Charge pump with adjustable frequency: what frequency to pick and why?

Question

I decided need a charge pump IC that I intend to use to get -3.3V from 3.3V to supply my op amps and diff amps to avoid running into output voltage swing limitations of those ICs (For true near zero or zero output).

In my specific application, I don't care much if output voltage is -3.3V or -3.0V or -2.0V - as long as the op amp can output down to 1mV and can produce a few milliamp max current (3mA max, maaybe 4mA in total). But my question is a little more general.

So I spent a fine afternoon looking at various inverting charge pump ICs on mouser, I've run into a few ICs that provide necessary current and have acceptable output impedance, and some of them have settable/adjustable frequency.

For example, this beefy charge pump SP6661 can work at 120kHz and 900kHz with caps from 1uF to 22uF.

Q: Why would I ever prefer one frequency over another? In what situations, for what use cases and with what external components (capacitors)? When would and wouldn't it matter?
Q2: Bonus question, if anyone knows: what's the behavior of an (inverting) charge pump with ultra-low load or no load at all? Should I have a pulldown on output to stabilize it when there is no load? (pulldown as "resistor to GND")

Any helpful insight will be appreciated.

Answer 1

It's explained by figure 1 and text above it in the datasheet you linked.

Higher frequency allows the use of smaller capacitors for same output impedance and ripple. If you don't need much current then you can also use smaller capacitors.

So it's you as the designer to balance between voltage drop, output impedance, circuit board area, voltage ripple, part count, total cost of parts etc to achieve what you want.

Higher frequencies may be easier to filter out but higher frequencies may also couple better to parts of the circuit where they cause problems.