Two Pole DC Rail Voltage Decoupler - John Dunn, Consultant, Ambertec, P.E., P.C.

The following sketch is of a two-pole rail voltage decoupling circuit.  

 
The circuit works by doing a current division between capacitor C1 and the remainder of the circuit whose impedance is examined as follows:   

The absolute values of the above circuit's series impedance versus frequency vary with the circuit elements. On the left in the above sketch, the effect of the variations in the emitter resistance R1 is seen. On the right, we see the effects of varying resistor R3.

When we feed power in from a DC source having a zero (negligibly low) source impedance, the load sees a rail-voltage source impedance consisting of the parallel combination of this series element and the aforementoned shunt capacitance, C1.

You want this filter to be either over damped or critically damped. If you let it be underdamped, there can be transient response voltage excursions undershooting or overshooting the final settling voltage which would not be a good thing.

The following table shows some examples of damping for value combinations of R1 and C1:   

 

Comments

[John Dunn]

Hi, Dave.

This circuit serves a different purpose. A linear voltage regulator, LDO or otherwise, would pass any change of load current transparently back to the input line. There would be no attenutation. Instead, this circuit would attenuate that load current to keep it from appearing at the input line.