I want to connect multiple LED drivers in cascade. Is it okay if the SCK waveform becomes shallow?

You may want to reduce EMI or circuit noise by slowly changing SCK, but doing so makes it difficult to meet its setup time requirement relative to SIN. If it is not met, data transfer will not be performed properly.
Be sure to meet the SIN-SCK setup time specified in the Electrical Characteristics table in the datasheet.

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