The PCB Board having LMZ31520 IC was kept for burnin of 192 hours.The IC was functioning properly till the end, but suddenly failed(non output). Please find the attached waveforms captured at the PH Pin of the LMZ31520 IC. Graph1 is for the IC having ouput voltage issue, and Graph2 corresponds to the working IC. As per the observation, Oscillator frequency in the working IC is 500KHz which is in accordance with the IC datasheet whereas in the IC with ouptut issue the internal oscillator frequency is varying. Also with changing the resistor between V5V and FREQ_SEL pin in the failed IC,waveform at PH pin not changing(i.e. Switching frequecny). Kindly help me out with issue
Note : Verified the input at PVIN pins, V5V Pin output,and the resistor values between VADJ and AGND,SS_SEL to PWRGD resistor value and all were found Ok.
Graph 1: IC having output issue (Captured @ PH Pin output)
Graph2 : Working IC(Captured @ PH Pin Output)