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Detecting USB device connection while unpowered

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oliver602:
I'm thinking about a USB charger that can be permanently installed in a car or motor home.

I want to minimise current draw while no USB device is attached. Tens of micro amps.

Perhaps by using a very low power circuit supplied by a linear regulator to disable the switching regulator when no device is attached. Id also like to include low input voltage shutdown, to avoid a flat car battery.

Is there a reliable way to detect when a USB device is connected while it is unpowered, without any mechanical switches?

reagle:
You might be able to just find one with very low quiescent current and rely on it  for detecting load.  Otherwise  maybe inrush current of the device getting connected? Typically there would be a sizeable cap on the power line

Psi:
The lowest idle power possible is zero, which you could do with a pushbutton to start the process.
Or even a tiny solar panel and supercapacitor to store enough charge to latch a relay or pull the IC's enable pin high.

However i'm sure there is an easy way to monitor for pullups/pulldowns present when a usb device is plugged in.
That would meet your tens of micro amps target.

Maybe get a few usb devices and see what resistance they have from D-/+ to ground.

Have a read of the USB standard, there might be an official way to do this.

joelby:
How about a microswitch in the USB socket?

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