SAN FRANCISCO, June 25 : Memory chipmakers have for decades been trapped in boom-bust cycles, with capacity buildouts hitting the market just as demand craters. Micron, Samsung and SK Hynix are now trying to convince investors this time is different, arguing long-term deals will keep cash flowing even if the datacenter boom bursts. Micron said on Wednesday customers such as Nvidia had committed $22 billion to lock in supplies of memory chips, playing up huge growth in five-year "take-or-pay"