Captain reported receiving an altitude alert from ATC while on visual approach after descending below profile. Flight crew returned to glide path and continued to landing.

Narrative

While on the Tipp Toe Visual 28L approach to SFO; NorCal Approach issued a descend and maintain 3000 clearance between EDDYY and SIDBY. The PF engaged LVL CHG after I set 3000 in the ALT SET window and the aircraft began a descent. We were subsequently cleared for the approach and told to maintain 180 knots until the San Mateo bridge. PF then asked to set TDZE. I set 100 in the ALT SET window. We finished configuring the aircraft for landing and completed the Landing Checklist. PF attempted to close the speed window by pressing the speed intervene button; when it didn't close; I tried to press it. I realized at that point that the selected mode was still LVL CHG. I pressed VNAV twice but it did not engage. The PF disengaged the autopilot and leveled the aircraft. At the same moment ATC issued an altitude alert with our call sign. The FMA (Flight Mode Annunciator) mode was in LVL change until about 950' AGL and we descended below glide path at approximately 3 mile final. Glide path was recaptured; the approach was stable at 500' and the landing was uneventful.