MC34708 VBUS pin

[ericmar81]

Hi,

May I know what is the use of this VBUS pin from theMC34708?

When I refer to the reference design of the i.MX53 Quick Start Board, it is connected to the External 5V USB power. Must I connect this pin like the reference design or I can leave it floating and not connected?

Thanks & regards,

Eric

[Yuri · Accepted solution]

  MC34708 VBUS  can serve as input for USB connect detection and as OTG supply
output. It may be recommended to follow the reference design, since during power up

VUSB regulator is only enabled if 5.0 V is present on VBUS (by default VUSB will be
supplied by VBUS). In this sense the VBUS may be considered as one of MC34708
power suppliers.

Have a great day,
Yuri

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[ericmar81]

Yuri,

You were saying "During power up VUSB regulator is only enabled if 5.0 V is present on VBUS".

If the VBUS is connected to External 5V USB power and there is  no USB host connected to the USB connector, does it mean VUSB regulator will not be enabled?

I did a measurement on my board and I found that even when no USB host is connected (i.e. VBUS is not supplied with 5V), the VUSB will still be outputting 3V3 after a while. Is this correct?

I did also try not to connect VBUS to the External 5V USB power, the board is still functioning properly. I just want to be clear on this to see if it is really necessary to connect up the VBUS pin or we can just leave it floating.

Thanks.

Eric

[Yuri]

  A problem of proper power up sequence of i.MX53 may take place, if MC34708 VUSB regulator
is used to supply the i.MX53.

Regards,

Yuri.

[ericmar81]

Hi Yuri,

In such case, I suppose there is no need to connect VBUS to External 5V USB power, right?

If the VBUS is not connected to External 5V USB power, the VUSB regulator will rely on the VINUSB, right?

Thanks & regards,

Eric

[Yuri]

Correct.

Regards,

Yuri.