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LT8705A SYNC external clock

Hello,

I am planning to use two LT8705A switchers in parallel, consequently I would like to SYNC them. I would like to use LTC6908-1 as external clock generator for both of them.
Because this clock will be provided by external device, my question is - is any problem if  clock is provided also if there is no Vin voltage on switcher regulators (clock is providing before Vin voltage is provided)?

Many thanks.
Dejan.

  • Hi, 

    Is the reason why you want to use two LT8705A in parallel is to achieve a higher output current? There is no parallel operation mentioned in the datasheet of LT8705. However, it mentioned something about the master and slave configuration which is the usual case for parallel operation. As per the datasheet, this is done by connecting the CLKOUT pin of the master to the SYNC pin of the slave. Note that this is not an active power sharing and you can't achieve a perfectly balanced output current for the two switchers. Therefore, in doing this configuration, each power supply must be designed to have an output power that is higher than 50% of the load.

    If you use the LTC6908-1 as an external clock for two switchers, then you will not know between the two which is the master or the slave. If you really need to use LTC6908-1, then you can try to connect it to the SYNC of the Master, and connect the CLKOUT of the master to the SYNC pin of the slave. The datasheet in page 20 tells more about the SYNC and the CLKOUT pin.

    Regards,  

  • Hello,

    I would like to connect them in parallel because I would like to charge the same battery from two separate (an different oriented) solar panels. Anyway I'm planing to control both switcher with external MCU and D/A converter...
    Is it important which switcher is master or which is slave?
    In case when just chain of solar panel is connected (other switcher is not active) there will not be master and slave switcher...

    In datasheet is mentioned parallel operation on page 21:
    The CLKOUT pin can be used to synchronize other devices to the LT8705A’s switching frequency. For example,
    the CLKOUT pin can be tied to the SYNC pin of another LT8705A regulator which will operate approximately 180°
    out of phase of the master LT8705A due to the CLKOUT phase shift.

    If I connect them outputs in a parallel, do I need any special filters (maybe LC type) on each output, or this is not needed because they work 180° out of phase?

    Thanks!

  • Can't you just connect the two solar panels (with reverse protection device) in parallel to supply power to a single LT8705A? 

    It is important to have a master and slave configuration when you are doing a parallel operation, because the latter has to follow the former for equal division of power to the load. You will also have lesser ripple voltage by having 180° phase shift between the two converters. You may need additional LC filters in the output as it can further reduce the ripple and avoid noise into getting from one supply to the other. In parallel configuration, you may also need common compensators for the two converters.

    Regards, 

  • I'm not completly sure if I will achieve higher efficiency of conversion if I use two separate LT8705A or if I just use one LT8705A (and connect solar panels as you suggested). What do you think?

    I understand now - with LTC6908-1 the both switcher will be driven by external clock, so no one switcher would not work as master from this view....But because LTC6908-1 generates a 2 clocks which are 180° out of phase, can I say that one switcher works as master and another as a slave? Or is it necessary that switcher generate a clock for another switcher to say that this switcher is a master?

    How the master/slave combination works in a solar (MPPT) applications? I can not imagine how the division of power to the load "works" here, because with each MPPT you would like to achieve the best efficiency as possible (at any time you would like to convert as much solar energy in to electrical energy as possible)?

    Thanks!

  • I guess you wouldn't need master-slave arrangement isfusing LTC6908-1.

    You may want to check a product that has MPPT feature LT8490 designed for solar panel input.