How to simulate variable resistance load of Buck converter

[vdavi81]

Hi,
I'm doing a postdegree stage with ST Microelectronis. I have to make a dc-dc design to integrate in 0.35u tecnology. My dc-dc is a step-down buck converter. For the simulations I need to know how to simulate variable resistance load. I'm using cadence icfb with ELDO simulator.
Somebody Can help me for this scope. I already know how to realize a parametric analysis, but I don't want it. Instead, i want to simulate loads effect during a simulation and see the graph of Vout variation.

P.S Excuse me for my english, but I speak english a little bit only.

Thank you very much
Davide.

 

[FvM]

I guess you want to use transient analysis and determine average DC input/output values. In this case, I don't see other chances than a parametric analysis. The only problem is, that it doesn't directly produce a plot.

 

[vdavi81]

I don't belive that only parametric analysis have this type of simulation. Instead I belive there is a method to have a time dependence resistance value.
I have to do a R(t). It's impossible that the cadence or ELDO or SPICE haven't this issue.

 

[FvM]

You didn't yet talk about time variable resistor. That's in fact an option to get the intended plots. It's available by behavioral modelling with most SPICE-like simulators.

 

[Dalibor]

I am not familiar with ELDO but hope it would be similar as in PSpice. Time-variable resistance can be simulated via controlled source. For example, here is the PSpice code of time-varying resistance R(t)=1k+100*sin(2*pi*1k*t), connected between nodes n1 and n2:
Eres n1 n2 value={(1k+100*sin(2*pi*1k*time))*I(Eres)}
This method is based on the fact that aresistor with the resistance R can be replaced by a voltage source with the voltage V=RI, with I is being current flowing through the resistor.
For modeling the time dependence of the resistance, you can use arbitrary mathematical functions which are provided by the simulator. I guess that for your case you would need step function for modeling a response of buck converter to jumply changed load resistance.
Similar approach can be also used for R(t) modeling via controlled current source.