differences in the usual suspects of mosfet specs. Nothing that’s jumping out as a fundamental performance difference between the two.
So how do they perform when hooked up to the 7136?

Well the 60N seems to be about the best performer I’ve tested so far with a pretty fast turn on while the 70N is by far the worst. It takes forever to fully turn on and as a result is completely useless in this application.

Here are the graphs of a 0-100% turn on capture.

NTD60N02RG

This fet actually switches on a fair bit faster than the standard small 30YLD fet I tested last time. It has about a 220uS delay from off to fully on compared with the 30YLD’s ~300uS

T70N03G

The 70N on the other hand is complete garbage, it takes so long to ramp up that the graph almost doesn’t fit in to the 100uS horizontal scaling. Just awful…

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So what’s causing this drastically different behaviour? No clue…
Total and individual gate charges are very close (and very low at only 9.5 and 13.2nC respectively) , they have identical threshold voltages (between 1v - 2v min-max on both) and capacitances are near as well.

The only major difference I found is that the 60N is apparently “Optimized for High Side Switching Requirements in High−Efficiency DC−DC Converters” and looking in the timing values, almost all the same again bar one; Tr or Rise Time.
Here there is a drastic difference between the 60N which has a Tr of 33ns and the 70N which has a staggeringly low Tr of 1.3ns! Haven’t come across a Tr this low up till now so I initially put it down to a datasheet error…but maybe this is really what makes it perform so poorly here?
I’m just speculating here but maybe having an exceptionally fast rise time is actually detrimental when combined with the 7136’s soft start signal output?

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Just for comparison here is the 30YLD fet again in the same test setup as above.

PSMN3R0-30YLD

And I also tested two other fets.

A philips PHD50N03LT


Looked promising initially but it’s a fairly slow one…strangely that fast initial start and soft ramp up plot actually seems to give a neater PWM trace on some levels. Non of the funky doubling seen on some modes with the “faster” looking fets like the 30YLD @4.7khz. But still nothing usable in terms of mode range.

DinTek DTU06N03

This looks like a variant of the DTU30N02 but with a higher Threshold voltage (1.2v-2.5v vs 0.6v-1.5v of the 30N02) and double the gate charge (total of 74nC vs ~30). Performance isn’t great unfortunately, had my fingers crossed it was all down to the DinTek name! :wink:

Just putting it out there really, see if you guys can make any connections.
I’m thinking an exceptionally low gate threshold voltage is pretty important, seeing as the ramp up of the 7136 keeps below 1.5v for quite a while. A fet like the DTU30N02 that can start opening up at that stage (<0.6v) will have some advantage over these fets which seem to only start cooking in the 1-1.5v range.

Also not sure what exact effect the gate charges have, I suspect at these slow PWM speeds it’s not that critical. Especially as the two fets with the biggest difference in performance both have a very low charge spec.

Also hooking all of these fets up directly to the Attiny 13A pwm pin they seems to slam on and off almost instantly with the PWM signal, nothing at all like the leisurely pace things operate at under the supervision of the 7136.

Here is a trace showing what the “useless” T70N03 does when driven directly from the MCU. Everything else the same.

Nothing at all slow about that :stuck_out_tongue: In comparison you can see what the open 7136 signal is doing in the comparison.

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My head is spinning…too many datasheets and still non the wiser… :Sp :weary: :arrow: Bed

Good work. I’d rather ask than read the manual: what method are you using to save screenshots?

There’s a manual? :stuck_out_tongue: I’ve just been randomly pressing all the buttons and twiddling the knobs until stuff started working right :slight_smile:

I just plug a usb stick in and the press the little green printer button below “Help” to save a screenshot. I’m sure you could hook the scope to a pc with the USB cable and capture even easier that way.
But this works ok and I haven’t played with the pc software yet.

Thanks! I thought that the little printer button was for screenshot-to-usb-flash-drive, but I hadn’t pressed it yet. My scope is a fair distance away from the PC, so I’d use Ethernet before I used USB-to-PC.

Speaking of op-amps got me thinking of an alternative solution to the intermediary transistor.
I just tried using one as a signal buffer between the 7136 output and the fet gate and it works as expected now with modes going all the way down to below 30pwm. That’s at 4.7khz with the standard 30yld fet!
Not a real solution to the problem obviously, as you say it’s kind of silly to use any additional component like this to fix an inherent issue with an IC… but…I’m pleased I got something working anyway! :slight_smile:

It seems to me like the soft start behavior of the 7136 can be outwitted to an extent this way. The op-amp input is such high impedance as to basically imitate an open output on the 7136. As a result it ramps up much quicker than with something attached. The op-amp output then handles charging the fet gate and it does that admirably, matching the 7136 ramp up behavior exactly. Timing of that ramp up is now down to around 30uS, very similar to the open output tests I did above.


Channels are same as above, except Dark Blue (Ch4) is now showing the gate drive signal coming out of the Op-amp.

30 PWM (Fairly low low, but not what I’d call a MOON mode yet)

60 PWM

100 PWM

180 PWM

Modes are nicely spaced as you’d expect from those traces. No issues at all, of course they’d also work quite well at a faster pwm frequency just with a bit more mode compression. And a faster fet with a lower threshold voltage would also still help in this regard.

Here is a close up of the trace @ 100pwm

Interesting. Can you post a schematic for what you used / did?

I’d probably get the schematic wrong if I tried to draw it properly, it’s very simple though.

I believe what I’m doing is called a Unity-Gain_Buffer config.

Vin is hooked up to the 7136 output, that’s the signal we want to amplify/buffer
Vout is fed to the gate pin on the fet.

Not shown on that diagram of an op-amp there’s also the Vdd and Vss pins, Vdd in my case connects to the postive output of the 5v LDO that also powers the rest of the driver. And Vss is connected to ground/battery negative.

It’s just functioning as a signal amplifier with the gain set to 1. So Vin = Vout. The very high input impedance of an op-amp fools the 7136 into thinking it’s not driving anything, so the ramp up behavior is very quick.
That’s my best stab at understanding what is going on anyway :slight_smile:


Here’s what the “schematic” looks like in reality, not quite so tidy :smiley:

Oh and don’t mind the resistor and the cap in that mess, they’re not connected to the circuit I tested, just there for tinkering.