LTC3872 question

Hi, I have a question about LTC3872: The carrying load ability for the output of the LTC3872 circuit is not good, I do not know what is wrong with my LTC3872 circuit configuration? can ADI engineer help me? And I will be very grateful!!!

1circuit configuration

Mosfet: Infineon BSZ018N04LS6 key performance parameters as follows

2Test result

1no load model, the circuit of LTC3785 working normally, and the output voltage is 15.5V

2the circuit of LTC3785 carrying 100mA load and it is working normally, the output voltage is 14.5V

3when the circuit of LTC3785 carrying 500mA load, it is operate abnormally. The output voltage is about 4.2V. the abnormal condition as follows:

(3.1)the voltage of run pin is about 880mvoscilloscope measurement error, the voltage of run pin maybe below 0.85v.according to LTC3872 datasheet, With the RUN/SS pin

below 0.85V, the chip is off and the input supply current is typically only 8μA. But my gate  pin still have drive waveform? What is wrong with the LTC3872 circuit?

(3.2) the voltage of run pin depends on the input voltage of LTC3872, the input voltage is still about 4V,but the voltage of run pin is about 880mvoscilloscope measurement error, the voltage of run pin maybe below 0.85v)?What is wrong with the LTC3872 circuit?

(3.3) according to LTC3872 datasheet, with the Iprg pin is gnd, the PK limit current of input is 70mv@LTC3872I. the rds(on) of mosfet is 1.8mΩ. so the PK limit current is about 38A. therefore, the circuit of LTC3872 operated abnormally not because of current limit.

(3.4) according to LTC3872 datasheet, I configured the RCR network of Ith PINbut the circuit of LTC3872 still operated abnormally.