How can I make power from usb ports conform to spec?

[Jamdan]

Hi all,

I have enjoyed reading this forum, but this is my first post.

I have a circuit I would like to power by plugging it into any USB port I happen to come across. The USB standard specifies output as 5 vdc (+/-.25 vdc) at 1 amp, which is perfect. But many ports are significantly (+/-.5 to .75 vdc) above or below this. I would like to take whatever output I am given from any USB port and then alter that output to be as close to spec as possible.

So, in short, how would I take the power from any USB port and then groom it to 5 vdc (+/-.25 vdc) at 1 amp?

Over-voltage is more of a concern than under-voltage.

I have looked at low-dropout voltage regulators, but I don't think I will always have room for the dropout. Maybe with a pmos voltage regulator? This one:

http://www.datasheetdir.com/TPS7150+LDO-Regulators

has a dropout of only 32 mV at 100 mA, but I think that increases with the current (?), and I have 1 amp of output so that would increase significantly (320 mV?), I'm guessing?

Another (lesser) consideration is efficiency, since many USB ports are on laptops, etc., which rely on battery power.

Thanks for any help you can offer.

 

[davenn]

You could use a DC to DC boost converter to guarantee a 5V output
Just beware with that 1Amp tho, that really is an absolute max. Personally, for continued operation of the USB port
I wouldnt really want to be drawing any more than around 0.75 Amp from it.

Dave

 

[misterT]

I think the standard says that you can draw maximum of 500mA from a USB 2.0 port, and that is only when one device is connected to the port. There is specification for dedicated charging ports which can deliver the 1A (or more), but if you want to power your device by plugging it to any USB port, I wouldn't expect to get more than 500mA. You can detect a dedicated charging port by reading the data wires (D+ and D-). They are usually grounded when the port is for charging only.

 

[Jamdan]

davenn, that sounds interesting, but there will be times of continuously drawing 1 amp so I agree with you about not going that route for that reason.

misterT, you are right about the USB amp specs being varied like that. I would accept whatever amps are offered by the port, but I would still like to get the voltage as close to 5 as possible.


Is there a way to run the voltage through [something (zener?)] with a breakdown/bypass at 5.25 volts --> resistor/diode that would subtract .25 volts, leaving me with 5 volts? (without changing the amps much). Basically, a method of determining if the voltage is over 5.25, followed by subtracting .25. That would get me roughly where I need to be.

 

[ericgibbs]

hi Jamdan,
Can you say why the 5.0V is so critical for your application.?

 

[Jamdan]

Hi Eric, I'm trying to figure this out for a relative and I don't know if he would want me to say. It is a very ordinary application, but there are reasons the power needs to be as close to 5v as possible.

 

[misterT]

Maybe this kind of regulator: https://www.linear.com/product/LTC3127

 

[Jamdan]

misterT, boy that looks great! Thanks very much.

I wonder if "Programmable (0.2A to 1A) ±4% Accurate Average Input Current Limit" means it can take more than 1A +/-4%, which it will then limit it to 1A +/-4%? Or does it mean it cannot handle more than 1A +/- 4%? If it cannot handle more than 1A +/-4%, maybe this is the same issue Dave was referring to earlier with the dc to dc boost converter? Unfortunately that would keep this from being a viable option. Just about all USB charging ports are 1A, and will probably come in a little over 1A +/-4%. Maybe I could just ignore the "Programmable... Input Current Limit" feature and not use it?

 

[ronsimpson]

A boost/buck switching power supply is built to output a voltage that is close to the input voltage. example: input 2 to 6 volts, output 5.0 volts.

TLC 3442, TLC3127, LTC3112/3
TPS63010/11/12, TPS63000/1/2

 

[Jamdan]

Thanks for the list, Ron. Some of those look very interesting, but I have no experience with these. I'm not asking you to do my research for me (the TPS63002 looks best to me so far), but if you happen to have any insight on which would be best for my application, I would appreciate your thoughts.

And the thoughts from anyone else here, as well!

 

[KeepItSimpleStupid]

You have to request the amount of current from the port. 1A is the most you will get on a root hub. IPad's can requst up to about 2.5A from a DEDICATED charging port.

ANY regulator will incur losses. A buck-boost regulator can regulate up and down from the input. There will still be ripple to deal with.

 

[ronsimpson]

Here is another.

 

[Jamdan]

Would there be any additional concerns if the incoming voltage was from a power supply/adapter instead of a PC? Do those things have power regulators? Would it matter that there might be two voltage regulator type devices in a row?

 

[KeepItSimpleStupid]

Here is one of the simpler devices available for USB charging: https://www.electro-tech-online.com/custompdfs/2012/03/MAX14529E-MAX14530E.pdf

Unfortunately, when you start mentioning Apple or hi current they are suddenly made from unobtanium: **broken link removed**

Motorola places resistors in their charging cable that determine the max charging rate basically forcing you to buy their charger.

There is no accepted standard, just a bunch of proprietary solutions.

 

[ronsimpson]

Two or three power regulators in a row is not a problem.
There are many 'USB chargers' that take 110/220 VAC to 5 volts DC. You want 5V at +/- 5% which is not that hard.

The first power supply has a voltage adjust pot so you can set the voltage at 5.00 volts.
The second power supply had a voltage adjustment that is +/-2%.

Edit: I see the data sheets are not attached. oops. I found 8 types of little "brick" power supplies that had adjustment for voltage. It is not hard to find supplies rated at +/-2.5%.

 

[Jamdan]

Thanks guys, great info!

@ KISS, that Apple stuff is nutty!

@ Ron, as always excellent answers.

 

[KeepItSimpleStupid]

See this app note: https://www.electro-tech-online.com/custompdfs/2012/02/slva464c.pdf

 

[Jamdan]

Good stuff, KISS. Many thanks.

 

[lzr]

You have to request the amount of current from the port. 1A is the most you will get on a root hub. IPad's can requst up to about 2.5A from a DEDICATED charging port....

@K.I.iS.S
I saw in apple website they say 2.1A max. Do you happen to have any link/reference for 2.5A?