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TPS62203 - open EN and FB pins

Aaron Chesir
Prodigy 30 points
Other Parts Discussed in Thread: TPS62203, TPS62230

One of the Army requirements before fielding any system that triggers the detonation of munitions is a Failure Mode Analysis. My responsibility is to go through each  component in the circuit, list each failure mode, and describe the likely consequences of such a failure. The goal of the analysis is to show that the likelihood of a failure that results in a premature or unpredicted detonation is not more than 1 part in a million.

The design whose Failure Modes I am analyzing contains a TI switching DC/DC converter: TPS62203. I need to know how the device will behave if either of the following occurs:

  1. Pin 3 open-circuits: The data sheet states only that the ENABLE pin must not be left unterminated. However, I need to understand what happens if the pin is unterminated.
  2. Pin 4 open-circuits: This is the feedback pin. I need to understand what happens if the pin is unconnected.

If the likely consequence of a failure is that the circuit will exhibit a behavior that is apparent to the user as abberant, then the user is trained to discontinue its use, and no detonation will occur. If, however, the circuit (including the firmware being executed by the ARM7 processor on-board) does not exhibit any abberant behavior, then I must document as such, and then calculate the probability of such a failure mode.

All help is appreciated.

Aaron Chesir

  • 0
    TI__Guru**** 151520 points

    Welcome to the e2e forums.  Please read the ground rules post at the top of this forum and re-title your post according to the instructions given there.

    These types of very application specific questions are best directed to your local FAE.  He knows the most about your application and can address your questions most quickly.

    You can test both of these questions on an EVM and see what the part does.

    The EN input tells the part to turn on or off.  If it is floating, the part might turn on and off sporadically as it floats above or below the logic high and low levels.

    The FB pin is the input to the control loop.  If this is floating, the IC has no input signal to regulate on and thus might output any voltage.  The voltage it outputs would be less than the input voltage.

    Finally, this is an old IC.  There are many newer ones that would likely give better performance, such as the TPS62230 family.

  • 0 in reply to Chris Glaser
    Prodigy 30 points

    Chris,

    It was in fact the folks at the TI Technical Support help line who advised me to post to this forum.

    What you stated above (basically that the outcome is unknown) is implied by the data sheet and the diagram in the data sheet. I really need an explanation from someone familiar with the part about what is likely to happen if either of the two pins are left floating.

    The circuit, as designed, was given to me. All I can propose as alternatives are drop-in-compatible replacement parts. On the other hand, if you can recommend a drop-in-compatible replacement part for which the answers I seek are explicitly documented, that would be a viable option.

    I appreciate the fact that you tried to help.

    Aaron

  • 0 in reply to Aaron Chesir
    TI__Guru**** 151520 points

    I described what happens if you float either of those pins.  If you leave inputs to the IC floating, you don't know whether they are read as high or low to the internal logic.  It also depends on external circuit's noise coupling into the pins--if any noise in the system drives them high or low.  You need to determine what that means to your system's operation.

  • 0 in reply to Chris Glaser
    Prodigy 30 points

    Chris,

    I accept what you state for the ENABLE input, but not for the FEEDBACK input. According to the drawing of the internal device architecture in the data sheet, if the FEEDBACK pin is left floating, then resistor R2 should make this input appear to the compensation network and the Gm amplifier as always low. What is the consequence? If the device is powered from a 6 Volt battery, does the output P-channel MOSFET stay always active - and therefore the device output voltage ramps up to the supply voltage of 6V, even though the TPS62203 device is internally configured as a 3.3-Volt output supply device? The resultant behavior is not obvious to me, since I do not understand exactly the interaction between the compensation network, the Gm amplifier, the sawtooth generator, the 1 MHz oscillator, the control logic, and the Driver Shoot-Through logic.

    I appreciate your patience and indulgence,

    Aaron

  • 0 in reply to Aaron Chesir
    TI__Guru**** 151520 points

    Ah yes, for the fixed voltage versions, the internal divider will pull down the FB pin which will cause the device to output Vin.