Electrical – Using a Nema 42 Stepper as a generator

You are creating a problem that does not exist. You are confusing current consumed with available current that can be consumed.

The IC and motors will only draw the current they need. The motors are rated to operate at 2.5 amps maximum current, so your power supply has plenty of current to spare. Its 15 amp rating is how much it can supply if all six motors 'request' that much current at the same time, which totals 15 amps (6 * 2.5).

What you should do for safety reasons is put a 5 amp fast-blow fuse or a 3 amp MDL slow-blow fuse in series with each ICs power input. This way a stalled motor or shorted circuit will blow the fuse instead of burning up in a bad way, and shut down only one motor.

EDIT: If you really need current limiting then:

Each motor driver IC has a current sense pin which has a resistor to ground. The voltage at that pin tells you the current drawn by that motor. To avoid using a MPU and complex software, just buy a LM339 quad comparator and set its trip point for 1.5 amps (I do not know what voltage that will be, as it is determined by your sense resistor). Use a P-channel MOSFET on the power input of the IC so the LM339 can shut it off by letting the gate-source voltage drop to zero.

A current sense resistor of 1.00 ohms 3 watts will give a voltage of 1 volt per amp, or 1.5 volts per 1.5 amps. The power per each IC goes through the fuse first, then the P-channel MOSFET which drives the +Vs connection in figure 6 of your data sheet. Set the LM339 to have the output go 'high' if (+) input is greater than 1.5 volts. Use a trim pot to put 1.5 volts on the inverting (-) pins.

The motor control IC can be driven by any MPU with a few pins to spare.

2.8A is the maximum current, but actually nobody uses motors on maximal values. All stepper controllers have the possibility to set up maximum current on each motor depends on your requirements. In most cases, 50% is more than enough. For example, 3D-printers usually working on 1-1.5A current. Additionally (for 3D-printing) Z-axis moves when other motors in idle state, so even with the maximum current you can save 25% of the expected power.