Lt1242 very simple ltspice simulation not working please help

obiwan

New member
Hi all, I am trying to understand why this simulation on LTSpice of an LT1242 which is a replacement for the UC3842 is not working.
The test consists to apply a linear voltage ramp to the COMP pin to see if the output produces a PWM signal with a variable duty cycle.
It seems to switch directly from 1% to 100% at about 1.18V input.
The same original model of the LT1242 from the LTSpice library seems to work fine in an example of a more complex SMPS.
Any ideas ?
Thank you !
 

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