Hi Rob,
Thank you for using WEBENCH for Power designs. TPS55340 has user selectable frequency between 100k - 1.2MHz. Are you sure, you are wanting to design for 1kHz?
In general you are right about Output impedance coming from a combination of Cout and Rload.
For your case of 12-24V @ 36V &0.5A. Rl=72 Ohms, Cout is 2.2e-6 with an ESR of 2.8mOhms. I designed at 1MHz.
So Zout = sqrt(Rl*Rl + w*w*Cout*Cout). To be more accurate, you can calculate impedance from capacitor so that it includes ESR.
Not taking ESR into account, and fixing freq at 1KHz with the current webench design, Cout has to be reduced to somewhere around 2.65nF. Let me confirm if it makes sense to do this with product line folks as well.
Meanwhile you can try edit design feature in WEBENCH to use a 2.5nF capacitance at Cout and try simulations.
Regards,
Pavani Jella
WEBENCH Applications