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LM34927: External soft-start circuit

Part Number: LM34927

Hi, team

My customer would like to add an external soft-start circuit to LM34027 as the datasheet shows. And I have done a simulation in TINA, the results are shown below. 

According to the waveforms, it seems in the soft-start period, the FB up and down between the Vref. And the slope of VB decreasing is slowed down.

I am wondering that in the soft-start period, the diode from VB to FB continues to being forwarded, so that VB and Vout contribute to Vfb at the same time?

If R11(from VB to GND) is small enough,  VB decreases very quickly. When VB reaches Vfb+VD, the diode seems to be reverse-biased immediately, then soft-start cannot be achieved. 

Is this the reason why R1 should be larger than others?