best switch for the job?

[justDIY]

I have an idea for a portable power source, designed to recharge devices that will accept a charge over the USB connection.

The heart of the system relies on a single cell boost converter, powered by a lithium cell, converting the lithium's substantial current into +5v.

To make the device super flexible, I also plan to incorporate a USB powered lithium charge controller, so the unit can recharge itself from any usb host (laptop, desktop, etc)

To keep things user friendly and 'simple' on the outside, I would like to use the same USB port to both receive power from the host for the lipo charging circuit, as well as provide power to a device from the boost circuit. I would like to make this as automatic as possible.

I know a simple single pole double throw mechanical switch would work here, diverting the +5 line from the usb connector to the charger or the supply ... the question is what is the best way to replace that switch with something electronic?

What I'm trying to figure out, is how to sense the difference between being connected to a host (source of power), and being connected to a device (load). To keep things simple, I want to avoid the software side / usb stack by all means.

Right now I'm working on the assumption that a proper device should never source current, where as the host will gladly source current.

Does anyone have some ideas?

I have attached a crude block diagram of the proposed setup.

 

[philba]

it runs in my mind that maxim has a buck regulator specifically targeted at USB+battery power based solutions.

 

[justDIY]

they have a couple, but they're designed to power a load (cell phone) off a battery, usb or ac adapter ... they're not designed to charge the battery.

unless I missed one, which is possible

if there's a single chip which is both a battery charger and a boost converter that can run off that same battery, that would be great.

 

[philba]

It was in a recent (approx 6 weeks ago?) maxim mailing. I think this it was both a charger and a buck regulator. unfortunately, I chucked, er recycled, it.

 

[justDIY]

i found one of the chips you speak of, mentioned under the app note "a ups for usb"

it can charge and discharge an onboard battery pack, and handle load switching between line and battery power

problem is, its terribly complex - lots of external components :/

I think I will use two jacks ... one Type A for the output, to recharge a usb device ... one Type B for the input, to recharge charger

 

[philba]

oh well, worth a look anyway... let me know how it works out. I've got a couple of USB projects coming up.

 

[philba]

I got currious and went looking. I must have imagined it but the max8606 looks pretty nice. You'd have to add a boost or buck regulator to get the right V if the Li-Ion wasn't the right output level. I'd try to contrive the system to run on the Li-Ion voltage.

edit: more stuff. LTC has some that maybe i got confused with the maxim ones.
for 3.3 and 1.8V output - https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1037,C1078,C1583,P2447,D1820
here's one with 3.3 and adjustable output https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1032,C1062,P9050,D6440

 

[justDIY]

Linear has some interesting stuff ... really small packages!

The problem I keep running into however, is the dual purpose for the single USB connector ... needing it to automaticly determine if it's sourcing current (powering an external usb device) or sinking current (recharging internal battery)

I'm thinking right now about using the MAX1811 as the charge controller, 500mah charge current should give approximately a 5 hr recharge time with a 2400mah cell.

For the boost converter, either the MAX1703 or the MAX1675 ... I need to look at the current draw of some typical loads ... the 1675 offers a maximum of 300mah output with a limit of 500mah switch current, the 1703 offers a maximum of 1.5a output with adjustable switch current upto 1a.

max1675:
**broken link removed**

max1811:
**broken link removed**

max1703:
**broken link removed**

I've attached my revised block diagram.