HELP: Integer -> Hex conversion (Overflow)

I am looking to convert a 20 digit integer to Hex, but I keep getting

an Overflow error. I am assuming there is a limit to the number of

digits in Hex format or the convesion. Is there a way around this?

If this involves the use of API, I will need an example as I am not

familiar with the API.

The reason for this is because I have a text field that has a maximum

of 15 characters. If I converted the 20 digit Int to Hex, it should

fit in nicely. But the problem above arises.

Even better, if anyone knows how to convert the large number to as

little number of characters (by way of algorithm, etc.) would be even

more valuable. Then I can use more than a 20 digits!

Any help would be greatly appreciated!