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The design of a 1.8V power supply, multi-frequency on-chip IC oscillator with a built-in low voltage detection (LVD) circuitry is described. Since the modules share the same bandgap cell, the system includes a new bandgap isolation strategy based on wide swing cascode current mirrors to reduce noise coupling into the LVD. The IC oscillator generates four selectable clock frequencies: 4 MHz and 8 MHz from 1.8 V to 5.5 V, 12 MHz and 22 MHz from 2.7 V to 5.5 V. After fine-tuning the oscillator via digital trimming its output frequency varies less than ±2.5% around the target frequency over supply ranges and from -40 to 125°C. The measured clock jitter is below 0.1% whereas the recover time from stop is 5 μs. The low voltage detection circuit monitors the supply voltage applied to the system and generates the appropriate warning or even initiates a system shutdown before the in-circuit SoC presents malfunction. The module was implemented in a 0.5 μm CMOS technology, occupies an area of 360×530 μm2 and requires no external reference or components.
Date of Conference: 7-11 Sept. 2004