LTC3536 Performance in Simulation

Hello,

In LTspice XVII, when I load & run the test jig for the LTC3536 (which has a fixed 3.6V input, 3.3V @ 1A output) it starts up in <50us, and reaches a stable output voltage in <1ms.  I modified the circuit to output 3.8V, changing R2 from 221k to 187k, and Rload to 3.8 ohms.  I also changed the input voltage source to a fixed 5.5V.  Once again, it starts up in <50us and reaches a stable 3.80V output in around 1ms.  So far, so good.

HOWEVER . . . If I change the simulation time to 7ms, and change the input voltage source from a fixed 5.5V to a pulse source, with an initial voltage of 5.5V and an 'ON' voltage of 1.5V, with a delay of 1ms, a 'rise' time of 5ms, and an on time of 1ms, the LT3536 output voltage doesn't begin to move from 0V until after 3.6ms.  Looking at the input voltage source current, the chip is doing something during the 3.6ms start-up . . .

What is causing the LTC3536 to have a delayed start when using a pulse voltage source to create a ramped input voltage from 5.5V to 1.5V over 5ms?  What am I missing?

TRS