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TPS7B7702-Q1: Faults and impact to VIN Pin

Matthias Terhorst
Expert 5140 points
Part Number: TPS7B7702-Q1

Team

page 12 in the datasheet of the TPS7B7702-Q1 shows the Fault Table (Table 1). How does each of the listed failure impact the input IN (pin 1) of the device ? Do you see other failure that have an inpact to IN (pin 1) ?

Best Regards

Matthias

  • 0
    TI__Guru 55940 points
    Hi Matthias,

    Any impact to the input rail for the LDO will largely depend on the power supply which is powering the TPS7B7702-Q1.

    Open load and Normal operating conditions should have no adverse impact to the Vin rail.

    Overcurrent and Short-circuit or current limit may trigger the upstream power supplies current limit or may look like a load transient to the upstream power supply. The current limit for TPS7B7702-Q1 will limit the current flowing from IN to OUT in order to protect downstream (load) devices.

    Thermal shutdown will look like a load transient to the upstream power supply as the LDO output turns on and off.

    Output short-to-battery and Reverse current should not have an adverse impact to the upstream power supply due to the TPS7B7702-Q1 reverse-polarity and reverse-current protections.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Expert 5140 points

    Hi Ryan

    thanks for your response and I see that the discussion is going in the right direction. Let´s have a more complex view on the failure modes of the device. 

    1) Output Short2Bat Condition at the Output of the TPS7B770x:

    As you already described, the device will protect the input from a S2B condition. I suppose the protection circuit will have a certain delay. What happens with the input when a S2B condition happens within the delay-time (means the protection switch is not yet turned off) ? What is the delay time of the protection circuit ?

    2) Output Short to GND

    Similar to 1) what happens when a hard short to GND happens  at the output but the protection circuit is not yet ready to limit the current ? What is the delay time of the OC protection circuit ?

    3) Reverse Current

    Similar to 1) what happens when a reverse current happens but the switch is not turned off because of the delay time ? What is the max. reverse current that might occur and how does it impact the imput of the device ? What is the delay time of this protection circuit ?

    4) Open Load and Normal Mode

    I suppose this should be fine and no impact to IN is expected

    5) Any other failure mechanism that might influence VIN ?

    Many thanks for your support in advance and I´m looking forward for your response.

    Best Regards

    Matthias

  • 0 in reply to Matthias Terhorst
    TI__Guru 55940 points

    Hi Matthias,

    The Short-to-BAT detection time is specified in the Switching Characteristics table as a typical of 5 us.  During this glitch time, it is possible for the LDO to be on.  As the title of this fault implies, before the protection circuitry engages, the input and output rails are shorted together.

    If the output is shorted to GND, the current limit for the LDO will limit the current.  As nothing in the real world is instantaneous, there will be a short delay before current limit engages.  This is also what allows inrush current to occur during startup.  This delay is not characterized; however, it will be similar to the short-to-BAT deglitch time.  Since the LDO output current is equal to the LDO input current minus Ignd, the increased output current will come from the upstream power supply.  How the power supply reacts to this current will be dependent on its current limit.  If the upstream supply cannot source the current, the input voltage will droop.

    If the output voltage exceeds the input voltage of the LDO, reverse current is possible.  The blanking time for reverse current detection is typically 16 ms.  During this blanking time, reverse current can flow in the OUT pin and out the IN pin.  This current will flow into any capacitors on the IN rail and raise the input voltage.  How your upstream power supply will react to this will again be dependent on the upstream power supply.

    All fault conditions with the LDO should exhibit as either additional current flowing into the IN pin of the LDO which would lower the input voltage rail voltage if the upstream power supply cannot source the additional current or it will exhibit as current flowing out of the IN pin as reverse current which would increase the input voltage rail voltage.

    Very Respectfully,

    Ryan