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LM5574-Q1: Syncing and inverting configuration questions

Part Number: LM5574-Q1
Other Parts Discussed in Thread: LM5574

I need to design a PSU that has several output rails generated from a single 40V unregulated DC input. The output voltages are +/-19V (0.2A each) and +48V (0.01A). I need all rails to switch in sync for minimizing comb frequencies and ease filtering of the supplies (which have to be very quiet). All rails should operate in CCM. It would be very convenient if switchers can be daisy-chained for sync operation.

For +/-19V rails I've found the LM5574 to be suitable. The +19V rail design is pretty straightforward and Webench does a good job with it. The -19V rail is problematic, because Webench generates values that result in DCM operation. I tried changing inductor value, but it looks like Webench can't do that.

Here're the questions I'd like to get answers to:

1. How do I calculate inductor value for inverting buck-boost circuit that will operate in CCM?
2. Should the switchers on all rails switch together or should I employ interleaving? If interleaving is substantially quieter, what are suggested part and circuits?
3. Is there any boost converter that can be synced the same way as LM5574 and that I can use together with LM5574 in my application?


  • Hello

    I think that the LM5574 should be OK for both pos and neg rails.

    I am not sure why Webench calculates DCM; I calculate CCM for your load of 0.2A.

    The best way to be sure is to use the EVM for this device and test.  It is easy to modify the EVM for IBB.

    Also, you will need a level shifter for the IBB.

    I have attached a short application note that may be helpful.

    I can not help with the boost; you will need to search the TI products to help you find a suitable boost.



  • Frank, thanks for your input.

    What inductor value did you assume? Or is it the same as for a buck converter?

    As for level shifter, is the impedance of the SYNC porl low enough to use a simple level shifter? Or should I use the one with MosFETs?

    Any thoughts on swithing timings?

  • Hello

    I use the inductor value from the Webench design.

    Regarding the level shifter, you will have to experiment to find a solution that fits your needs.