Redesigning the old 555 [timer circuit]

摘要

It is rare that an integrated circuit stays on the market for 25 years. And it is surprising that, except for a CMOS derivative, the 555 timer has never been updated. It has been shrunk somewhat, but the original schematic and layout have remained intact. For a long time this situation could be explained by the versatility of the 555, whose speed, current draw and voltage range sit right in the middle of a vast field of applications. But in the last few years, supply voltages have moved downward, well below the 4.5 V at which the 555 cuts off, and the unit's 5-10 mA current requirement now seems wasteful. A better design is clearly needed. The author describes the features of the original timer before discussing the redesign. The voltage versus current design and output stage are discussed.