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AMC7836: AVDD/ DVDD backflow voltage issue

Part Number: AMC7836
Other Parts Discussed in Thread: TPS7A7001

 hi team:

 My customer uses AMC7836 device and found that AVDD/DVDD has some backflow voltage causing  LDO(TPS7A7001) shutdown. The backflow voltage reach to -0.2V. if there any comments about this case?

 here are some waveform and schematic show as below:

LDO output waveform: (if disconnect the ADC ,then LDO return to normal)

 LDO Schematic:

two ADCs shematic:

  • Hi Allen,

    Can you explain a bit more? What supplies inputs are provided to the AMC7836 during this event? Are you disabling the supply input to the LDO?

  • hi Paul:

     As we can see as above SCH, the supplied inputs are 3v3 to IOVDD , 5V to AVDD/DVDD and -8V supply for DAC. if there any power on timing requirement of this device?

     Base on this SCH block diagram, the AVDD/DVDD have some backflow voltage cause the LDO fault, and no output to this ADC. if i disconnect the AVDD/dvdd ,the LDO will work normally.

  • Hi Allen,

    We address that the when the AVSS/AVEE supplies are provided first, there can be a negative bias on the AVDD/DVDD supply.  This might cause the "backflow" you are seeing. 

    We recommend that AVSS/AVEE is the last supply provided during supply sequencing. 

    Thanks,

    Paul