uSupply USB Rev B Update

[xani]

A possibility with having "too big" micro would be adding a bunch of extra features, like "battery charger" mode, ramp-up/ramp-down voltage  ( to test "brown-out" detection) or adding artificial noise/50Hz hum to output for testing audio circuits. But that's easy to for modders . Kinda sucks to have LED (no decent menus, hard to display other units like watts etc). but I guess one can always desolder it and connect some LCD

It would be nice to have some connectors/jumpers for isolated side of RS232, it would allow for connecting cheapo bluetooth<>RS232 converter and use it from tablet or phone. (or laptop few meters away).

[Bored@Work]

Wrong. TTL UART signals idle high. RS232 idles low.

I think I should take up plumbing instead of EE. Would keep me out of embarrassment and would help fixing the broken heat pipe we have since yesterday. Win-win situation.

[mariush]

I'm curious how - and if - the Vr voltage will affect the opamps.... the LM324 seems to need 3v at least and if i remember correctly it's not a rail-to-rail opamp, and you're feeding it from the tracking pre-regulator... it's not clear from the schematics what's the lowest voltage of that regulator. I assume it has to be larger than the lt3080, since that's a ldo and assuming this psu is a 0-15v therefore the tracking pre-reg needs to be as close as possible to keep efficiency high...

Would the accuracy of the opamp change if you give it 5v or 10v, is it an accuracy/stability issue for the current/voltage limiting?

[hlavac]

Couldn't you use something like MCP1603T-250 or REG711EA-2.5/250 instead of the lossy linear MCP1700-2502E for the 2.5V rail?

[EEVblog]

Couldn't you use something like MCP1603T-250 or REG711EA-2.5/250 instead of the lossy linear MCP1700-2502E for the 2.5V rail?

a) You wouldn't be saving much if anything in terms of efficiency at say a nominal 3.7V input.
b) Extra cost, complexity, and noise.
c) The linear reg has a low temp co. I'm using it as a quasi ADC reference.

Dave.

[hlavac]

I see, power efficiency is a bit of a lost cause there when powered from USB.
First the isolating DCDC converter steals 20% of the power, then the linear battery charger, then the linear 2.5V reg, at this point whatever is powered from the 2.5V has already power efficiency of 40% even before the actual losses are considered...

[EEVblog]

I see, power efficiency is a bit of a lost cause there when powered from USB.
First the isolating DCDC converter steals 20% of the power, then the linear battery charger, then the linear 2.5V reg, at this point whatever is powered from the 2.5V has already power efficiency of 40% even before the actual losses are considered...

Nothing to do with the isolating DC-DC converter as such, that just for charging. It's simply the low input battery voltage.
Assume 3.7V batt with a linear reg @ 50mA. That's 0.185W input power.
A 2.5V DC-DC converter with an ideal 100% efficiency at the same 50mA output current is an ideal 0.125W input power.
So any DC-DC converter over 68% efficiency will gain you an advantage over the linear reg.
For say an 80% efficiency, it's now about 0.16W input power, saving you not a huge amount in the scheme of things.
And when you compare it to the fact the PSU will usually be supplying a load, any saving becomes pretty insignificant.

Dave.

[hlavac]

You are right, if the USB is used just for charging, and not as in the original uSupply concept of completely USB powered lab supply.
I guess the merge between the battery powered supply and the old uSupply left us closer to the battery powered one...

Speaking of USB charging, the FT230XS-R has functionality for detecting 5V/1A chargers (which have D+ and D- shorted together),
maybe you want to use it to enable fast charging?
Is it worth an extra optocoupler?

[fenclu]

Still not sure if I get the idea...it's a battery powered, portable PSU and the USB is just for charging? Why not get rid of the DC-DC converter and just note on the silkscreen that the PSU cannot be used while charging? That would be a significant cost advantage...

[robrenz]

Still not sure if I get the idea...it's a battery powered, portable PSU and the USB is just for charging? Why not get rid of the DC-DC converter and just note on the silkscreen that the PSU cannot be used while charging? That would be a significant cost advantage...

And a significant loss of usefulness.

[klinkerstein]

Dave, you need an output clamp or similar on the 2W SIP7 DC/DC Converter or use a regulated one. Without any load, you easily get 20V out of a 5V/5V sucker.

Secondly, I think the Couplers are very battery inefficient. have you considered using an ADUM12xx from Analog Devices for this purpose? They are pretty easy to handle and usually easy to get.
http://www.analog.com/en/interface-isolation/digital-isolators/adum1200/products/product.html
But it doesnt seem as they would support 2.5V.. Hm

Other than that, it really looks pretty good. See, that you choose the right ATmega, which supports 2.5V when buying. And choose a ceramic resonator, that is accurate (drift, tolerance and tempco) enough for RS232.

[mikeselectricstuff]

Secondly, I think the Couplers are very battery inefficient. have you considered using an ADUM12xx from Analog Devices for this purpose? They are pretty easy to handle and usually easy to get.

Once they get reconnected the right way round, they will only draw power when communicating, which will be rarely if ever, so current draw is not an issue.

[klinkerstein]

Secondly, I think the Couplers are very battery inefficient. have you considered using an ADUM12xx from Analog Devices for this purpose? They are pretty easy to handle and usually easy to get.

Once they get reconnected the right way round, they will only draw power when communicating, which will be rarely if ever, so current draw is not an issue.

Yeah, that is absolutely correct.

EDIT: Oh, about the 120R resistor at AVCC on the microcontroller: Do you know, that the entire Port A is supplied through AVCC and not only the internal ADC circuitry?

[EEVblog]

Speaking of USB charging, the FT230XS-R has functionality for detecting 5V/1A chargers (which have D+ and D- shorted together),
maybe you want to use it to enable fast charging?
Is it worth an extra optocoupler?

The DC-DC converter is only 2W.

Dave.

[KeithBrown]

I have a real problem with the "insert tab A into slot B and solder" connection method. My problem is based on the experience of several of these becoming open circuits with no visual cues. I'd feel a lot better if there were some plated through holes on both the main board and the riser board so that solid jumpers could be soldered in as well. This is especially a risk if the banana jack board is not to be fastened VERY firmly mechanically to the main board.

[mikeselectricstuff]

I have a real problem with the "insert tab A into slot B and solder" connection method. My problem is based on the experience of several of these becoming open circuits with no visual cues.

Were the slots plated and the tabs double-sided ?

[KeithBrown]

Were the slots plated and the tabs double-sided ?

The tabs were double sided but different signals on both sides. Not sure if the slots had plating through, however I guess they could not have because of the last sentence.
I understand that  Dave's design is a much better situation, however I still cringe at using solder as a mechanical connection and expecting it to never crack. I suppose the clearances would be critical: allowing just the right amount of solder to flow into the slot beside the tabs. Too much space (and solder) and the mechanical connection would be soft, too little and there could be no solder there at all!
Anybody know if there are guidelines (IPC or somebody else) in regard to this connection method? I was taught a looong time ago that the first rule of soldering is a good mechanical connection, and this violates that teaching, again especially if it is also the mechanical connection. I understand that all SMD soldering also violates this--unless the components are glued.

[EEVblog]

I was taught a looong time ago that the first rule of soldering is a good mechanical connection, and this violates that teaching, again especially if it is also the mechanical connection. I understand that all SMD soldering also violates this--unless the components are glued.

Correct. By definition, a solder joint is a mechanical method of holding components together. That's one of it's jobs!
It's used every day on big bulky components, like PCB mount transformers for but one of countless examples.
If it makes you feel any better, there are also tabs in the top side of the vertical board that will also slot into the top case cover. The paranoid could also glue those to create a very rigid box type structure.

Dave.

[baoshi]

Speaking of USB charging, the FT230XS-R has functionality for detecting 5V/1A chargers (which have D+ and D- shorted together),
maybe you want to use it to enable fast charging?
Is it worth an extra optocoupler?

The DC-DC converter is only 2W.

Dave.

On the contrary, I think the "USB enumerated" signal can be used to stop charging if the PSU is connected to a bus powered hub

[KeithBrown]

Correct. By definition, a solder joint is a mechanical method of holding components together. That's one of it's jobs!

But I would not attach a trailer to my car with solder: sure cold solder is a mechanical connection, but not a very good one. We have all come across very tight banana plug/socket combinations where a lot of insertion or extraction force is required. And, if the sockets are exposed, as I expect they must be, a drop on the floor will possibly impart a large impact force on the solder joint...of course you would never drop any piece of electronics on the floor on purpose, would you!

[konfu]

Hi Dave,

it might be a nice feature to allow the LT3080 to go down right to zero volts output. To do this there is the need for a negative feed into the constant current load ground connection as shown in one of my last posts How to bring down the LT3080 to 0V output. Easy to do and just a nice feature and not too expensive (about 0.88 EUR plus 2 elkos).

I'd like to understand what value and type you are using for F1 - the ferrite bead between the pre-regulator and the LT3080.

Are you using precise values for R38, R39, R40 and R41 (Self Test Dummy Load) so the load is somewhat stable on it's current draw?

Nice work though!

Cheers

Florian

[Hobgoblin612]

EDIT: Oh, about the 120R resistor at AVCC on the microcontroller: Do you know, that the entire Port A is supplied through AVCC and not only the internal ADC circuitry?

In any case all of port A is used as analog inputs anyway. (except maybe Spare2)

[amspire]

Hi Dave,

it might be a nice feature to allow the LT3080 to go down right to zero volts output. To do this there is the need for a negative feed into the constant current load ground connection as shown in one of my last posts How to bring down the LT3080 to 0V output. Easy to do and just a nice feature and not too expensive (about 0.88 EUR plus 2 elkos).

If you are going to add a negative supply rail, there are better or cheaper ways to do it then using the temperature sensitive LM334. Like a 2N2222A transistor with collector connected to the output, base to 0V and emitter connected to a resistor to the negative supply. The LM334 has a 0.3%/C temp coefficient whereas the transistor/resistor pair have a 0.05%/C temp coefficient if you have a stable -5V rail. Also the transistor will not let the output voltage fall below -0.7V. With an extra diode and resistor to bias the transistor base at 0.6V, you can reduce this minimum to less then -0.1V. This is important as you cannot let most opamp inputs (eg the voltage regulator) get below -0.3V if they are using 0V as the opamp negative supply.

[konfu]

Hi amspire

I really like that hint. Gonna test it out during the next days and smash a rough circuit together for it. Thanks a lot!

[rec]

Greetings --

I stumbled onto this project yesterday.   I was looking for a way to make a USB recharged battery supply for a radio.  Google led me to the Turnigy charger tear down episode, which led to the uSupply project, or at least parts of it.

(It would be really nice if http://www.eevblog.com/projects/usupply/ were updated so it actually linked to all the videos about this project and the forum topics which continue the project.)

But I'm dubious whether the uSupply can actually be used with a radio as it's designed.  I expect it's going to have ample noise from the switching frequency, harmonics, and mixing products.  But it will be interesting to try.