Ludacris -- I'm That Kid's Daddy But I Can't Pay Much

Ludacris didn't use protection when he banged some chick, but he's taking precautions now ... because he wants to legally establish his paternity and then set a hard limit on his child support.

Ludacris has been in a long-term relationship with Eudoxie -- pronounced kinda like the book in the Bible (don't ask). But Luda took a break from Eudo long enough to get Tamika Fuller preggo.

Cai Bella Bridges was born December 9th. And here's where the plot thickens.

It seems Ludacris knows he's the daddy, but he's worried Tamika will go in for the kill. So he's filed legal docs -- obtained by TMZ -- claiming he's only pulling in $25,842.41 a month on average. Under Georgia law ... his monthly child support payment would be set at $1,754.66.

FYI ... Forbes says Luda is worth $12 mil ... but that seems real low.