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CD74HCT4051: S0~S2 and EN ciriteria? at power up

KANJI SHIBATANI
Mastermind 6205 points
Part Number: CD74HCT4051

 

Hello

My customer is using CD74HCT4051 for their new design FY2018.

However, there are problem ( oscillation at COM output  ) for their ES borad in case by case.(cause unknown),

but customer is concerned about their power up sequence  S0~S2, Enable  and Vcc,Vee.

I had attached the schematic and also their problem as ppt file.

 

Q1  Criteria

Do you have criteria ( timing ) about relationship     

between  control input S0~S2 , /Enable  and  Vcc, VEE in case  Power Up( and Down )?      

We can not find the criteria those in data sheet.

 

Q2  Need help as Your opinion 

In customer’s desig n, Enable  ( and S0~S2) start up before VEE goes t- -4.5V. 

Do you think this is problem?

 

Q3  Need help as Your opinion

In customer’s set, VEE goes +1V at a moment.( cause unknown)      

Do you think this is problem?

With my Best Regards

 

CD74HCT4051_power_up.pptx

  • 0
    TI__Expert 3900 points
    Hi Kanji,

    We are currently out of office on the 4th of July holiday. We will be back on the 5th and can help address your question.
  • 0 in reply to Dakotah
    TI__Expert 3900 points
    Hi Kanji,

    1) There is no time sequencing criteria for this part. However, it is always good practice to have your supply pins ramp before your enable pin.

    2) This answer is related to the first. Again, the sequencing should not be an issue but you can rule it out by have an external source control your enable and debugging in that manner.

    3) VEE temporarily goes to +1V. this is outside the Abs Max spec of +0.5V . I would not recommend operating in this manner. However, this would not be my first assumption of why the oscillation occurred.

    Common oscillation problems can occur from unstable switching regulators or op amps with insufficient amount of phase margin. One way to rule out the the CD74HCT4051 is to remove the device , blue wire your signal path, and retest.
  • 0 in reply to Dakotah
    Mastermind 6205 points

    Dear Dakotah-san

    Thank youf your advise precisely.

    Thank you again