USB_VBUS and USB1_VBUS

hirai — Tue, 19 Mar 2019 12:32:59 GMT

The LPC546xx datasheet says as follows.

When USB1_AVDD3V3, USB1_AVDDTX3V3 and VDD are connected to 3.3V LDO, is USB1_VBUS 5V tolerant?

I have another question about VBUS, If USB1 is only used for host mode, should it be connected to VDD instead of VBUS according to table 5. termination of unused pins?

Re: USB_VBUS and USB1_VBUS

xiangjun_rong — Fri, 24 May 2024 06:28:00 GMT

Hi, Maki,

Q1)When USB1_AVDD3V3, USB1_AVDDTX3V3 and VDD are connected to 3.3V LDO, is USB1_VBUS 5V tolerant?

>>>>>yes, the USB1_VBUS is an digital signal input pin for LPC546xx family, the USB1_VBUS 5V tolerant means that the pin can accept both 5V logic and 3.3V logic signal. When USB1_AVDD3V3, USB1_AVDDTX3V3 and VDD are connected to 3.3V LDO, USB1_VBUS is 5V tolerant.

Q2)I have another question about VBUS, If USB1 is only used for host mode, should it be connected to VDD instead of VBUS according to table 5. termination of unused pins?

>>>>>>If the USB1 is only used for host mode, and the host USB1 connector provide 5V power for the USB device which inserted into the USB1 connector, in the case, I suppose that the VDD power supply(3.3V) is from the 5V power supply via a regulator, the USB1_VBUS signal pin is not used, you can connect to either VDD or GND or float.

Pls refer to section 13.7 in data sheet.

Hope it can help you

Xiangjun Rong

13.7 Suggested USB interface solutions
The USB device can be connected to the USB as self-powered device (see Figure 48) or
bus-powered device (see Figure 49).

"
On the LPC546xx, the USB_VBUS pin is 5 V tolerant only when VDD is applied and at
operating voltage level. Therefore, if the USB_VBUS function is connected to the USB
connector and the device is self-powered, the USB_VBUS pin must be protected for
situations when VDD = 0 V.

PLS note above line.

If VDD is always at operating level while VBUS = 5 V, the USB_VBUS pin can be
connected directly to the VBUS pin on the USB connector.
For systems where VDD can be 0 V and VBUS is directly applied to the VBUS pin,
precautions must be taken to reduce the voltage to below 3.6 V, which is the maximum
allowable voltage on the USB_VBUS pin in this case.
One method is to use a voltage divider to connect the USB_VBUS pin to the VBUS on the
USB connector. The voltage divider ratio should be such that the USB_VBUS pin is
greater than 0.7 VDD to indicate a logic HIGH while below the 3.6 V allowable maximum
voltage.
For the following operating conditions
VBUSmax = 5.25 V
VDD = 3.6 V,
the voltage divider should provide a reduction of 3.6 V/5.25 V or ~0.686 V.

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