BQ25886: Unable to achieve desired input current limit and charge current

Hi Chris,

With D+=D- shorted, the charger uses its ICO routine to determine who much current it can pull without collapsing the input voltage to VINDPM=4.3V.  Is the IC VBUS pin at slightly above 4.3V with 300mA input current?  If so, then the connection from your input power supply to VBUS is highly resistive.  If you have an oscope, you can watch ICO in action as explained in the appnote below:

Regards,

Jeff

Hi Jeff,

I've spent some time testing things, considering the possible issues you mentioned.  With the DMM I'm measuring 5v at the VBUS pin (measured from the directly on the VBUS and chip GND). On the scope I'm seeing something, but I'm not sure if it's the ICO functionality or just noise.  Here is a scope shot of VBUS.  As you can see from the scope shot the Vmin=4.7v.

I added a testing header to my board so I can power it up with my lab power supply.  I get the exact same result when I test with lab DC power supply as I do with the wall adapter.  Here's my board layout.  Let me know if you see anything obvious that would cause the current limit to be 1/10 of what it should be.  As I mentioned before, I've shorted the D+/D- pins temporarily for troubleshooting.  I'm also using my lab supply hooked up to the pads labeled "Alt Charger", and not using the usb recepticle (not that it makes a difference).  The system load is not connected, I've isolated the charging circuit from everything else on the PCB.

Last thing...Is there any way to disable the ICO for troubleshooting?

Thank you,

Chris

Hi Chris,

Can you attach the input power and the D+/D- lines to a USB or QC2.0 adapter with known input current capability?

Regards,

Jeff

I'm not sure I understand the question, but if you're asking if I've attached the adapter to other devices that draw similar current the answer is yes.  I've been able source 5v @ 3A using the same adapter.  Of course my bench power supply is capable of that as well.  

I didn't short the D+/D- lines until I had tried several different adapters, double checked my design, and checked the connections on the PCB.  After that I shorted D+/D- and used the bench power supply as my power source.  It doesn't seem to matter whether I use the usb adapter or the power supply, nor does it matter whether I have D+/D- shorted, The input current is ~300mA.

Does the D+/D- need to be shorted through a resistor?

If I misinterpreted your question can you please elaborate?

Hey Jeff,

Quick update:  Through a happy accident I believe I figured out my issue.  While checking voltages yet again, my DMM probe slipped and I shorted the ground pin of one of the SYS caps to the ground pin of one of the PMID caps, which was directly connected to the chip ground pins (pins 19 and 20).  The input current shot up to 3A, and the charge current went up to 1.5A.  I did it several more times to verify that it wasn't just a fluke.  When I looked at my PCB artwork I found that the ground pour for the chip was connected to the ground pours for the rest of the board through a small trace.  Admittedly sort of a dumb mistake on my part, but if you look at the artwork for the board you'd see how it was easy to miss.

Regards,

Chris