Page 4 of AN1792 (below) shows how the ZCD pin of the L6562 PFC controller can be used in making the L6562 operate in Constant Off Time mode.
Do you think that the L6564 can be used in the same way?
The ZCD pin of L6564 is certainly identical but we just wondered is there any hidden function which may mess things up here?
I haven't studied the two parts in depth, but I do see one difference in the ZCD between them.
Although the text descriptions of the ZCD pin are the same in the two data sheets, the block diagrams are different.
Figure 2 of the L6562 shows the ZCD pin connecting to a Disable block, in addition to the Zero Current Detector.
Figure 1 of the L6564 shows no such block.