Start-Up Circuit for a Ring Oscillator
Original Publication Date: 1985-Dec-01
Included in the Prior Art Database: 2005-Feb-19
This on-chip ring oscillator starter circuit insures that the substrate bias voltage generator starts every time in the absence of a substrate bias. The illustrated circuit is effective in holding one of the ring oscillator input stages 20 to a solid down level until VDD has risen to a high value above the device thresholds, permitting oscillation to occur. The circuit has enough gain to create the transition at this oscillator node 20 quickly, even with a slow, smooth VDD ramp. Devices T1 and T2 form a voltage divider between VDD and ground, causing node A to rise at approximately a third the rate of VDD. Thus when node A reaches the threshold of T4, VDD is well above the device thresholds. This transition is amplified by each of the inverter stages formed by devices T3 through T8.