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TPS54231: Non-liner output response

Part Number: TPS54231

I use TPS54231 for two buck switcher designs  :5V-to-1.2V and 5V-to-1.0V .There have same problems.

Below is the output. Green is 5V.Yellow is ouput.Pink is the SS pin.Blue is the PH pin.

Below is the schematic.

Step1 :I noticed the SS capacitor is too big.But the article on the forum told that is ok. I change the capacitor to 22nf. The problem remains.

Step2 :I added the 1 Meg resistor in parallel with the SS capacitor(0.1uf). The problem remains.

If the chip boots up repeatedly and frequently, It will reduce a large overshot.

I wonder if there is a reasonable explanation. 

Thanks.