Shield Charge

Shield Charge
Type of Feat: General Prerequisite: Shield Bash, Knockdown, Base Attack Bonus + 3 Use: Automatic.

If you hit an opponent with your shield as part of a charge action, in addition to dealing damage normally, you will perform a trip check without provoking an attack of opportunity. If you lose, the defender does not get to try to trip you in return. Size increasing or diminishing effects can affect your trip. (The +4 size bonus from Powerful Charge applies as well as a +4 for Improved Knockdown)

Gameplay Notes
The knockdown feat is a second feat prerequisite, though it is undocumented elsewhere.