High Current, 5KHz Switching............. Darlington Array or MOSFET?

Projects General Guidance
2
  1. Instead of the switches shown in schematic, a chip is required to perform high current, high speed switching (5.1KHz).

By no stretch of the imagination is 5KHz high speed.

Darlington drivers loose voltage across them when switching, look it up on the data sheet. Use a MOSFET.