Circuit for generating a centered reference voltage for a 1T/1C ferroelectric memory
View Patent ↗A ferroelectric reference circuit generates a reference voltage proportional to (P+U)/2 and is automatically centered between the bit line voltages corresponding to the P term and the U term across wide temperature and voltage ranges. To avoid fatiguing the reference ferroelectric capacitors generating (P+U)/2, the reference voltage is refreshed once every millisecond. To eliminate the variation of the reference voltage due to the leakage in the ferroelectric capacitors during this period of time, the reference voltage generated from the reference ferroelectric capacitors is digitized when it is refreshed. The digital value is fixed and converted to an analog value which is then fed into sense amplifiers for resolving the data states. The reference voltage is automatically at the center of the switching (P) and non-switching (U) signals and therefore the signal margin is maximized.
1. A reference circuit for a ferroelectric memory comprising:
a reference generating circuit for generating a ferroelectric reference voltage; and
an updating circuit coupled to the reference generating circuit for providing a periodically updated version of the ferroelectric reference voltage comprising:
a counter;
a DAC coupled to the counter;
a comparator having a first input coupled to an output of the DAC,
a second input for receiving the ferroelectric reference voltage, and an output for controlling the counter; and
a sample and hold circuit having an input coupled to the output of the DAC, and an output for providing a periodically updated ferroelectric reference voltage.
2. The reference circuit of claim 1 wherein the reference generating circuit comprises at least two ferroelectric reference cells, each including a transistor and a ferroelectric capacitor.
3. The reference circuit of claim 1 wherein the reference generating circuit comprises:
a reference word line;
a reference plate line;
at least two reference bit lines; and
at least two ferroelectric reference cells each coupled to the reference word line, the reference plate line, and one of the reference bit lines, wherein one of the reference bit lines provides the ferroelectric reference voltage.
4. The reference circuit of claim 1 wherein the sample and hold circuit comprises:
a transistor having a first source/drain forming the input of the controlled buffer circuit, a second source/drain, and a gate for receiving an update control signal;
a capacitor coupled between the second source/drain of the transistor and ground; and
a buffer stage having an input coupled to the second source/drain of the transistor, and an output forming the output of the controlled buffer circuit.
5. The reference circuit of claim 3 further comprising an equalizing circuit for shorting the reference bit lines together.
6. The reference circuit of claim 5 wherein the equalizing circuit comprises at least one transistor for shorting the at least two reference bit lines together.
7. A method of generating a reference voltage in a ferroelectric memory comprising:
providing at least one ferroelectric reference cell including a ferroelectric capacitor;
periodically accessing the at least one ferroelectric reference cell to generate a reference voltage corresponding to a (P+U)/2 charge component to avoid fatiguing the reference ferroelectric capacitor and to compensate for variations in temperature and voltage; and
using a sample-and-hold circuit to hold a previous reference voltage while a new reference voltage is generated.
8. The method of claim 7 further comprising generating the reference voltage about once a millisecond.
9. The method of claim 7 further comprising:
digitizing the reference voltage corresponding to the (P+U)/2 charge component to create a digital value; and
holding the digital value until a next value of the reference voltage is generated.
10. The method of claim 7 further comprising providing a valid reference voltage with no interrupts while generating a new reference.
11. A reference voltage circuit for a ferroelectric memory comprising:
at least one ferroelectric reference cell including a ferroelectric capacitor;
circuitry for periodically accessing the at least one ferroelectric reference cell to generate a reference voltage corresponding to a (P+U)/2 charge component to avoid fatiguing the reference ferroelectric capacitor and to compensate for variations in temperature and voltage; and
a sample-and-hold circuit to hold a previous reference voltage while a new reference voltage is generated.
12. The reference circuit of claim 11 further comprising a digitizing circuit to obtain a digital value of the reference voltage.
13. The reference circuit of claim 12 in which the digitizing circuit comprises a counter.
14. The reference circuit of claim 12 in which the digitizing circuit comprises a digital-to-analog converter (DAC).
15. The reference circuit of claim 12 in which the digitizing circuit comprises a comparator.
16. The reference circuit of claim 12 in which the digitizing circuit does not include an analog-to-digital converter (ADC).
17. A method of generating a reference voltage in a ferroelectric memory comprising:
providing at least one ferroelectric reference cell including a ferroelectric capacitor;
periodically accessing the at least one ferroelectric reference cell to generate a reference voltage corresponding to a (P+U)/2 charge component to avoid fatiguing the reference ferroelectric capacitor and to compensate for variations in temperature and voltage; and
providing a valid reference voltage with no interrupts while generating a new reference.
18. The method of claim 17 further comprising generating the reference voltage about once a millisecond.
19. The method of claim 17 further comprising:
digitizing the reference voltage corresponding to the (P+U)/2 charge component to create a digital value; and
holding the digital value until a next value of the reference voltage is generated.
20. The method of claim 17 further comprising using a sample-and-hold circuit to hold a previous reference voltage while a new reference voltage is generated.
21. A reference circuit for a ferroelectric memory comprising:
a reference generating circuit for generating a centered ferroelectric reference voltage; and
an updating circuit coupled to the reference generating circuit for providing a periodically updated version of the ferroelectric reference voltage.
22. A method of generating a reference voltage in a ferroelectric memory comprising:
providing at least one centered ferroelectric reference cell including a ferroelectric capacitor; and
periodically accessing the at least one ferroelectric reference cell to generate a reference voltage corresponding to a (P+U)/2 charge component to avoid fatiguing the reference ferroelectric capacitor and to compensate for variations in temperature and voltage.
23. A reference voltage circuit for a ferroelectric memory comprising:
at least one centered ferroelectric reference cell including a ferroelectric capacitor; and
circuitry for periodically accessing the at least one ferroelectric reference cell to generate a reference voltage corresponding to a (P+U)/2 charge component to avoid fatiguing the reference ferroelectric capacitor and to compensate for variations in temperature and voltage.