How can I generate all permutations of alphabets that add up to a sum value?

A=1,B=2,C=3,etc...

So if I input the sum value of "24", I would like it generate all combination of letters in:
1. Ascending Sequence only so no sequence will repeat itself.
2. Fixed number of letters of my choosing.

So, if I input the sum value of "24" and the number of letters as "3",

It should generate a list of alphabet combinations including the word:

"ACT"

Because 24 = 1+3+20 = ACT

I don't need it to...

Generate All Combination of Alphabets that Add up to a Sum Value?