LTM4676A; using spread spectrum clock

From the LTM4676A operation point of view, it can track to 1-2kHz rate change. In your application, what’s the switching frequency and the modulation rate? If the modulation rate is higher than 1-2 kHz so that the LTM4676A couldn’t track it. That's why you see the PLL fault.

Switching frequency is 600kHz and generated by LTC6909.

This 600kHz +/-10% with 9.3kHz modulation rate clock are supplied to LTM4676A.

It seems LTM4676A will not be able to track higher modulation rate clock.

Thank you for your help.