# Patent

## Title
Level shifter for low voltage operation

## Abstract

A voltage level translator boosts the gate voltage of a transistor, and increases the gate to source voltage, to allow operation over a wider range of supply voltages. The P/N ratio of transistors in the voltage level translator is therefore increased, and control of the flipping of nodes is dependent on gate voltages as opposed to P/N ratios.

## Background
RELATED APPLICATION
This application is a continuation of U.S. patent application Ser. No. 10/931,587 (Allowed), filed Sep. 1, 2004, now U.S. Pat. No. 7,200,053, and titled “LEVEL SHIFTER FOR LOW VOLTAGE OPERATION,” which is commonly assigned and incorporated by reference in its entirety herein.


FIELD
The present invention relates generally to integrated circuits and in particular the present invention relates to level shifting for low voltage operation of integrated circuits.
BACKGROUND
Normal operation of integrated circuits requires less and less operating voltage as technology continues to improve. As operating voltages for integrated circuits continue to shrink, voltage differences previously relied upon to be available for operating integrated circuits are becoming strained. Where once an operating supply voltage Vcc was on the order of 5 volts, supply voltages have dropped into the range of 1.2 volts. Supply voltages at this level are close to the threshold voltages of the devices that are being operated in a circuit.
When systems with low supply voltages are operated, current capabilities of devices are sometimes reduced due to the lack of sufficient voltage to fully turn on or turn off transistors and the like. When this happens, unreliable operation of the devices, and therefore the circuits, can create numerous issues, including failure of the part to perform as desired and the like. In memory devices, such an improper operation can result in lost data or incorrectly written data.
In order to provide sufficient operating voltages when supply voltages continue to drop, level shifters are used. Typical level shifter circuitry operates on a fairly narrow range of supply voltages. A previous level shifter 100 is shown in FIG. 1. Level shifter 100 comprises cross coupled p-channel metal oxide semiconductor (PMOS) transistors 102 and 104, and NMOS transistors 106 and 108 gate coupled to complementary inputs. Output is taken at node 110 and complementary output node 112.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved low level voltage shifting.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is circuit diagram of a typical prior art level shifter;
FIG. 2 is a circuit diagram of a level shifter according to one embodiment of the present invention;
FIG. 3 is a circuit diagram of a level shifter according to another embodiment of the present invention;
FIG. 4 is a circuit diagram of a level shifter according to still another embodiment of the present invention;
FIG. 5 is a circuit diagram of a level shifter according to yet another embodiment of the present invention;
FIG. 6 is a circuit diagram of a level shifter according to yet another embodiment of the present invention; and
FIG. 7 is a block diagram of a memory and processing system on which embodiments of the present invention are practiced.



DETAILED DESCRIPTION
In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.
The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.
FIG. 2 is a diagram of a voltage translator 200 according to one embodiment of the present invention. Voltage translator 200 comprises a pair of cross coupled PMOS transistors 202 and 204 in first and second branches, as well as NMOS transistors 206 and 208 connected in series with transistors 202 and 204, respectively, between Vccp and input bar signal 203 and input signal 201, respectively. The gates of transistors 202 and 204 are cross-coupled to nodes 212 and 210 of the other series branch, respectively. Boosting capacitance element 214 is connected between input 201 and precharge element 218, with a node 224 between boosting capacitance 214 and precharge element 218 connected to the gate of transistor 206. Boosting capacitance 216 is connected between input bar 203 and precharge element 220, with a node 226 between boosting capacitance 216 and precharge element 220 connected the gate of transistor 208.
In one embodiment, Vccp is approximately 3 volts, and Vcc is approximately 1.2 volts. Input signal 201 ramps from 0 to 1.2 V, and input bar 203 ramps from 1.2 to 0 V in an oscillating input. At those voltage ranges, the voltage at nodes 224 and 226 are at potential Vcc-Vt of the respective transistors of precharge elements 218 and 220. When input 201 equals 0, transistor 208 has a gate voltage sufficient to turn it completely on with the capacitance from boosting capacitance element 216. In that instance, node 212 is pulled to the value of input signal 201 (0 at this time), and transistor 202 turns on. This passes Vccp to node 210, turning transistor 204 completely off. As the input signal 201 ramps to 1.2V, the current at transistor 208 slows, and transistor 206 begins to turn on. As this happens the lowering voltage of complementary input signal 203 is passed to node 210, ramping the voltage down at node 210 to 0, turning transistor 204 on, which in turn passes Vccp to node 212, completely shutting off transistor 202. Because of the boosting at nodes 224 and 226, the normal gate to source voltage of transistors 206 and 208, which is typically 1.2V, is increased to 1.2+ΔV, increasing the operating range of Vccp.
As the boosting capacitance of a boosting element drops, the transistor to which the boosting capacitance is connected is clamping. The very low current under initial conditions through transistor 206 because of the almost insufficient voltage at its gate means that node 210 cannot flip at such a condition. In a typical level translator such as that shown in FIG. 1, the P/N ratio of the transistors must be approximately 1:50 in order to provide proper operation, that is the n-type transistors are on the order of 50 time larger than the p-type transistors. The boosting capacitance and precharging of the present embodiments allows the size differences between the p-type and n-type transistors to drop, or in other words, the P/N ratio of the transistors is increased, to on the order of 1:3 or 1:4. Because of this, as supply voltages continue to drop, the boosted gate to source voltage at the transistors 206 and 208 allow for operation well beyond the normal operating range of typical level translators such as that shown in FIG. 1. Node flips in this embodiment do not rely heavily on P/N ratio. Instead, node flips in this embodiment are dependent on gate voltages. For a supply voltage Vcc of 1.5 V, a typical level translator, such as that shown in FIG. 1, and the embodiment of FIG. 2 operate properly (provided the P/N ratio is sufficiently low in the typical level translator). However, if Vcc drops to 1.2 volts, the typical configuration will not work, but the present embodiment does.
More detailed circuits showing further embodiments of the present invention are shown in FIGS. 3, 4, 5, and 6. FIG. 3 is a diagram of a circuit 300. In circuit 300, the boosting capacitances 214 and 216, as well as the precharging elements 218 and 220, are shown in further detail. In FIG. 3, the element 214 comprises a transistor 302 with its drain and source connections electrically connected and its gate connected to node 304, and a transistor 306 gate connected to the source of transistor 302. Transistor 306 is connected between node 210 and ground. The element 216 comprises a similar drain to source connected transistor 308 with it's gate connected to node 310, and a transistor 312 gate connected to the source of transistor 308. Transistor 312 is connected between node 212 and ground.
Element 218 in one embodiment comprises a thin oxide low threshold voltage diode connected transistor 314 connected between Vcc and node 304. Element 220 in one embodiment comprises a thin oxide low threshold voltage diode connected transistor 316 connected between Vcc and node 310. Thin oxide devices have a lower threshold voltage Vt than thick oxide devices. If a thick oxide device is used, Vcc is on the order of 0.2 to 0.3V. In the embodiment of FIG. 3, the thin oxide devices are boosted with the boosting capacitance. Initial conditions for operation are therefore around 0.8 to 1.2 volts.
The boosting capacitance allows the sizes of the p-type and n-type transistors to be closer than previous level shifters, that is, the ratio of PMOS transistors 202 and 204 to NMOS transistors 206 and 208, respectively, is lowered. Typical P/N ratios in the prior art are on the order of 1:50. In contrast, the ratios of the present invention are reduced because of the boosting to on the order of 1:3 or 1:4.
FIG. 4 shows another embodiment 400 of a level translator circuit. Circuit 400 is substantially identical to the circuit 300 of FIG. 3, with the exception of the connection of the transistors 314 and 316 to Vcc. The connection to Vcc for transistors 314 and 316 in circuit 300 is through p-type transistor 402 which is electrically connected to pass Vcc through to the transistors 314 and 316 because of its gate connection to ground. On power up of this circuit embodiment, the supply voltage Vcc is provided to the gates of the diode connected precharge transistors, and blocks early latching.
FIG. 5 shows yet another embodiment 500 of a level translator circuit. Circuit 500 is substantially similar to circuit 300 of FIG. 3 with the exception of the configuration of the capacitive boosting elements 214 and 216. The boosting capacitance elements 214 and 216 each comprise in this embodiment a drain to source connected transistor 502 or 504 that is gate connected to node 304 or 310 respectively. Node 304 and node 310 are connected respectively to diode connected transistors 506 and 508 which are in turn connected to Vcc. This limits the voltage at the nodes 304 and 310 to approximately Vcc +Vt.
The embodiments of FIG. 5 use clamping circuits to limit the voltages at the nodes between the boosting capacitance and the precharge elements.
FIG. 6 shows yet another embodiment 600 of a level translator circuit. Circuit 600 has boosting capacitance elements 214 and 216 substantially identical to that of FIG. 3. Circuit 600 has nodes 602 and 604 connected, respectively, to the gates of precharge element transistors 606 and 608.
The embodiments of the present invention have wider acceptable Vcc ranges because of the boosting capacitance and precharging applied to the inputs.
FIG. 7 is a functional block diagram of a memory device 700, of one embodiment of the present invention, which is coupled to a processor 710. The memory device 700 and the processor 710 may form part of an electronic system 720. The memory device 700 has been simplified to focus on features of the memory that are helpful in understanding the present invention. The memory device includes an array of memory cells 730. The memory array 730 is arranged in banks of rows and columns.
An address buffer circuit 740 is provided to latch address signals provided on address input connections A0-Ax 742. Address signals are received and decoded by row decoder 744 and a column decoder 746 to access the memory array 730. It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends upon the density and architecture of the memory array. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.
The memory device reads data in the array 730 by sensing voltage or current changes in the memory array columns using sense/latch circuitry 750. The sense/latch circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array. Sense/latch circuitry 750 in one embodiment includes a driver circuit for the pass transistors of the sense/latch circuitry, such as that described above. Data input and output buffer circuitry 760 is included for bi-directional data communication over a plurality of data (DQ) connections 762 with the processor 710.
Command control circuit 770 decodes signals provided on control connections 772 from the processor 710. These signals are used to control the operations on the memory array 730, including data read, data write, and erase operations. The flash memory device has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.
A voltage level translator circuit 780, such as is shown in circuits 300, 400, 500, or 600 above, is shown in FIG. 7 connected to address circuitry 740 and I/O circuitry 760. The voltage level translator circuit 300 is used in various embodiments in a memory device and in a processing system including processor 710, to provide proper voltage translation to circuits having different voltage requirements. It should be understood that any circuit in such a memory device or processing system that needs a voltage different than the supply voltage, for example a boosted voltage, may use the embodiments of the present invention, and that while not all connections are shown, such connections and use of the voltage level translator circuit embodiments of the present invention are within its scope. It should also be understood that while a generic memory device is shown, the voltage level translator circuit embodiments of the present invention are amenable to use with multiple different types of memory devices, including but not limited to dynamic random access memory (DRAM), synchronous DRAM, flash memory, and the like.
It should also be understood that while specific levels of voltages are discussed herein, the embodiments of the present invention are amenable to use with other voltage levels, and the concepts for those other voltage levels are within the scope of the present invention.
CONCLUSION
Level translator circuits have been described that include boosting capacitance and precharge elements to affect the gate to source voltages of transistors to provide operation with smaller NMOS transistors and over a wider Vcc voltage range. NMOS transistor gates for pulling down a drain node is controlled by the boosting technique.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

## Claims

1. A voltage level translator, comprising:
a pair of cross coupled p-type transistors;
a pair of n-type transistors connectable between complementary inputs and a gate of the one of the p-type transistors; and
a pair of boosting circuits, the gate of the n-type transistors connected to one of the boosting circuits to boost the gate voltage of the n-type transistor, each boosting circuit comprising:
a first drain to source connected transistor gate connected to the gate of its respective n-type transistor; and
a second, diode connected, transistor connected between the gate of its respective n-type transistor and a supply voltage.

2. The voltage level translator of claim 1, and further comprising:
a precharge element to precharge the gate of each of the pair of n-type transistors.

3. The voltage level translator of claim 2, wherein each precharge element comprises:
a thin oxide n-type transistor gate to drain connected between a supply voltage and the gate of one of the first or the second n-type transistors.

4. A voltage level translator, comprising:
a pair of cross coupled p-type transistors;
a pair of n-type transistors connectable between complementary inputs and a gate of the one of the p-type transistors;
a pair of boosting circuits each to boost a gate voltage of the gate of its respective n-type transistor, each boosting circuit comprising:
a first drain to source connected transistor gate connected to the gate of its respective n-type transistor; and
a second, diode connected, transistor connected between the gate of its respective n-type transistor and a supply voltage; and

a precharge element to precharge the gate of each of the pair of n-type transistors.

5. A voltage level translator, comprising:
a first and a second p-type transistor;
a first and a second n-type transistor, the first n-type transistor connected between an input signal and a gate of the second p-type transistor, and the second n-type transistor connected between an input signal and the gate of the first p-type transistor; and
a pair of boosting circuits, the gate of one of the n-type transistors connected to one of the boosting circuits to boost the gate voltage of the n-type transistor, each boosting circuit comprising:
a first drain to source connected transistor gate connected to the gate of its respective n-type transistor; and
a second, diode connected, transistor connected between the gate of its respective n-type transistor and a supply voltage.

6. The voltage level translator of claim 5, and further comprising:
a precharge element to precharge the gate of each of the pair of n-type transistors.

7. The voltage level translator of claim 6, wherein each precharge element comprises:
a thin oxide n-type transistor gate to drain connected between a supply voltage and the gate of one of the first or the second n-type transistors.

8. A voltage level translator, comprising:
a pair of cross coupled p-type transistors;
a pair of n-type transistors connected between complementary inputs and a gate of the one of the p-type transistors;
a pair of boosting circuits, the gate of the n-type transistors connected to one of the boosting circuits to boost the gate voltage of the n-type transistor, each boosting circuit comprising:
a first drain to source connected transistor gate connected to the gate of its respective n-type transistor; and
a second, diode connected, transistor connected between the gate of its respective n-type transistor and a supply voltage; and

a precharge element to precharge the precharge the gate of each of the pair of n-type transistors.

9. The voltage level translator of claim 8, wherein each precharge element comprises:
a thin oxide n-type transistor gate to drain connected between a supply voltage and the gate of one of the first or the second n-type transistors.

10. A memory device comprising:
an array of memory cells;
control circuitry to read, write and erase the memory cells;
address circuitry to latch address signals provided on address input connections;
input/output circuitry coupled to read and latch a row of data from the memory array; and
a voltage level translator circuit connected to at least one of the control circuitry, the address circuitry, or the input/output circuitry, the voltage level translator comprising:
a pair of cross coupled p-type transistors;
a pair of n-type transistors connected between complementary inputs and a gate of the one of the p-type transistors; and
a pair of boosting circuits, the gate of the n-type transistors connected to one of the boosting circuits to boost the gate voltage of the n-type transistor, each boosting circuit comprising:
a first drain to source connected transistor gate connected to the gate of its respective n-type transistor; and
a second, diode connected, transistor connected between the gate of its respective n-type transistor and a supply voltage.

11. A processing system, comprising:
a processor; and
a memory coupled to the processor to store data provided by the processor and to provide data to the processor, the memory comprising:
an array of memory cells;

control circuitry to read, write and erase the memory cells;
address circuitry to latch address signals provided on address input connections;
input/output circuitry coupled to read and latch a row of data from the memory array; and
a voltage level translator circuit connected to at least one of the control circuitry, the address circuitry, or the input/output circuitry, the voltage level translator comprising:
a pair of cross coupled p-type transistors;
a pair of n-type transistors connected between complementary inputs and a gate of the one of the p-type transistors; and
a pair of boosting circuits, the gate of the n-type transistors connected to one of the boosting circuits to boost the gate voltage of the n-type transistor, each boosting circuit comprising:
a first drain to source connected transistor gate connected to the gate of its respective n-type transistor; and
a second, diode connected, transistor connected between the gate of its respective n-type transistor and a supply voltage.

12. A method of boosting gate to source voltage of an n-type transistor in a voltage level translator, comprising:
boosting a gate to source voltage of each of a pair of pull down n-type transistors in the translator, wherein boosting comprises boosting with a drain to source connected transistor having its gate coupled to the gate of its respective n-type transistor and a diode connected transistor connected between the gate of its respective n-type transistor and a supply voltage.

13. The method of claim 12, and further comprising:
clamping a clamping transistor to which the boosting capacitance is also connected.

14. The method of claim 13, wherein clamping comprises:
limiting a voltage at the gate of each n-type transistor to approximately the supply voltage plus a threshold voltage of the clamping transistor.

15. The method of claim 12, and further comprising:
precharging a node connected to the gate of each of the pull down n-type transistors.

16. The method of claim 15, and further comprising:
limiting the voltages at the nodes between the boosting capacitance and the precharge elements.

17. The method of claim 16, wherein limiting is performed with a clamping circuit.

18. The method of claim 12, and further comprising:
precharging the gate of each of the pair of pull down n-type transistors.

19. A method of translating voltage level, comprising:
boosting a gate voltage of the gate of each of a pair of n-type transistors in a level translator; and
precharging the gate of each of the pair of n-type transistors;
wherein boosting comprises boosting with a drain to source connected transistor having its gate coupled to the gate of its respective n-type transistor and a diode connected transistor connected between the gate of its respective n-type transistor and a supply voltage.

20. The method of claim 19, and further comprising:
clamping a clamping transistor, a single clamping transistor connected to each of the n-type transistor gates.

21. The method of claim 20, wherein clamping comprises:
limiting a voltage at the gate of each n-type transistor to approximately the supply voltage plus a threshold voltage of the clamping transistor.

22. The method of claim 19, and further comprising:
limiting the voltages at the nodes between the boosting capacitance and the precharge elements.

23. The method of claim 22, wherein limiting is performed with a clamping circuit.

24. The method of claim 19, wherein precharging comprises:
providing a precharge voltage to the gate of each of the n-type transistors through a gate to source connected thin oxide transistor, the precharge voltage being approximately the supply voltage minus a threshold voltage of the thin oxide transistor.

25. The method of claim 19, and further comprising:
blocking early latching of the level translator.

26. The method of claim 25, wherein blocking comprises:
providing a precharge voltage to the gate of each of the n-type transistors through a gate to source connected thin oxide transistor, the precharge voltage being approximately the supply voltage minus a threshold voltage of the thin oxide transistor.

27. A method of increasing an operating range of supply voltage in a voltage level translator, comprising:
boosting a gate to source voltage of each of a pair of pull down n-type transistors in the translator; and
precharging the gate of each of the pair of n-type transistors;
wherein boosting comprises boosting with a drain to source connected transistor having its gate coupled to the gate of its respective n-type transistor and a diode connected transistor connected between the gate of its respective n-type transistor and a supply voltage.

28. The method of claim 27, and further comprising:
clamping a clamping transistor, a single clamping transistor connected to each of the n-type transistor gates.

29. The method of claim 28, wherein clamping comprises:
limiting a voltage at the gate of each n-type transistor to approximately the supply voltage plus a threshold voltage of the clamping transistor.

30. The method of claim 27, and further comprising:
limiting the voltages at the nodes between the boosting capacitance and the precharge elements.

31. The method of claim 30, wherein limiting is performed with a clamping circuit.

32. The method of claim 27, wherein precharging comprises:
providing a precharge voltage to the gate of each of the n-type transistors through a gate to source connected thin oxide transistor, the precharge voltage being approximately the supply voltage minus a threshold voltage of the thin oxide transistor.

33. The method of claim 27, and further comprising:
blocking early latching of the level translator.

34. The method of claim 33, wherein blocking comprises:
providing a precharge voltage to the gate of each of the n-type transistors through a gate to source connected thin oxide transistor, the precharge voltage being approximately the supply voltage minus a threshold voltage of the thin oxide transistor.

35. A method of expanding an operating voltage range of a level translator circuit, comprising:
boosting a gate voltage of the gate of each of a pair of n-type transistors; and
precharging the gate of each of the pair of n-type transistors, wherein boosting comprises boosting with a drain to source connected transistor having its gate coupled to the gate of its respective n-type transistor and a diode connected transistor connected between the gate of its respective n-type transistor and a supply voltage.