These two devices are exactly the same except that MAX17559 has a Soft-Stop feature which is not present in the MAX17558.
MAX17558
Production
The MAX17558 is a dual-output, synchronous step-down
controller that drives nMOSFETs.
The device features a peak-current-mode, constant-frequency
architecture...
Datasheet
MAX17558 on Analog.com
MAX17559
Production
The MAX17559 is a dual-output, synchronous step-down controller that drives nMOSFETs. The device uses a constant-frequency, peak-current-mode architecture...
Datasheet
MAX17559 on Analog.com
These two devices are exactly the same except that MAX17559 has a Soft-Stop feature which is not present in the MAX17558.