# Recommended bulk capacitance used for the VDC_5V_LOCAL_USB rail for a USB 3.0 device?

Source: https://forum.modalai.com/topic/2433/recommended-bulk-capacitance-used-for-the-vdc_5v_local_usb-rail-for-a-usb-3-0-device
Category: VOXL 2 and VOXL 2 Mini (https://forum.modalai.com/category/26/voxl-2-and-voxl-2-mini)
Tags: voxl-2
Posted: 2023-07-17 17:54:40 UTC by Moderator (ModalAI staff)
Replies: 1 · Views: 438

## Moderator (ModalAI staff) · 2023-07-17 17:54:40 UTC

Do you have a recommended bulk capacitance used for the VDC_5V_LOCAL_USB rail for a USB 3.0 device? I experienced a brownout issue when connecting a USB 3.0 ethernet adapter and I'm trying to figure out what caused it.

## Reply by Vinny (ModalAI staff) · 2023-07-17 19:44:53 UTC

@Moderator 
That port is capable of providing 2A at 5V from a SW controlled GPIO (GPIO_157).
I'm surprised you would experience a brown-out from a USB to ETH adapter.
On Voxl2, we provide a 10uF and a 1uF cap right at the 60-pin J3 connector.
If you are doing a custom adapter, and to meet USB3 specs, you should not need much more capacitance.

However, if you have a long length before your mating connector, a good 10-22uF would be wise.
We provide a 100uF cap on all of our USB ports that mate to Voxl2 (M0090 5G Modem Plug-in card, for example), but it is also due to a  USB switch to help manage the ON time during enumeration.

Back to your specific issue, I would double check you are not toggling GPIO_157, or that your connection is solid and not intermittent. 
I cannot find specific ETH dongle power consumption specs online, but it is likely using a LAN7801 or similar type device plus a PHY, which would not exceed more than a few hundred mA combined with magnetics:  
![a50456c1-db0d-4702-ab52-693715163dfb-image.png](https://forum.modalai.com/assets/uploads/files/1689623049484-a50456c1-db0d-4702-ab52-693715163dfb-image.png) 
![b4d29526-d5e9-4dff-aeb6-760e4808daa0-image.png](https://forum.modalai.com/assets/uploads/files/1689623071606-b4d29526-d5e9-4dff-aeb6-760e4808daa0-image.png) 

So a brown out does not seem likely here. Even a 500mA USB2 rated device has no issue powering these types of dongles.