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Thread: External VAREF of TLE9879

  1. #1

    External VAREF of TLE9879

    There is a scenario as follows:
    An externel reference voltage is supplied to pin VAREF (within the max Parameters, that is: -0.3V < V[AREF] < V[DDP]+0.3V). Then the TLE9879 is (re)started with VAREF already being present.

    Question1:
    What happens if the internal reference is enabled (by default or by software) during an external V[AREF] is supplied to the VAREF pin? Will that damage the TLE9879?

    Question2:
    Is the internal reference enabled or disabled by default after starting the TLE9879 (or after restarting the TLE 9879 after reset)?

    Question3:
    Is it necessary (e.g. in order to avoid damage) to develop a logic which assures that the external reference voltage is only switched to the VAREF pin once the internal voltage is switched off by software?

    Neither the datasheet nor the user manual nor the manuals to the eval boards provide a clear answer to my questions. Or did I miss something?

    I could rephrase my three questions as a single question: What are the absolute maximum ratings for an external voltage supplied to VAREF in case the internal supply of 5V to VAREF is enabled?
    Last edited by werner_d; Mar 7th, 2018 at 10:22 AM.

  2. #2

    Infineon Employee
    Infineon Employee
    Mc-Key will become famous soon enough
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    Hi werner_d,

    I have one question to your setup:
    Do you use VDDP or VDDEXT as the external voltage reference for VAREF?

    A1:
    The answer to your question 1 depends on your connection between the external voltage and the VAREF.
    The VAREF Pin has two safety functions for protection.
    1st: hardware overload protection (VREF5V_OVL_STS Flag)
    2nd: the post-processing unit of ADC2, monitors VAREF and can trigger interrupts for overvoltage and undervoltage conditions (ADC2 CH6).

    A2:
    In reset condition, the internal VAREF voltage regulator is disabled.
    VAREF regulator is enabled by default, after reset.

    A3
    VAREF can be disabled in the user code, during initialisation phase.
    If VDDEXT is used as an external reference, the switch on sequence can be controlled without external circuitry.

    The electrical characteristics of VAREF are mentioned in chapter 29.9.1 in the datasheet.
    The input resistance at the VAREF PIN is intended protect the internal regulator from beeing damaged by external supply.

    Best Regards,
    M.K.
    The views expressed here are my personal opinions, have not been reviewed or authorized by Infineon and do not necessarily represent the views of Infineon.

  3. #3
    New Member New Member haryy-th is on a distinguished road
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    Hi M.Kilian,
    I was the original requestor for this questions.
    To answer your question: I am using an external generated reference voltage which I want to feed into pin VAREF.
    If I understand right, there is a 100kOhm input resistance to protect the internal VAREF supply.
    So if I connect an external 5V reference voltage to the pin there should be no damage till SW is able after startup to switch internal VAREF off?

    Thanks in Advance

    Best Regards

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