Sensing state of memory by variable gate voltage

一种用来读出存储阵列(214)中浮栅存储单元的状态的方法和设备。由于其稳定性和准确性、读出设备(220)可用来读出多位浮栅存储单元的状态。存储单元的状态借助于向浮栅存储单元的顶栅施加一个可变栅压并将单元电流同固定参考电流进行比较而被读出。一个电路探测单元电流何时等于参考电流。当二电流相等时，可变栅压的值就表明存储单元的状态。对于一个实施例，模数转换器(210)将可变栅压转换成数字值，当二电流相等时，此数字值被锁定。被锁定的数字值表明了存储单元的状态。对此实施例，斜波电压或其它适当的可变电压可用作可变栅压。对另一实施例，数模转换器被用来产生可变栅压。 A method and apparatus for sensing the state of floating gate memory cells in a memory array. Because of its stability and accuracy, the sensing apparatus may be used for sensing the state of multi-bit floating gate memory cells. The state of a memory cell is sensed by applying a variable gate voltage to the top gate of the floating gate memory cell and comparing the cell current to a fixed reference current. A circuit detects when the cell current is equal to the reference current. When the currents are equal, the value of the variable gate voltage indicates the state of the memory cell. For one embodiment, an analog-to-digital converter converts the variable gate voltage to a digital value that is latched when the currents are equal. The latched digital value indicates the state of the memory cell. For this embodiment, a ramp voltage or other suitable variable voltage may be used as the variable gate voltage. For another embodiment, a digital-to-analog converter is used to generate the variable gate voltage. A counter generates digital values to step the variable gate voltage. When the cell current equals the fixed reference current, the digital counter value is latched to indicate the state of the memory cell.