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This is the DC-DC converter: http://cds.linear.com/docs/en/datasheet/37911fb.pdf

There is a clockout pin in the chip and in the datasheet it says:

CLKOUT (Pin 33): Clock Output Pin. A 180° out-of-phase clock is provided at theoscillator frequency to allow for paralleling two devices for extending output power capability.

This is the block scheme of the chip: scheme

I searched in application notes and example designs but there were no use of this feature in them. I need a clear explanation of the working principle and how to do this.

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    \$\begingroup\$ This extra 180° phase-shifted clock output is probably for interleaving purposes but with such complex controller, the best would be to contact LTC's technical support. \$\endgroup\$ – Verbal Kint Jul 11 '17 at 12:57
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    \$\begingroup\$ I'm not 100% confident, but the description of the eval board suggests you would tie the CLKOUT pin of one converter to the SYNC pin of another to interleave them. Source: digikey.com/catalog/en/partgroup/lt3791-evaluation-board/28331 \$\endgroup\$ – Chris M. Jul 11 '17 at 13:06
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My first reaction was that you should use two of the controllers in parallel and connect the CLKOUT pin of the one to the SYNC pin of the other. It believe it is clear enough:

SYNC (Pin 34): External Synchronization Input Pin. This pin is internally terminated to GND with a 90k resistor. The internal buck clock is synchronized to the rising edge of the SYNC signal while the internal boost clock is 180° phase shifted.

and

Frequency Synchronization The LT3791-1 switching frequency can be synchronized to an external clock using the SYNC pin. The falling edge of CLKOUT corresponds to the rising edge of SYNC thus allowing 2-phase paralleling converters. The rising edge of CLKOUT turns on switch M3 and the falling edge of CLKOUT turns on switch M2.

You basically use the CLKOUT clock as the external clock to which the other controller is synchronized.

By searching a bit online, I also came across this in the LT journal, issue October 2012.

enter image description here

That should clear any doubt!

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