Transaction Logs are getting full frequently

Hi,

I have 2 nodes participating in AlwaysON. Problem is i am getting transaction logs full frequently.Prime prerequisite of AG is to have databases in full recovery model.

I cannot change the recovery model to simple when DB's are participating in AG.

How to prevent my transaction log from getting full.Please help.

The transaction log for database 'XXXXXXXXX' is full due to 'LOG_BACKUP'.  This error is coming on primary replica.