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UCC27537: UCC27537 sinking current/impedance during UVLO, Enable function clarification

user434532
Prodigy 20 points
Part Number: UCC27537

The UCC2753x datasheet calls out the feature "Output Held Low When Inputs are Floating or During VDD UVLO".  However, nowhere is the behavior specified.  Ideally, the datasheet should provide a curve that shows sinking current vs. Vdd for voltages below the UVLO threshold, assuming this function is self-biased, and not a simple resistor.  Please provide whatever info is available that would allow one to determine whether this driver has sufficient sink capability to ensure that the driven device remains off when it is subjected to high power up dv/dt.

Additionally, the Enable Function description, diagrams, and truth table are inconsistent.  Section 9.3.3 states "If desired, the Enable can also be driven by low-voltage logic to enable and disable the driver."  However, it appears that the driver is never actually disabled (there is never a case when both high and low side devices are high impedance).  Rather, it appears that all variants pull down when the device is not enabled - please confirm.  

Lastly, the truth table for the UCC27537 appears to be wrong.  The chip block diagram indicates that the second to last row (both enable and input high) should result in a high output - please confirm.  Note that another poster requested clarification on this, but the question went unanswered.

  • 0
    TI__Mastermind 22285 points
    Hello,
    Thank you for the interest in the UCC27537. I am an application engineer with Texas Instruments and will address your questions and concerns.

    The behavior of the driver output with low VDD is as follows.
    At very low VDD: The internal high side driver MOSFET has a body diode which effectively clamps the driver output to VDD, the output is clamped ~0.6V above the VDD level.
    At VDD ~1.5V, the voltage available is adequate to start driving the internal low side MOSFET and actively clamp to ground, at this point the driver output will be clamped close to ground.

    The enable function is implemented such that when the part is disabled, the driver output is driven to a low state. In most applications, this is the desired behavior. The function you mention would be a tri state or high impedance output during disable.
    You are correct in the statement of the part is never disabled to a state of high impedance output, but this is the typical implementation of enable or disable in most driver devices.

    For the logic table of the UCC27537. It does appear that row 4 (EN Pin H, IN+ Pin H, OUT Pin L) is not correct. With enable high and the non-inverting input high, the driver output should be high.

    Please confirm if you feel we have addressed your questions.

    Richard Herring
  • 0 in reply to Richard Herring
    Prodigy 20 points
    Thank you for the prompt and detailed response. The part works as required for my current design.

    I have a suggestion for a possible future variant. For IGBT drive applications, it can be necessary to place the driver in a high impedance state (true enable function). One would do this when a soft turn-off function in response to de-sat detection needed. We normally implement the soft turn-off drive with a small N-FET and a resistor. Very few drivers accommodate this. The split output part is good for IGBT apps, and probably the best configuration to include this functionality on.