UCC2800: UCC2800 Sync operation with UCC28089

Someno-san,

What is the target free-running freq & sync freq?

What is the VDD rail, I assume 12 V since that the SYNC pulse amplitude shown?


The size of the required sync pulse will depend on the freq difference between the free-running osc and sync freq. The free-running freq (+ max tol) must always be < sync freq (- max tol), the SYNC cannot work if it is lower than free-running freq.

Since the osc ramp goes from 0.2 V to 2.65 V (see fig. 16 of UCC2800 datasheet), the SYNC pulse must elevate the RC pin above 2.65 V in order to terminate the current cycle. A new cycle can start once the SYNC pulse is removed, and the internal pull-down can pull the RC pin back down to 0.2 V.

Assuming that the free-running freq is 80% of the sync freq as recommended, the RC pin will be ~ (0.2 + 80%*2.45) = ~2.16 V when the SYNC pulse arrives, so the SYNC pulse magnitude must be > (2.65 - 2.16) = 0.49 V. SYNC pulse 1-V amplitude is recommended to give margin for when free-running freq is at min tolerance but the sync freq is at max tolerance.

If there is a larger difference between free-running and sync frequencies, a larger SYNC pulse amplitude is required. If the SYNC pulse amplitude is very large, the only concern is that it should not increase the RC pin above the recommended max level of 6 V.


I reviewed your circuit and it should work ok. However, my only concern is that the current out of the SYNC pin of UCC28089 is quite high, ~23 mA. The UCC28089 abs max is 50 mA, and the recommended max is 25 mA.

You could consider increasing the resistance values of the 50-ohm and 470-ohm to 100-ohm and 1-k - this will still give 1 V SYNC pulse at the UCC2800 RC pin, and will limit the UCC28089 SYNC source current to ~11 mA. The only penalty of increasing the 50-ohm resistor is that the internal osc of UCC2800 will take slightly longer to discharge the timing cap Ct, which will increase the dead-time slightly and reduce the Dmax slightly.


I hope this answers your question, if so please click the "verify answer" button.

Thansk,
Bernard