Browse Prior Art Database

Precharge of Bootstrap Circuit

IP.com Disclosure Number: IPCOM000038605D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Cranford, HC: AUTHOR [+3]

Abstract

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.

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Precharge of Bootstrap Circuit

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. This is accomplished by bringing input node X low which isolates node BOOT from ground by turning off device T2. Input W is then brought high, turning on device T1 which allows node BOOT to start charging. Node CBOOT is held low until node BOOT has precharged to about 4 volts. When BOOT has precharged to this level, the sense amplifier (devices T5-T17) causes node CBOOT to charge high. The charging of node CBOOT causes node BOOT to bootstrap to around 7 volts. This is sufficient to allow the source of T3 (node SP) to charge to +5 volts. The sense amplifier insures that node BOOT has precharged to approximately the same voltage level (approx...