TPS54620 Vout waveform with soft start function

I will attach TPS54620 Vout waveform depend on SS/TR capacitor value.
When soft start capacitor is put, Vout waveform has flat range during start-up.
Do you have any idea about this cause?

This circuit condition is the following,
-Vin: 5V, Vout: 1.15V
-L: 5.2uH, 22m ohm
-Cout: 100uF (ceramic)
-Fsw: 480kHz

TPS54620 Vout.pptx

Best Regards,
Kohei Sasaki