Quoting Books Online:
BACKUP DATABASE and BACKUP LOG permissions default to members of the sysadmin fixed server role and the db_owner and db_backupoperator fixed database roles.
Which is a bit of a pity, as you may have a junior DBA that should be able run backups without being sysadmin.
What you can do is to write a stored procedure that runs the backup commands needed, and where you check the input parameters. Then you sign that procedure with a certificate from which you create a login that you add as member to sysadmin. Finally, you grant permission on this procedure to the junior DBA.