building a dsn for use with CI

I am trying to connect via a dsn to a remote database. I am confused as to how to build the dsn.

From php.net

<?php
$dsn = 'mysql:host=localhost;dbname=testdb';
$username = 'username';
$password = 'password';
$options = array(
    PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8',
); 

$dbh = new PDO($dsn, $username, $password, $options);
?>

from the CI manual pages:
$dsn = 'dbdriver://username:[email protected]/database';
$this->load->database($dsn);

I have connected to this databse via psql on the command line with:
psql --host=imagesdatabase.cwymdn16xxxes.us-east-1.rds.amazonaws.com --port=5432 --username=richb2 --password --dbname=postgres

How do I go from my working psql version above to the CI example? 

I tried this but I am getting a 500 error from the load database command below, which mean invalid driver
$dsn = 'jdbc:postgresql://imagesdatabase.cwymdn16xxxes.us-east-1.rds.amazonaws.com:5432/postgres -Urichb2';
$DB1 = $this->load->database($dsn, TRUE);

proof that an old dog can learn new tricks