What is the major difference?
LT8641
Recommended for New Designs
The LT8641 step-down regulator features Silent Switcher architecture designed to minimize EMI/EMC emissions while delivering high efficiency at frequencies...
Datasheet
LT8641 on Analog.com
LT8641A
Recommended for New Designs
The LT8641A step-down regulator features Silent Switcher architecture designed to minimize EMI emissions while delivering high efficiency at frequencies...
Datasheet
LT8641A on Analog.com
What is the major difference?
Mostly just a packaging change. The LT8641A is in a flip-chip package.