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LM5146: Maximum current in to SS/TRK clamp

Liam Powell
Prodigy 40 points
Part Number: LM5146

Tool/software:

The SS/TRK pin on the LM5146 contains clamping circuitry to limit the voltage to 115mV above the FB pin voltage. What is the maximum current that this clamp can handle? We are planning to connect SS/TRK to a DAC to create a variable voltage supply so we need to size an input resistor according to this limit.

  • 0
    TI__Guru 72782 points

    Hi Liam,

    The SS/TRK pin will not consume a DC current if the voltage doesn't exceed 915mV (800V FB reference plus the offset).

    By the way, another option here is to couple a current into the FB pin to adjust the VOUT setpoint. This is done with an adjustable voltage source coupled through a resistor into FB.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 40 points

    Is this also the case if FB is not at 800mV? We have a wide input voltage range and have selected the FB resistors such that 800mV corresponds to the maximum input voltage, therefore the SS/TRK pin may be at 800mV while FB can only reach 400mV in some scenarios.

    Using SS/TRK means that if our MCU resets then the DAC output will go to 0V and the regulator will shut down, as opposed to current injection where the regulator would go to its maximum output voltage. We could also connect the MCU to the EN pin or add some extra circuitry, but using SS/TRK seems like the lowest cost and simplest option if the clamp won't cause issues.

  • 0 in reply to Liam Powell
    TI__Guru 72782 points

    The FB voltage is a divided-down version of Vout and should not change with Vin. I doubt you would get complete shutdown if SS/TRK went to zero...probably a very low output voltage with some residual switching. If the DAC voltage went to zero, then yes it would go to the max Vout setting (the injection resistor would effectively be in parallel with the lower feedback divider resistor).

  • 0 in reply to Timothy Hegarty
    Prodigy 40 points

    Below is a simplified version of our schematic. At VIN=24V and close to 100% duty cycle FB will reach 800mV, but at VIN=12V FB will only reach 400mV. Based on the datasheet it appears that the clamp will activate at 515mV in this case. We could constantly check VIN on the MCU, but we'd prefer to have a design where no DAC output can cause damage to the LM5146.

  • 0
    Prodigy 40 points

    I ended up finding this within the text of the datasheet:

    > The clamp circuit requires a soft-start capacitance greater than 2 nF for stability and has a current limit of approximately 2 mA.

    It would be useful to have in the specification tables rather than just the text.

  • 0 in reply to Liam Powell
    TI__Guru 72782 points

    Thanks for finding that in the text.

    Yes, agreed, although some parameters are generally not added to the EC table, as it would then require testing by ATE.

    Regards,

    Tim