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LTC7149 behavior

Hi everyone. I have some problem with the LTC7149. In the picture there is the SW to Vout-=-5V (red) pin and the Vin=6V to SW pin (blu) The RT resistance is 249k so the fsw =400kHz. However the bottom switched is turned off before the next clock cycle!!! The load is 0Amp. However when I rise the load nothing change until about 500mA. With this load the SW to Vout=-5V became the white in the picture. Why with low load there is such behavior?
PS: the time division is 1us.