Output Capacitance in LDO and SMPS Circuits

[electronx]

In LDO or SMPS circuits, the maximum capacitive load is generally specified. Is the capacitance at the output of the LDO or SMPS circuit used after the SMPS LDO output also included in the output capacitance of the main SMPS or LDO?

If there are additional capacitances downstream from the LDO or SMPS output, such as capacitors at various stages of a power distribution network, are nthey typically included in the specified maximum capacitive load of the LDO or SMPS itself?

For easier understanding, should Capacitors C2 and C3 be included in the output capacitance load of Smps1?

[xvr]

No

[AnalogTodd]

In LDO or SMPS circuits, the maximum capacitive load is generally specified. Is the capacitance at the output of the LDO or SMPS circuit used after the SMPS LDO output also included in the output capacitance of the main SMPS or LDO?

If there are additional capacitances downstream from the LDO or SMPS output, such as capacitors at various stages of a power distribution network, are nthey typically included in the specified maximum capacitive load of the LDO or SMPS itself?

For easier understanding, should Capacitors C2 and C3 be included in the output capacitance load of Smps1?
[picture snipped]

The way to look at all of this is to think about the impedances each presents. A load capacitor on the output of SMPS1 presents a low impedance at frequency and is often a major part of the stability equations for the circuit.

From an ideal perspective, SMPS2 and SMPS3 do not have their input impedance be a function of their output capacitance. They actually show a negative impedance as their current changes in proportion to the voltage (think dR = dV / dI). This is independent of their output capacitance and as such their output capacitors are not included on the output capacitance of SMPS1.

[ejeffrey]

The downstream switcher does not look like a capacitance but you can't always just ignore it either.  It's definitely possible with cascaded systems, especially cascaded SMPS to get stability issues because of the negative impedance of the downstream regulator.  But it's a much more involved analysis than just an effective capacitance.

An LDO looks like a constant current (high impedance load.  So for the case of an upstream switcher and downstream LDO the stability criteria are easier.