I have am using an LM5017 to step down a relatively high voltage solar panel (VOC over 90V) to 5V. I have bench tested my circuit and it is providing good buck regulation with input voltages up to 95V. I connect this to a type A USB port to provide a charging port for USB devices. The LM5017 is only capable of supplying 600mA. I would like to emulate a Standard Downstream Port so that the client will draw up to 500mA from the charger.
I tried tying D+/- to ground via a 15kohm resistor. But the port is not being recognized by some devices as SDP. I assume I am doing something wrong.
does anyone have any advice.
I could try shorting the data pins together to emulate a dedicated charging port, but I do not want the client to attempt to draw greater than 500mA.
In general is there a circuit which will emulate a USB Standard Downstream Port without implementing logic?
Thanks
Best Answer
As noted in this Maxim whitepaper (Tutorial 4803: The Basics of USB Battery Charging: A Survival Guide):
Some devices do not use the standard. Apple for one, created a different standard (before BC 1.1 was released) for their own devices, allowing them to distinguish between non-supported chargers, 500mA standard chargers (enumerated usb), 1A high current chargers, and 2.1A highest current chargers. This is done with varying voltage dividers on the DP and DM pins. Some other manufacturers have adopted that, and the regular standard, at the same time. Some IC manufacturers have created USB host/battery charger detection ICs which figure out what kind of charger is being used and adapt accordingly.
iPhones are one type of device that will not charge on an unenumerated standard downstream port. The Sony Playstation3 Wave Controller is another. Another method is that some require the ID pin on the mini/micro-usb connector to be grounded or otherwise connected.
You have two options if you need to limit to 500mA. 1, if the device requires enumeration, you will need to enumerate. 2, use a usb charging/host ic that can current limit (or a current limiting regulator or a current limiting ic)