Vybrid Power Sequencing wrt USBx_VBUS

[dougplymale]

Please reference Vybrid Data Sheet, Rev 4.1, 12/2012 chapter 8.1 Power sequencing, Table 26 Power sequencing (Pg. 28 & 29).

I have a couple of questions about this table and notes:

1)  How is "All supplies grouped together e.g. 1, 2, others.  These have no power sequencing restriction in between them." interperted:

     a)  The individuals of the groups do not have power up/down restrictions in reference to the other members of the same group.

     b)  The groups do not have power up/down restrictions between members of other groups

     c)  Other?

2)  In the tower reference design, the USBx_VBUS can be powered outside of the restrictions based on table 26, i.e anytime powered up/down as device bus-powered.  (5V is applied to the device which uses U11 to make the 3.3V.  However 5V is available and up before the 3.3V.)   Is the Data Sheet correct with respect to the sequencing of the 5V USB power?

[naoumgitnik · Accepted solution]

Dear Doug,

1. The "All supplies grouped together e.g. 1, 2, others.  These have no power-sequencing restriction in between them." phrase is only applicable to the power rails inside the same group. [By the way, the easiest way to meet this requirement is to power the entire group from the same voltage source, with possible additional filtering for part of the rails (e.g. analog ones).]
2. The Tower Board design has additional components in the power management section; due to that, the Board as a whole may behave differently than the “bare” Vybrid IC.

Sincerely, Naoum Gitnik.

Later addition:

New details of the power-up sequence will be clarified in the Datasheet:

• There is no longer a sequencing requirement for USBx_VBUS.
• There is still a recommendation to not power USBx_VBUS when the device is in standby mode because there will be a stand-by current on USBx_VBUS of  ~ 300-500 uA. This, however, is only important when USBx_VBUS is powered from a battery. For USB bus-powered devices, this is not an issue as the USB spec allows up to 2.5 mA on VBUS during suspend.

/N.G.