Analog circuit to maintain a minimum current?
Add a Resistor in parallel. For 50 mA at 5V you need a resistor of 5/0.05 = 100Ohm and it will dissipate 0.25W.

In other words a quarter Watt 100 Ohm resistor. Though using 2 200Ohm resistors in parallel will reduce the power dissipation per resistor.