Voltage reference generator using big flash cell
View Patent ↗A voltage reference generator includes multiple closed loop voltage references. Each of the closed loop voltage references uses a flash cell with a variable threshold voltage and a feedback loop to trim a reference voltage. The voltage reference generator includes sample and hold capacitors in output stages to allow reference voltages to be refreshed during a standby mode of operation.
1. A multi-level cell memory device comprising:
a plurality of closed loop voltage references that include programmed flash cells, wherein reference voltages produced by the closed loop voltage references are controlled by threshold voltages of the flash cells; and
a plurality of output stages, at least one of the plurality of output stages being a pull down output stage, and at least one of the plurality of output stages being a pull up output stage;
wherein the pull down output stage comprises a p-channel current source transistor and a first sample and hold capacitor coupled to a gate node of the p-channel current source transistor, and a p-channel source follower transistor and a second sample and hold capacitor coupled to a gate node of the p-channel source follower transistor.
2. The multi-level cell memory device of claim 1 wherein the pull up output stage comprises an n-channel current source transistor and a third sample and hold capacitor coupled to a gate node of the n-channel current source transistor, and an n-channel source follower transistor and a fourth sample and hold capacitor coupled to a gate node of the n-channel source follower transistor.
3. The multi-level cell memory device of claim 1 further comprising load capacitors coupled to output nodes of the plurality of output stages.
4. The multi-level cell memory device of claim 3 further comprising switches to isolate the plurality of output stages from the load capacitors.
5. The multi-level cell memory device of claim 1 further comprising switches to isolate the plurality of closed loop voltage references from the plurality of output stages.
6. The multi-level cell memory device of claim 1 wherein each of the plurality of closed loop voltage references includes a flash cell and a source follower transistor, a gate node of the flash cell being coupled to a source node of the source follower transistor.