I know at my institution they just make up the difference between my current stipend and the ford. It usually varies per institution. Maybe ask an administrator in the grad school at your institution.
@kiki_2_4 This happened to me last year. My only recommender from my current institution was dealing with personal health issue and misread that it was 5pm eastern standard instead of pacific. I was devastated because my feedback mentioned the lack of recommender from the current institution. This year I managed to get 5 recommenders just in case and told them the deadline was the day before the actual deadline.