What dictates the A320's 'emergency ram air inlet' operation?
(wikimedia.org)

You have misinterpreted the text. Given that the DITCHING pb is not pushed, the RAM AIR pb opens the emergency ram inlet.

The differential pressure (DP) being above or below 1 psi is for the outflow valve (not the inlet). If the DP &lt; 1 psi, the outflow valve opens—this is the opposite to the former title (now the first line) of the question.

This serves two purposes:


Clear smoke from the airplane
Bring in fresh air if the packs fail.


When the system operates in this mode, there is no fine tuning for the outflow valve. By testing, they would have figured out with the fixed ~50% open position, any DP above 1 psi results in rapid decompression.

Note: the protection is via a check valve, so no system logic is involved. It's mechanical.