Hi,
Look for MOSFET charge pumps, lots of app notes and ICs with examples. Capacitative and basically like clockwork 'alternating between bucket filling and emptying'. Efficiency is poor, compared to e.g. Buck converter. Charge pumps are high Zout, and Cout influences this also.
See 7660 - typical negative charge pump IC, max load = - 20mA (but be saddened by - Vout droop at that load). Others can reach - 100mA.
Challenge would be to use clk and !clk to charge output capacitor (so other 50% of clk cycle no longer 'wasted' and Vout therefore allowed to droop less) with error amplifier feedback using Vin as Vreg.