Post Go back to editing

Two stage +/-15V supply with LT4320 bridges

Is there any plans to support the below signal chain power?

  • (50Hz AC) -> (LT4320) -> (LT3081) -> (LT3045)
  • (50Hz AC) -> (LT4320) -> (LT3091) -> (LT3094)

Preferably with room for large bulk caps in all three stages

The reason is, it is a more or less bullet proof high PSRR setup useful for ultra low noise designs (such as audio) and LTspice is having a hard time simulating it. Justification for two linear regulators is that the first takes care of attenuating voltage ripple allowing it's input cap to be relatively low value to decrease current spikes and increase bridge conduction angle while at the same time ensuring LT3045/94 Vin max and tracking 3V above the output by stacking VSET on top of VOUT. Using a switcher as preregulator would mean a stiffer input and it's duty cycle would be modulated by rectified mains frequency which has increased risk of traveling where it shouldn't due to the higher switching frequency.

  • Hi! Thanks for the question.

    Are you asking if there are any plans to incorporate the LT4320 and LT3081 into the SCP portfolio in the near future, or is this an applications question about the performance with those parts in that order?

  • Hi James

    My pleasure.

    My question (here) is whether they will appear in the SCP portfolio to facilitate others building such a design. I added the particular Apps info to provide background and reason why SCP should contain such building blocks (also quad LT3045/94 in parallel - please). Adding those blocks would also remove the somewhat rigid "must contain a boost to provide negative output" limit which one of your other EZ questions highlighted.

  • Thanks for clarifying. The "must contain a boost" is only for the dual output converter option in SCP Configurator. One can always choose a positive and negative rail in single converter/rail mode and the software will independently pick the appropriate single output topology. 

    The LT3045-1 in quad configuration is available as a standard demo board. The SCP single output board standard form factor will not support a quad output. The LT4320 has its own version of support boards modeled after the SCP hardware evaluation platform. Please contact your local FAE for information regarding these boards. 

  • That would be myself (I'm the local FAE)...

    I get it how i can have SCP draw the positive rail albeit not with the parts I want. So far so good, but I don't get how to have SCP accept an external negative voltage and regulate it to e.g. -15V. I get the error "Input Voltage must be positive or >0" when trying to input -18V

  • I understand the issue now. Yes, the SCP program anticipated a positive input for nearly all cases (a dual input directly from a bridge was not one of them). Most AC-DC cases are a PFC flyback of some kind that would be a positive intermediate bus. This is a special case. We don't have plans to add a negative buck (with associated level shift components) any time soon, but it is possible to do what you want.

    The 1x5 and 1x2 breakout boards both support a negative input path on the connector. You can take any standard 3 pin header and connect to the output of the bridge boards. Make sure to follow the SCP polarity standard. You can plug this directly into the 1x2 breakout board and then modify and plug in the LDOs directly. We don't have the LT3081, but You might try the same trick modifying a DC1870A/DC2276 combo using the same method (with pins offset for clearance) to test the concept.