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TL7700: Tolerance estimation of Output Pulse-Duration Time(tpo)

Shota Mago
Expert 7260 points
Part Number: TL7700
Other Parts Discussed in Thread: TPS37

Hi team,

My customer will use TL7700 for their application and they're asking about tolerance(min-max) estimation of tpo.

According to datasheet, tpo can be calculated as tpo=Ct*10^5 second.

[Questions]

Is tpo influenced by below parameters except for tolerance of Ct capacitance?

Tolerance of ICT (Timing-capacitor charge current)

Tolerance of threshold voltage of Ct comparator

If yes, could you please provide how we can estimate tolerance of tpo including above parameters?

Best regards,

Shota Mago

  • 0
    TI__Mastermind 40675 points

    Hi Shota-san,

    The the TL7700 is an old part.  Is the customer open in using a newer part?  A new part, such as TPS37, would be a good suggestion for them to use in their application.

    Below are my answers to your question:

    Shota Mago said:
    Is tpo influenced by below parameters except for tolerance of Ct capacitance?

    Correct, the changes to the current and the threshold voltage will affect the absolute time of tpo.  For ICT, the spec is shown below where the change of current is +/-26.67%:

    The change of tpo is 50% and if the ICT variation is +/-26.67%, the VCT voltage variation will be +/-23.33%.

    Ben

  • 0 in reply to Ben C
    TI__Expert 7260 points

    Hi Ben-san,

    Thanks for your quick reply! Unfortunately, the customer already decided to use TL7700 and designed some boards. So they can't change this device...

    Datasheet states that "The TL7700 has an internal 1.5-V temperature-compensated voltage reference from which all function blocks are supplied". If we assume threshold for CT comparator is this 1.5V reference, equation(3) can be derived as follows.

    tpo=Ct*Threshold/ICT=Ct*1.5V/(15uA)=Ct*10^5 seconds

    If above understanding is correct, could you please provide rough tolerance of 1.5V reference(not guaranteed)?

    Or can we use ±1% tolerance for the 1.5V reference from Vs(SENSE input voltage) of section 6.5?

    Best regards,

    Shota Mago

  • +1 in reply to Shota Mago
    TI__Mastermind 40675 points

    Hi Shota-san,

    You are correct, I forgot that the change in tpo also includes the variation of the capacitor.  The 1.5V temp compensated voltage will not vary much.  As you mentioned, I would use the SENSE input voltage variation (1%) and apply that to the VCT voltage.

    Ben

  • 0 in reply to Ben C
    TI__Expert 7260 points

    Hi Ben-san,

    I understood. Thank you for clarification!

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Mastermind 40675 points

    Hi Shota-san,

    No problem!  Glad that we were able to provide answers to the customer.  Good luck to you!

    Ben