1. A method for updating a check node in a low density parity check (LDPC) decoder, comprising: transmitting log-likelihood ratio (LLR) messages from variable nodes to a plurality of check nodes; decomposing the LLR messages into a plurality of node messages for each check node; and updating each check node using a function comprising a function g′(x) that is an approximated exponential function based on a node message, wherein the function g′(x) comprises a sum of a first exponential term and a second exponential term.