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TUSB8044A: vdd before vdd33

Soppera Francesco
Prodigy 40 points
Part Number: TUSB8044A

goodmorning, i will use the TUSB8044A in a  pcb design but there is one thing that is not completely clear to me.

in the datasheet this is written: 

"There is no power-on relationship between VDD33 and VDD unless GRSTz is only connected to a capacitor to GND. Then VDD must be stable minimum of 10 µs before the VDD33"

and also: 

"The 1 µF capacitor on the GRSTN pin can only be used if the VDD11 supply is stable before the VDD33 supply. The depending on the supply ramp of the two supplies the capacitor size may have to be adjusted.".

so far so good, but in the evaluation board schematics there is nothing like a delay circuit or anything, moreover vdd regulator input is actualy connected to the vdd33 regulator output, so for sure vdd  is stable after vdd33. it could also be that since the two regulators are different (with different turn on time ) this condition can be somehow accomplished, but can you clarify it?

  • 0
    TI__Guru 60870 points

    Hello Soppera,

    The notes in the datasheet are included so that users are aware of the dependencies of the voltage ramps and GRSTz.  The concern is that if a customer is using the passive reset circuit: external capacitor on GRSTz and internal pullup to 3.3V, if 3.3V ramps before 1.1V the pullup on GRSTz will already have pulled to 3.3V before the 1.1V rail turns on and the device will not get a proper reset.  On the EVM, the 3.3V and 1.1V rails ramp almost simultaneously and we verified the passive reset timing was not impacted.

    Regards,

    JMMN