Accessory power with Lightning Camera Adapter: Electrical Engineering question

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Accessory power with Lightning Camera Adapter: Electrical Engineering question

Hi,

I'm an electronics hardware developer. The latest version of my hardware gives a power consumption warning whereas the previous version does not. I'm seeking advice and details on what is actually behind the message "This accessory requires too much power."

The accessory is a class-compliant USB 2.0 audio DAC connected with a USB-C port. It works when I connect a PC. When I connect my iPhone 13 with the Camera Adapter and a generic USB A-C cable, it enumerates and gives the power message.

See below for the tests I have performed. It seems that the error message does not correlate with either DC current on the VBUS line or the inrush current. What then triggers the iPhone to give the error?

The same iPhone, Camera Adapter and A-C cable work perfectly in the previous hardware prototype. The two prototypes use the same firmware. They are both externally powered.

When I replace the iPhone and Camera adapter at the USB-C plug with a lab DC supply at 5V, I get a DC current consumption on the VBUS line of 608µA on the working one. The one with the error message has been tested at 640µA and 297µA. I have a fast current-to-voltage converter that I use to monitor inrush current. The working and failing device are very similar. VBUS capacitance in the working device is 5.6µF. The failing device has been tested with anywhere from 0.9 to 5.6µF. Both devices use the same TVS diodes on USB.

When connected to the iPhone again, the failing device gives the same error message when the failing device is tested in different hardware configurations: both with and without TVS, with and without USB common-mode filter, with the different DC currents, with the different inrush currents etc.

The only practical difference that remains is that the two devices use a different USB data multiplexer. The failing device uses an improved MUX with lower capacitance on the USB power lines.

I have yet to test the devices with a USB hub (powered and passive) and powered USB3 Lightning Camera Adapter.

I wish to make the final product work with the USB 2 Camera Adapter. It is not an option to permanently use USB3 powered Camera Adapter or USB hub.

Best regards,

Børge

Posted on Apr 2, 2024 4:36 AM

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Answer 1

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Borge-san Author

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Apr 3, 2024 3:27 AM in response to Borge-san

Problem solved!

Discovering that it was probably not due to current consumption and VBUS, I explored D+ and D- in detail. It turned out that a USB multiplexer IC was improperly soldered.

On a multilayer circuit board the ground pads . In this case the solder on the pad was hot enough to deform but not hot enough to wet the pad on the multiplexer IC.

[Edited by Moderator]

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Answer 2

Borge-san Author

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Apr 3, 2024 2:52 AM in response to Borge-san

More results are in:

I have measured how VBUS voltage and current behave when the USB 2.0 Camera Adapter is plugged into the iPhone. The DAC was already connected to the CA USB-A port. The VBUS rise is very clean and identical on iPhone 7 and 13. The inrush current looks very similar on the DAC device that works well and the one which reports over-power. I struggle to see how the error message can come from the voltage and current on VBUS.

However, with a USB 3.0 hub connected between the Camera Adapter and DAC, everything works as expected with no error messages on either iPhone 7 or 13. Device performance is great! The hub does have a power input but is not powered in this test.

With the USB 3.0 Camera Adapter and Lightning power cable, everything also works as expected with no error messages. It does the same on iPhone 7 and 13. Device performance is great!

This leads me to believe there is an issue with the data lines. Something other computers ignore, something the iPhone reacts to through the USB 2 Camera Adapter, something that is hidden from the iPhone's attention when there is a hub or USB 3 Camera Adapter in the loop.

Thanks,

Børge

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Accessory power with Lightning Camera Adapter: Electrical Engineering question

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