National Semiconductor Corp. plans to cut over 1,700 jobs, about 1/4 of its workforce, as the recession eats into sales at the maker of mobile-phone chips. The company will eliminate 850 positions now and 875 over the next few quarters, according to a statement today. The reductions will occur in areas including manufacturing, sales and marketing, National said. The company also will shut plants in China & TX. Sales this quarter will drop as much as 10% from the previous period as customers curb spending to cope with the economic slump, National said. The global economy may shrink for the first time since World War II as trade declines, according to a report this week from the World Bank. National