LT8362 SEPIC supply: Poorer regulation when externally clocked from an LTC6908-1

Hi

I was wondering if someone at Analog could advise on the following.

I am currently designing a 24V 0.2A SEPIC supply utilizing an LT8362 for the effect.

The SEPIC forms part of a multi output SMPS design featuring out of phase sync lines feeding every pair of PWM switchers towards achieving lower EMIs.

The out of phase syncs are generated by two LTC6908-1 spread spectrum clocks operating at different frequencies. 

CASE 1:

When using the int osc, i.e.  MODE pin NOT synced to an ext clock, the simulations on LTspice show a flat, well regulated 24V output with good ripple (about 60mV or so) across a voltage range of 20 to 60V. This is my baseline in terms of output quality.

CASE 2:

However when syncing the LT8362 directly from the LTC6908-1 device, the sims show the output varying as much as 4V across the 20 to 60V range i.e about 19.5V at 20V in, and 23.7V at 60V in, which from a requirements perspective is not acceptable. 

CASE 3:

Found a possible workaround which is to feed the ext sync from the LTC6908-1 to the MODE pin via a 10k/100k voltage divider. 

I am however uncomfortable with the idea of commiting an "unknown" to a design to be commercialized without fully understanding its operation/implications. The sync is reestablished without the output drop yet I am none the wiser why it seems to work?! 

Given that the datasheet doesn't offer much added detail on the internal circuitry and/or examples on how to drive the MODE pin from an external clock source apart from a straight wire between devices, I would like to ask for some more clarity on the best way to interface the two ICs together as they are both Analog products supposedly designed to work well together (I would kindly refer you to the TI TPS54160 datasheet as an example where a similar application is comprehesively illustrated).

Thanks in advance for your help

Fernando Fonseca

Senior Hardware Engineer.

  • 0
    •  Analog Employees 
    on May 28, 2019 2:13 PM

    Hello, Mr Fonseca,

    The application you have described should not be done with the LT8362. This converter has a switch voltage rating of 60V. A 24V output with a 60V input will exceed the switch rating by at least 24V. In a SEPIC application Vsw=max Vin+Vout.

    I would recommend you limit the input voltage to 30V in order to allow some switch voltage margin. If that is not possible, then, consider using the LT8361, a converter with a 100V rated switch. 

    If you decide to use the LT8361, please ensure Vin stays within 60V, as this is the max Vin allowed.

    Then, if you still need support with syncing the converter, please post your schematic and sync frequency details, amplitude and frequency.

    Cheers!

  • Hi dulcevida,

    Thanks for your quick reply and yes, you are absolutely correct. I should have spotted Vsw exceeding the 60V rating!

    The LT8361 seems to work fine under the same design limits (24V out, Vin_max just below 60V to a Vsw of about 85V). 

    As for sync issues, so far so good. There is no loss of output when syncing the 8362 straight from the LTC6908-1.

    Again, top reply! Cheers!

    Nando