How can I use Laplace in constant current source instead of a general voltage source?

[ruwan2]

Hello,
I am familiar with DSP in recent years, but not in analog circuit analysis. When I try to simulate a charge pump circuit in a PLL, I have a few questions on the charge pump.
1. A RC network behaves different for a voltage source and a current source. For a current source, the voltage cross the capacitor increases linearly while it is not linear for a voltage source. The same Laplace transform can be used for a current source driven RC network? Or, some modifications are needed?
2. How do I simulate a current source charge pump? I don't see a mechanism or library component for the charge pump simulation in Matlab/Simulink. There is a current source component in "Electrical Sources", but it is not for this PLL simulation purpose. I don't see it is useful here.

Can you help me on the questions?

Thanks,

 

[KlausST]

Hi,

You need to understand voltages and currents in RC series connection and RC parallel connection.

In a series connection the current through each device is the same.
Thus with a constant current source in an RC series connection the current through the C is independent of the value of R.
...and thus the voltage across the C is independent of R. You even may omit the R.
Then the voltage across the C is just the integral of I/C over time. (No need for Laplace).

Similar if you have a voltage source and an RC parallel connection.
The current through C is independent of R.
Maybe here it is more obvious (or more commonly known) that the voltage across the C is independent of the value of R.

Klaus

 

[FvM]

Feeding with RC with current versus voltage source results in a different circuit, different differential equations and thus different Laplace transfer function. Calculate yourself.

It's always possible to model the charge pump circuit behaviorally with a combination of linear and non-linear function elements. Electrical components library is for your convenience, but is must not necessarily cover all application cases.

 

[pancho_hideboo]

When I try to simulate a charge pump circuit in a PLL

Models in Mathworks Simulink are signal-flow model,
so there is no concept of kirchhoff's rule.
Off course, there is no concept of voltage and current.

However Loop filter of charge pump PLL is a block where input is a current and output is a voltage.
So if you will model charge pump and Loop filter separately, it is very difficult to model them without energy consevative modeling concept.
Use https://mathworks.com/help/physmod/simscape/electrical-models.html

If you will model charge pump and loop filter as one block, you can model it by signal-flow modeling concept.

1. The same Laplace transform can be used for a current source driven RC network?

Of course, No.
Transfer functions are different, Hv=Vout/Vin, Zt=Vout/Iin.

2. How do I simulate a current source charge pump?
I don't see a mechanism or library component for the charge pump simulation in Matlab/Simulink.

Use https://mathworks.com/help/physmod/simscape/electrical-models.html

If you don't use energy consevative modeling concept, you have to express all signal as voltage and express loop filter as Vout/Vin.
Or you have to model charge pump and loop filter as one block.