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LT3579 synchronization for positive and negative voltages?

Category: Hardware
Product Number: LT3579


I am planning on using two LT3579 DC/DC converters to generate ±17V power rails out of 3.0V - 4.2V input. The datasheet describes that the converters can be synchronized via CLKOUT and SYNC pins, and shows an example where converters can work out of phase to generate higher output voltage and current. 

In my application I am using a dual inductor inverter topology and a sepic converter topology to generate the power rails.

1) Is there any benefit in synchronizing the converters in this case?

2) If yes, the only line that needs to be connected to achieve this is the CLKOUT from the main to the SYNC pin of the subordinate?

3) As the converters turn on, they sip quite a lot of current. To limit the current, I could turn them on sequentially. I assume I need to fire up the main first and then the subordinate? And when powering down, the subordinate first and then the main?

Please see my suggested schematic below.


  • If you don't synchronize the converters you can get a beating effect where frequencies that are close to each other produce an audible noise when the difference in frequencies is in the audible range. This can also adversely affect other circuitry. All of your other statements are correct including how to synchronize the two supplies. You don't need to sequence them if you add a ~47k resistor on the sync pin to gnd of the slave converter. The slave will then startup using the RT resistor and when the CLKOUT clocks the SYNC pin it will then synchronize. The SYNC frequency can always be higher than the free-running oscillator frequency (as set by the RT resistor), fosc, but should not be less than 25% below fosc.

  • Thank you! To the point answer. 

    One thing I forgot to ask is that should one of the converters be of the LT3579-1 model, instead of the LT3579 (without -1). Is there any benefit to this? As far as I understand the only difference between these models is that the CLKOUT is 180 degrees out of phase. Would this help to even out the momentary current consumption or does it not matter?

  • Yes, using the LT3579-1 to feed the second LT3579 or second LT3579-1 will have them operating 180 degrees out of phase. This will reduce the peak input current and will help with EMI.

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