Why does USB power need 4.4V min for LDO to 3.0V??

[dluberger]

I want to have all power connections to the processor to be downstream of a single fused/regulated supply line. The only peripheral for which this seems to be a problem (based on the sabre board schematic) is the USB_OTG1_VBUS (I'll only be using one USB port to serve as a mass storage gadget).  The datasheet for the processor says that the internal regulator "LDO_USB" creates 3.0V, presumably for peripheral logic and comm signal generation (though I thought USB signaling is 3.3V differential); however the datasheet also shows that USB_LDO input (using either USB_OTG1_VBUS or USB_OTG2_VBUS) has a supply input range of 4.4V minimum. Why is that?? This creates a problem for me because I don't want to use the 5V usb power in from the microB connector, and everything in my power "island" is 3.3V max. It would be way better for my design if I could simply tie USB_OTG1_VBUS to my 3.3V supply, along with my other peripheral supply power inputs (such as NVCC_SD, etc.).  Why is USB_OTG1_VBUS at 4.4V minimum? Is this simply stating the USB standard, and therefore the actual LDO input can be lower? I realize I can cut and jump on the sabre board to the 3.3V supply to test it out, but I thought I would ask here first. I need USB power draw to be as low as possible, and I cannot support >4.4V at 50mA max draw, and I only have 3.3V max available on my design.

[CarlosCasillas · Accepted solution]

Hi David,

The USB module could operate at 3.3V without requiring the external 5V (VBUS) power supply for using the LDO_USB. However, it would be required to configure the USB module as Device (only Device mode) as self-powered.

The issue may happen when it is intended to have USB Host or USB OTG functionality, as it is part of the certification providing VBUS voltage to external Devices, with the minimum value of 4.4V (although some devices may present malfunctions due to the lower voltage level).

So, if you don’t want to have 5V on your design, please ensure powering the USB modules without requiring the LDO_USB, and configure the USB module (from driver side) just as Device, also modifying the descriptors to be declared as a self-powered device, and also considering the proper hardware for Device-only operation.

For complementary related information, you could take a look to the following Community thread:

https://community.nxp.com/message/968013

 
Hope this will be useful for you.
Best regards!
/Carlos
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[dluberger]

Thanks. I suspected the datasheet was written with the 4.4V min in order to meet requirements as a host. I will only ever use the USB port as mass storage device.  I'm not planning to make my own drivers for the device, and testing using the g_mass_storage module works fine with USB_OTG1_VBUS connected to 3.3V.