UCC28070: Synchronization problems

Hi,

Thank you for the query on UCC28070.

From your specifications of Fsync of 213khz and Tsync=250nS, using equations 9 and 10, we get the value of Rdmx = 15000*(2*0.95-1-0.05325)/213 = 59.63k for Dmax=0.95

There was no issue in synchronization when 62k was used because the max duty cycle clamp (Dmax) can vary between 92% to 98% with a typical value of 95%. But when you increase Rdmx for the same synchronization pulse width such as with 68k and 75k, Dmax theoritically is 1.01 and 1.06 which can create timing issues internally. Hence it is advisible to chose Rdmax corresponding to the max. duty cycle (corresponding to typical value) and stay within 3% of this value for best results.

Also please check the Rrt value corresponding to equation 8. 100kohm seems different from the value of 77kohm as per this equation.

Please let us know if you have any questions.

Regards,

Harish

Hi,
Thank you for your answer.

When Dmax = 0.98, Rdmx is 63.2kΩ.
Does this mean that a resistance value smaller than this value is OK?
Where can I find out about the maximum Duty cycle value that can be set?

Also, please explain why the synchronization function does not work properly when the maximum duty cycle setting is set too large.

I think the general idea is that the frequency is not involved, just that it cannot be expanded beyond the maximum duty cycle limit.

Hi,

Thank you for the reply.

The Dmax can be between 0.92 and 0.98 as described in section 6.5 of the datasheet. (standalone case i.e no sync)

But the above need not be true for the case where external synchronization is used. I checked some equations in the design document mentioned below:

So going by the above equations for the case of Fsync =213khz, tsync=250ns the value of Rdmx comes to 61.693kohms and Dmax comes to 0.9026. So max duty cycle is limited in external synchronization case. That is why synchronization does not work when duty is increased for this case. So as given in the screenshot above, Rdmx is strictly restricted to value less than 61.6Kohm.

Regards,

Harish

Thank you for your answer.

Am I correct that for external synchronization, the resistance must be less than or equal to the resistance required by equation (1) above?
Please let me know the source of this equation. Or please submit it as a formal document.

Hi,

Thank you for the reply.

Yes you are correct that the value of resistance must be less than or equal to the number defined by equation 1.

Currently we do not have a formal document. You can use this as reference (these are hand written notes from an old design which I happened to locate but no derivations) together with the datasheet equations (9) and (10) and they should more / less give the same value. 

Regards,

Harish

We should not design with what is not a formal document.
Can I request that it be submitted as a formal document?