Electronic – Input and output oapacitor for PoE + DCDC controller

I see three things that are hurting your charge pump circuit.

The capacitors are aluminum electrolytic instead of ceramic, so they have very high dissipation factor and low self-resonant frequency. These charge pumps really do require ceramic capacitors.

Second, they are radial-leaded and axial-leaded capacitors instead of surface-mount. That adds some series inductance, which tends to defeat the capacitance.

Third, the whole thing is built on a white solderless breadboard, which adds even more inductance. You'd be better off using a PCB breakout board (like a Surfboard) and solder capacitors with trimmed short leads, or better yet, surface-mount capacitors.

Not all capacitors are created equal, and not even all ceramic capacitors are created equal. The X7R and X5R types give fairly large capacitance values stable over a good temperature range. The cheaper Y5V, Y5U, and Z5V types can lose a lot of their capacitance over temperature, so sometimes we have to compensate for that loss by using higher nominal starting capacitance. (The NP0 and C0G types are highly stable over temperature, but usually are only available in very low capacitance values.)

Back in the day, ceramic capacitors were only available up to about 10uF, but nowadays there are 100uF ceramic capacitors. So the need for using tantalum and aluminum electrolytics is somewhat diminished; now it's driven by a cost vs performance trade-off.

The self-resonant frequency (SRF) is important in switching circuits, because at frequencies higher than SRF, the capacitor stops behaving like a capacitor and starts behaving like an inductor. Surface-mount ceramic capacitors are usually higher SRF and thus closer to "ideal" capacitors.

Aluminum electrolytic capacitors are great for providing a large amount of capacitance and accepting large inrush current, these are typically seen on power supply inputs. But they don't work well for the "flying capacitor" of a charge pump.

Take a look at MAX619EVKIT http://datasheets.maximintegrated.com/en/ds/MAX619EVKIT.pdf for an example of a good charge pump layout, using surface-mount ceramic capacitors.

Disclosure: I am a Maxim employee and designed the MAX619EVKIT way back when great lizards roamed the earth.

The input capacitance value is not critical. What is critical is that you have some way of preventing the input voltage from dipping during operation over a wide frequency band.

The connection between the power source(battery, generator, AC power, etc) and the regulator input will have some inductance L. In a linear regulator current passes from the input of the regulator, goes through a pass element (usually a BJT, MOSFET, or IGBT), and then flows to the load. The input and output current are typically about equal except for a small amount of extra input current used to run the regulator internal circuitry (reference, error amp, gate drive).

Suppose that the regulator input current increases at a rate of di/dt (due to the load current changing). Then without any input capacitance the input voltage to the regulator would dip by an amount V_dip = L * di/dt. Clearly if the voltage dips too much then the regulator will stop working and the output load voltage would drop.

The datasheet will usually recommend a minimum required capacitance on the input, but you can always use more. Ceramic capacitors tend to work well over a wide frequency band but have lower capacitance values. Electrolytic capacitors have larger values but work only at lower frequencies. Typically a combination of both types is used to get both high capacitance and wide frequency operation.

Linear regulators typically have an error amplifier and pass transistor inside of them. Both of those components have limited bandwidth. If the output current changes too rapidly then the regulator will not be able to adjust to the demand changes quickly enough and the output voltage may dip.

The amount of output dip at frequencies much higher than the regulator bandwidth is approximately dV = I/(2 * pi * f * C). For example if you had a regulator with a bandwidth of 100kHz, and you were running some digital electronics that drew 100mA spikes at 1MHz and had a 0.1uF output capacitor then the output ripple would be 100mA/(2*pi*1MHz * 0.1uF) = 15.9mV peak.

Typically you would try to pick a capacitance that leads to an acceptable ripple voltage (using the above equation) at the peak load current at the frequency corresponding to the regulators bandwidth given in the datasheet.

Another factor to consider for the output capacitor is stability. The error amplifier in a linear regulator typically uses feedback and can oscillate if too much or too little capacitance is used. Many linear regulators are stable with a wide range of output capacitances. The datasheet will often specify that the capacitance must be below or above a certain value for stable operation.

You cant really calculate how much capacitance is required for stability without knowing the characteristics of the error amplifier (phase and gain margin vs. frequency). Since the manufacturer often doesn't tell you that information you sort of have to take the manufacturer at their word on that one.