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LM2574: Output Voltage Pin Transient Undershoot Specification

Michael Monaghan
Prodigy 20 points
Part Number: LM2574

On the LM2574 datasheet (page 4) in the absolute maximum specification it has the output voltage to ground steady state voltage rating.

I'm wondering if there is a transient output voltage pin undershoot maximum? For most switching power regulators I've seen use a transistor to switch power and transistors have a maximum Vce rating.

I'm concerned in my design if the output voltage has enough undershoot before the Schottky diode switches on it will break some part of the circuit. Is this generally an issue to be concerned about? I haven't found much specific data on catch diode switching time requirements.

  • 0
    TI__Mastermind 23815 points
    Hi Michael,

    There is no transient output voltage undershoot maximum rating on this part, I believe as long that your output voltage is not below than -1V, you should be ok.

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 20 points

    Hi Arief,

    I'm currently meeting the specification for output voltage to ground, steady state is a max of -1V. However, the time it takes to reach steady state is not specified either.

    I'm a little confused on how the -1V specification is determined. Is it from the switching transistor's breakdown voltage? However, this is just a basic model of a power regulator with external components is there other components embedded in the IC on the output of the power regulator(LM2574) that have a lower reverse breakdown voltage / max voltage rating?

    See the source image

    Thanks,

    Michael

  • 0 in reply to Michael Monaghan
    TI__Guru 57555 points

    Maybe I can be of some help.  We normally refer to the node connected to the inductor as the SW node.  Unfortunately, for this 

    device we call it the VOUT node.  The voltage on the SW node of any buck will go negative during normal operation.  Many times you

    will see a fast spike on this node.  Since this spike is just a transient, it is allowed to exceed the maximum steady state limit in the data sheet.

    The danger from an excessive negative voltage on the SW node is that a parasitic device inside the IC may turn on and cause damage. 

    Unfortunately, many of our older devices do not specify a maximum spike voltage on this pin.  Typically, we can say that a negative spike of a 

    few volts should be OK if it lasts for only 10s to 100s of ns.  But of course we can not grantee that.

    The other issue is allowing the output voltage (across the load) to fall below zero volts during normal operation.  If this voltage becomes more negative 

    than the diode voltage, then the inductor can not discharge during the switch off portion of the cycle.  In this case the inductor current may run away; the current limit would not be able to protect the device or the diode.  If this negative output voltage is a transient, then it may not be a problem.  However, if some kind of active load tries to pull the output voltage below ground for many switching cycles, then this may become a problem.