Allegro MicroSystems falls as Onsemi pulls $6.9 billion offer

** Shares of Allegro MicroSystems ALGM.O slump 11% to $19.61 in extended trading, Onsemi ON.O jumps about 4% to $37

** U.S. chipmaker ON pulls its all-cash $6.9 billion proposal to buy Allegro for $35.10 per share

** "We have decided to withdraw our acquisition proposal given the reluctance of Allegro's board to fully engage and explore our proposal" - company says

** Semiconductor solutions provider Allegro had rebuffed the takeover approach earlier in March, saying the offer was "inadequate"

** So far this year, ON down 43.5%, ALGM largely flat as of last close