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Requirement for well regulated input for switching controllers and regulators - what does it mean in practice?

Denis Kotlar
Intellectual 820 points
Other Parts Discussed in Thread: LM5085, TPS54560, LM5576, OPA548

I'd like to test how TPS54560 or LM5085 works as possible pre-regulator for a bench power supply project. I found in datasheet for both device (pg.35 for former, and pg. 22 for the later) the following statement: This input supply should be well regulated. In the case of e.g. LM5576 I cannot find such "disclaimer".

I'd like to know what does it mean in practice. Does for example a rectified voltage from the main transformer, and filtered with few thousands of uF elco before Cin that is ceramic could be considered as good/well enough regulated?

Another question is what could be the end result if such input is used as a source?

Many thanks.

  • 0
    TI__Mastermind 31580 points
    Hi Denis,

    This post belongs in the non-isolated dc/dc forum. I will move it for you and one of my colleagues will answer it.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Guru**** 191445 points
    That datasheet statement is a new addition. it generally means that the input voltage should be stable and not contain too much ripple, such as the output of an up stream dc/dc converter. It is intended for exactly the situation you describe. It is up to the designer to ensure that the input voltage is sufficiently filtered. TPS54560 can operate with a significant amount of ripple voltage on the input. There are three primary concerns; you may see ripple voltage on the output at the input voltage ripple frequency as high PSRR is not a feature of TPS54560, if the ripple should cause the input to fall below the minimum operating voltage the device will shut down, wide input voltage swings may cause irregular switching currents and possible instability if not accounted for in the design.
  • 0 in reply to JohnTucker
    Intellectual 820 points

    Thanks John for prompt reply. Does the third mentioned concern could result in switching controller damage? I'd like to use 35VAC torodial transformer output that is rectified and filtered with min. 2x1500u elco and 2x10u ceramic to supply switching controller or regulator that should deliver 3-33VDC (up to 3A) on output for driving an OPA548 based final stage (post-regulator). Does it make sense?