Using get_where to get data between today and last x days from database

This is working:

PHP Code:

$this->db->select('*');
$this->db->where('payment_timestamp BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()');
$this->db->where('vendor', $id);
$data['dash_sale_d'] = $this->db->get('sale'); 


This is not:

PHP Code:

$data['dash_sale_d'] = $this->db->get_where('sale', array('vendor' => $id, 'payment_timestamp' => 'BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()')); 


What am I missing?