FRANKFURT, Dec 15 German chipmaker Dialog Semiconductor Plc slashed its revenue guidance on Tuesday, citing weaker-than-expected demand for chips used in mobile phones such as Apple Inc's big-screen iPhone. The statement sent shares in Dialog, which is trying to buy U.S. rival Atmel Corp for about $4.6 billion, 10 percent lower in pre-market trade, according to prices quoted by brokerage Lang & Schwarz.