Li-ion charger with USB OTG

[kkbb]

Hello All,

 I am looking for a IC that does Li-ion charging, and powers a USB OTG device

 I narrowed down on the bq25703a ( http://www.ti.com/lit/ds/symlink/bq25703a.pdf  )

 The USB otg device plugs into a A-type socket, and gets its power from the bq25703a (or so I think)
 I am unsure about where to wire up the power pins of this usb A socket.

 The charger plugs into the mini-USB B socket. I understand the power pins of this mini USB-B wire up to the 'adapter' on the PDF.

 Does the attached drawing make sense?

 I would appreciate any comments.

Thanks

[jbb]

You'll need a couple of power switches, so that you can switch between the USB A input and USB B output.

The BQ25703 is quite complicated - it needs 4 external FETs (or 5 for best results) and may be overkill.  The buck-boost converter is useful for overlapping voltage ranges (especially when USB PD comes in).

If you're using a single LiIon (or LiPo) cell, the cell voltage will be under the USB bus voltage and it might be easier to deploy a buck battery charger and a separate boost converter - with output disconnect function - for USB OTG output.

[krho]

I suggest that you go through the TI charger section once again. There are chips that support OTG without external components. We'll you'll need the MCU to communicate via I2C but that's about it except from some passives.

One example is bq25890

[malavon]

I hope it's ok if I resurrect this topic. I'm currently in the process of designing with this very IC (enthusiast-level, not professional) and noticed the difficulty of hooking this one up as well.
While googling I noticed I'm not the first person to experience this. So I decided to chime in in case someone else has the same question (or if OP still has it).
Disclaimer: I'm just a hobbyist and might very well be missing something here.

I might however be able to shed some light on why someone would use this charger instead of the other ones made by TI. As far as I know (I'm still in the process of checking out the other
chargers) this is the only TI charger that supports powering VBUS (USB) from the battery. The other chargers use VBUS as power input, not output.
e.g. as taken from the BQ25890 datasheet (p6): "VBUS: Charger Input Voltage."

A direct extract from the BQ25703 (or BQ25713) datasheet (p5):
"In the absence of an input source, the bq25703A supports On-the-Go (OTG) function from 1- to 4-cell battery to
generate 4.48 V to 20.8 V on VBUS. During OTG mode, the charger regulates output voltage and output current."

At the same time, this IC also manages the power path to prioritize the output voltage (SYS) over VBUS. This is an important thing to consider and it's basically why someone would use this
IC over a simpler charger with a separate boost converter. I know it would be possible to do this in a discrete solution, but I personally don't know if it would be smaller/easier/cheaper to do.

A related question was actually posted on the TI E2E forum (http://e2e.ti.com/support/power-management/f/196/t/717133), which might help as well.

Since I myself am still looking to improve my design (and knowledge) as well, it would be helpful if anyone else could correct me or provide some more details into an alternate solution.

[jbb]

For USB OTG, I think the usual method is to use a micro AB connector and then the ID pin tells the device to be a host or a slave.  A special OTG cable connects up the ID pin to tell your devise what’s going on.

If you want a USB host port and a USB drive port, is that really USB OTG?

I haven’t got all the part numbers on me, but if you want a single cell LiIon or LiPo there are options from Analog, Maxim and TI with the switches built in. (LTC4155 comes to mind but may need an external FET. MAX77818 maybe?)

[malavon]

I dug some more through Analog's and Maxim's offering and found the following when it comes to powering USB from the battery, thus needing a boost converter.
Below are some devices that can fill this requirement and are more or less comparable. Like @jbb said, most of them have switches integrated.
Information provided as-is, without any warranty 

Texas Instruments
BQ25703/BQ25713

Analog Devices/Linear Technology
LTC4155
LTC4160
LTC4156

Maxim Integrated
MAX77818
MAX77596