A320 FLT CREW IS ADVISED BY SCT APCH OF A LOW ALT ALERT ON THE LOC 27 INTO SAN; CA.

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I WAS ACTING AS PNF ON FLT TO SAN. EXPECTING LOC 27 ON ARR; WE HAD BRIEFED THE CDAP PROC. AFTER ENTERING SOCAL APCH AIRSPACE; WE BEGAN GETTING VECTORS AND VARIOUS SPD RESTRS FOR THE LOC 27 APCH IN SAN. SOCAL APCH CLRED US TO DSND AND MAINTAIN 4000 FT UNTIL ESTABLISHED ON THE APCH AND CLRED FOR LOC 27 APCH. XING VYDDA WE BEGAN FURTHER DSCNT AND THE CAPT CALLED FOR LNDG GEAR AND FINAL DSCNT CHK. SHORTLY AFTER VYDDA; WE WERE ISSUED A LOW ALT WARNING AND TOLD TO CLB TO 3600 FT. WE WERE AT ROUGHLY 2800 FT AND 11.2 DME FROM UBR (LOC 27). THE CAPT INITIATED AN IMMEDIATE CLB AS WE TOOK A QUICK GLANCE AT THE APCH PLATE TO VERIFY OUR POS. WE WERE ESTABLISHED INSIDE OF THE SWATT STEPDOWN OF 3600 FT AT 12.6 DME; SO AFTER TOPPING OUR CLB AT ROUGHLY 3100 FT; THE CAPT BEGAN DSNDING VIA THE APCH PROFILE AGAIN. WE WERE ESTABLISHED LATERALLY AT ALL TIMES; BUT SEEMS TO BE SOME DISCREPANCY AS TO OUR MEETING THE 3600 FT AT SWATT. WE LANDED UNEVENTFULLY; BUT WERE CONTACTED BY ATC. WE WERE TOLD WE WERE 600 FT LOW AT SWATT. WE HAD THE APCH DISPLAYED ON OUR NAV DISPLAYS AS WELL AS RAW DATA IN THE FORM OF DME AND LOC. HOWEVER; SWATT ISN'T BUILT INTO THE APCH AS PART OF OUR DATABASE AND SHOULD HAVE BEEN ADDED TO HELP INCREASE SITUATIONAL AWARENESS. THIS IS A TECHNIQUE I SEE AND USE REGULARLY FLYING THE LINE. AMIDST THE VECTORING AND VARIOUS SPD RESTRS ISSUED TO US; THIS WAS MISSED. WHILE WE BOTH HAD ADEQUATE RAW DATA DISPLAYED; I REALIZE HOW BENEFICIAL THAT ADDED DISPLAY CAN BE TO THE PNF WHEN TRYING TO COMPLETE CHKLISTS AND MONITOR THE APCH AT THE SAME TIME. SUPPLEMENTAL INFO FROM ACN 786470: I DON'T KNOW THE EXACT ALT WE REACHED BY THE TIME WE CROSSED VYDDA BUT RECALL TELLING THE FO TO SET THE NEXT ALT AS WE WERE GOING TO BE ABOVE 4000 FT BY THIS POINT. AROUND THIS TIME I BELIEVE WE WERE AGAIN TOLD TO SLOW -- THIS TIME TO 150 KTS -- AND SAID TO THE FO THAT 'THEY ARE REALLY MAKING IT HARD TO GET DOWN.' NORMALLY; I BUILD ALL POINTS THAT THE FMC DOES NOT PLACE ON THE MAP BUT; IN THIS CASE; I HAD NOT PLACED SWATT ON THE NAV DISPLAY AND WE WERE SIMPLY USING THE UBR DME TO ESTABLISH THE POINT. MY THOUGHT IS THAT IN FOCUSING TOO MUCH ON MAKING THE 2000 FT BY REEBO; I MAY HAVE MISREAD THE PLATE OR THE DME AND WAS TOO AGGRESSIVE WITH MY DSCNT AFTER VYDDA.