Read circuit which uses a coarse-to-fine search when reading the threshold voltage of a memory cell
US5748533A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | Mar 26, 1996 |
| Grant date | May 5, 1998 |
| Priority date | — |
| Expiry date | Mar 26, 2016 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG11C7/16
- WIPO fieldComputer technology
- WIPO sectorElectrical engineering
Abstract
A read circuit includes a driver which changes a gate voltage of a memory cell and a sense circuit which identifies when the memory cell trips. The driver searches for the threshold voltage of the memory cell using stages which ramp up gate voltage and stages which ramp down the gate voltage. Each stage ends when the sense circuit senses that the memory cell trips, i.e. begins or stops conducting. Initial stages of the search have high ramp rates so that the gate voltage reaches the threshold voltage. These initial stages can give inaccurate threshold voltage readings because high ramp rates change the gate voltage during the period between the transistor tripping and sensing the trip. Later stages ramp the gate voltage slowly to provide an accurate threshold voltage reading. The low ramp rate of the last stage provides accuracy, and the high ramp rate of the initial stages reduces read time. To further reduce read time, the search process can begin at a median voltage for possible threshold voltages.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.