Precharge of Bootstrap Circuit
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
This circuit uses a sense amplifier to determine when a node is fully precharged and can be efficiently bootstrapped. Whenever an enhancement or low threshold device is used as a pullup device, bootstrapping is necessary to allow its source to reach the power supply level. To ensure efficient bootstrapping, the gate of this pullup device must be precharged to a minimum level (approximately +4 volts) before the bootstrapping process begins. Important factors in determining precharge are: accuracy in the precharge voltage level, reliability of the precharge voltage level, and space constraints for the precharge circuit. The figure shows the bootstrap circuit. When node SP is to be charged to +5 volts, the gate of device T3 (node BOOT) must be bootstrapped to at least a low threshold above +5 volts.