LED drivers and Accessories you want, but don’t exist

Hmm, well 2.8V LDO's are looking worse for option f, but they help option a.

Yeah, with a 2.8V LDO option F leaves you at about 3.0V LVP, you understand that right, assuming the 2.0V AREF floor is hard. But option (a) gives you 2.8V LVP or close.

Now I like option a) again. 2.9 V LVP isn't bad. I was worried it might push up to 3.1 or 3.2. 2.9 is ok.

OK, now from all the different options I’m a little confused. What do I need to do for option a) again?

Umm, don’t count on that. The whole substrate can be treated as a (reverse-biased) diode, and hitting one pin with reverse voltage can essentially apply the whole battery voltage through that one pin and out one/both power pins.

Same danger with applying an input voltage to a pin higher than Vcc.

I think that's true if it's not tri stated. My experience with these little buggers is they are tough as nails. I don't in principle yeah giving negative 3V to a tristated pin is probably beyond spec, but I'd guess it survives it. I'm pretty sure I've done just about every bad thing it's possible to do to things with 4.2V several times, and not managed to harm one yet. Although most of that was brief contacts.

nothing. And I probably confused you, I meant a) and I think g) (not f) the last one we were talking about, that probably never got a letter. Anyway, a is the leave it alone option.

The concern with a) and a low LDO is OTSM performance being poor at that low of a voltage. But you're just running up against all the limits. If the LDO is higher, like 3.0V and you use option G to get LVP lower, the boost still won't control quite right at 2.7V, although, that's probably pretty minor. It will still control, just start dimming a bit.

I think you should give option a shot and see how it works out. You've already got the boards coming anyway. I'll package an OTSM debug hex for you so you can test OTSM timing on it. Oh yeah, reflashing isn't exactly easy, well that's the price of the qfn.

Getting info from avr freaks on the 2.0V AREF limit. Looks like there's a little cushion beyond that in practice, but it does eventually quit working, in the right direction though.

Update: So by spec the absolute minimum on any pin is -0.5V. Indeed reversing the battery would be out of spec on any pin. Actual damage is still up for question. Well, I may as well just test it. I figure if it stays under 40mA it should be ok, so I'll crank it down slowly and see what happens. Lightbringer's point make sense though. It's like if the pcb board becomes conductive, the resistors don't matter. Bigger problem is I guess I should desolder 7135's first. You squint the wrong way and those things go belly up, at least mine do.

If the order has not been sent to fab you can email them at support@oshpark.com and either ask for cancel or send them new updated gerbers. I’ve done both quite a lot. They are very flexible up to the point when they are sent to fab, then it’s too late.

Glad to hear from you.

In the end, nothing had to be changed, so I didn’t need to cancel my order. But good to know. I think about making another version of the board, where just the pwm signal is connected to the LDO. Not sure if it fits though, need a diode and two transistors for that.

Back in town again so to say… Was a nice computer and internet free two month climbing vacation, but now back to reality…

I have those boards based on the GXB17 design waiting for some attention. Before I dig into them, how have things been going with 17mm single cell boost drivers? Are there any readily available and tested boards yet? In other words, is the GXB17 design obsolete?

Not tested, and not readily available, but I got some boards on the way, already have the parts.

Sorry to ask, you’ve probably addressed these questions earlier in the thread.
I see that you are not using the same boost IC as the GBX17 design. May I ask why you chose this one?
What kind of max output are you aiming for?
I don’t see a digi-pot in schematics you have linked to earlier in the thread. How will you control modes/output?
If your design works, would you mind if I rip it off and make a ATtiny1634 based version of it? I don’t use any other MCU now days.

I chose a MP3431, because of 19A switch limit. I’m aiming for up to 3A with an XHP35, and a bit over 6.5A with an XHP50. Don’t know if it can do that, on paper it should. I’ll try my best.
No, there is no DigiPot, the control circuit of the H2-C driver is pretty good, so I’ll use that for my driver. In short, the pwm signal from the MCU gets smoothed, divided with a voltage divider, and then compared with the current sense voltage. An OpAmp will bias the feedback loop on the boost converter to get constant current.

If you can fit an ATTiny1634 on it, sure you can. I’ll try to make a little different version though, which doesn’t need an LDO to supply the MCU, but just the PWM signal. It may be tricky to use bistro-HD like it is now.

I can answer a few of these as I helped a little in the planning stages.

1 - Boost IC is the MPS MP3431, we chose this IC as it has twice the input current limit as TPS61088, and it has a higher maximum output voltage, which means we can easily drive 12V emitters like XHP35. TPS61088 is on the threshold of working with XHP35 and can’t do much current since its maximum output voltage is 12.5V

2 - Heat transfer from driver to host will be the real determining factor for maximum output, so will vary by application.

3 - The control is done by PWM through an RC filter into the op-amp. The circuit design was ripped off from H1-A and H2-C. This method is better IMO because it allows the use of already existing firmware which control PWM.

Thanks for the answers both of you.

I can of coarse understand that compatibility with existing firmware is of interest, for me it isn’t though. I am however interested in the actual boost part of your design as it would be community tested. Loneoceans appears to have left the forum and too my knowledge no one else has actually built one of his boards, so I’m some what reluctant to start working on that design if you guys are coming up with a working alternative.

Schoki: May I ask why you would need an LDO to supply the MCU in the first place? The single cell voltage should be able to power the MCU prior to boost?

The PWM “amplitude voltage” (don’t know how to call it, but it is 0V to the max. voltage) that comes out of the ATTiny varies with the battery voltage. If there’s just a small load (which is the case), the amplitude voltage is pretty much the same as the battery voltage.

Now take a 50% PWM signal from a fully charged cell: 2.1V
And from a pretty empty cell (3V): 1.5V

So the voltage average varies, and we don’t want that. The current output to the LED would decrease, when the cell gets discharged.

But I don’t know if I can remove the LDO on the input, there’s a lot of ripple voltage (we’re pulling up to 19A from a single cell, pulsed). Maybe the ATTiny doesn’t work if there’s too much of it.

The LDO also allows 2S input to power XHP35 or other 12+V emitters. 3S is also possible if the emitter voltage is 12.6 to 16V. If you use digipot method instead of the PWM method you could remove the LDO without any adverse effects using 1S voltage, just make sure to put a decoupling capacitor very close to the MCU pins.

Yeah, but I ordered the wrong LDOs, so for now it’s 1S only. I ordered the TPS706 instead of the TPS709. They are the same, just the maximum input voltage is different.

This was also pretty much exactly the control scheme we came up with for the Texas buck. If you're going to have analog output that gets compared to an external reference (it does), of course you're going to need an LDO. Well, I suppose you could compensate PWM value for the battery voltage in software, but, and I think Schoki pointed this out in other discussion, it's likely to not be very smooth or could even oscillate, and of course you need the software, a pretty large table to get good resolution, or likely some math libraries, either way it would probably need that better chip and more work. And how do you compensate 30% battery level changes at a PWM value of 2? Probably ok if the light gets dimmer with low bats in moon you might not mind, but it's a bit sad if a boost driver can't even do moon without voltage sag.

Hey Mike C, are there any major gotchas with moving software to that chip? I assume one should review all the matchups between I/O pins and clocks, ORCX etc ADCMUX etc, mostly just header define stuff aside from possibly clock modes and speeds, Obviously there are some new sleep modes available. It doesn't seem like porting software should be painful, no?

Are you using for those this Attiny package?

https://www.mouser.de/ProductDetail/Microchip-Technology-Atmel/ATTINY85-20MUR/?qs=sGAEpiMZZMvqv2n3s2xjsduPWajY%252b7cdD%2FyGmvLk9iQ%3D