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LM5022: Using the Sync Pin to synchronize LM5022 switchers

Chris_CBC
Intellectual 470 points
Part Number: LM5022
Other Parts Discussed in Thread: LM5122

I have read that someone on this forum is synchronizing several LM5022 switchers using the sync pin. Is there an application note somewhere describing how to do this. I looked to the LM5022 page and didn't see anything.

  • 0
    TI__Guru* 78200 points
    please refer to Figure 13 of the LM5022 datasheet.
  • 0 in reply to Youhao Xi
    Intellectual 470 points

    Right. I did read the datasheet (page 13). There is very little information (none) there about synchronizing multiple switchers.It talks about how to send a sync pulse to a LM5022, but this is not what I would like to understand. 

    Specifically,

    Why would you want to synchronize multiple power supplies? 

    I have read that the LM5122 has a master/slave mode of operation that can have two switchers work out of phase to produce more current or lighten the load on any one switcher. I would use the LM5122, but it is not powerful enough for what I need (according to Webbench).

    Can this be done with the LM5022 as well if external logic is used?

    How is this done on the LM5022?

    If so, how are the outputs combined, does it require special circuitry, or are the output just tied together? 

    Furthermore, would you want them to work in phase or out of phase? 

    For the LM5022 the datasheet says "The external clock must have a higher frequency than the free-running oscillator frequency set by the RT resistor" on first read this seems counter intuitive to me. Why does it need to be faster?

    How often do you need to sync them. Do you have to sync every cycle, or can you sync and them let them run for a while?

    So that was the root of my original question. Do you have a app note that talks about why you would want to sync multiple switchers? Can you combine their outputs? how? What are some example implementations.  

       

  • 0 in reply to Chris_CBC
    TI__Guru* 78200 points
    If multiple converters are not synchronized in a system, a beat frequency can be produced which is unwanted in some systems, then synchronization is needed.

    You can make multiple LM5022 in master slave mode and you can configure them like the LM5122 if you have external logic to have them interleave the switching to lower the input ripple current and output ripple voltage.

    How is done: Use individual clock from your external logic circuit (phase delays between multi clock signals), and feed into the circuit as shown in Figure 13.

    You can tie the outputs together, but make sure only the master one controls the Vout regulation, and for all slaves, ground the FB pins and tie their COMP pin to the master's COMP.

    The external clock must be faster to overdrive the internal clock.

    We don't have an application notes on this, but this is a conventional way done in industrial for many years.

    Hope I answered all your questions.
  • 0 in reply to Youhao Xi
    Intellectual 470 points

    OK, so is the following topology correct?

    And for the sync signals, would the following be correct. The LM5022 as configured has a clock of around 200KHz. So If I produce a clock of say 250KHz and sync the master on the rising edge, and the slave on the falling edge that should work? Using the correct pulse width of 50 to 150nS and peak voltage level (greater than 3.8 V) of course. 

  • 0 in reply to Chris_CBC
    TI__Guru* 78200 points
    The connection is correct.
  • 0 in reply to Youhao Xi
    Intellectual 470 points
    What about the clocking/sync'ing question

    And for the sync signals, would the following be correct. The LM5022 as configured has a clock of around 200KHz. So If I produce a clock of say 250KHz and sync the master on the rising edge, and the slave on the falling edge that should work? Using the correct pulse width of 50 to 150nS and peak voltage level (greater than 3.8 V) of course.
  • 0 in reply to Chris_CBC
    TI__Guru* 78200 points
    Do you mean your master clock pulse is aligned with your 250kHz square wave clock rising edge and the slave with the falling edge? If this is the case, it is okay.
  • 0 in reply to Youhao Xi
    Intellectual 470 points

    One more question. Is it important to sync them every cycle, or can you sync and them let them run for a while?

  • 0 in reply to Chris_CBC
    TI__Guru* 78200 points
    Its up to your performance requirement. Most designs I believe will require always sync if sync is used.