# Patent

## Title
Level-shifting circuitry having “high” output impedance during disable mode

## Abstract

A level shifting circuit includes a level-shifting section responsive to an input logic signal, which varies between a first voltage level (e.g., ground) and a second voltage level (e.g., 2.1 V). The level-shifting section provides an output logic signal at an output terminal. The output logic signal varies between the first voltage level and a third voltage level (e.g., 2.5V). The circuit also includes an enable/disable section with a first portion coupled between the level shifting section and a first reference voltage node (e.g., ground) and a second portion coupled between the level shifting section and the third reference voltage node. The enable/disable section causes the output terminal to be placed at a relatively high output impedance condition independent of the logic state of the input logic signal in response to a disable mode indication from an enable/disable signal.

## Background
BACKGROUND OF THE INVENTION
This invention relates generally to level-shifting circuitry.
As is known in the art, level-shifting circuitry is used to shift lower voltage signal levels to higher voltage signal levels. One example of such circuit is shown in FIG. 1. Such circuit 9 is formed on a semiconductor chip and includes a pair of N type Metal Oxide Semiconductor Field Effect Transistors (MOSEETS) N1 and N2 and a pair of P type MOSFETs P1 P2 arranged as shown. The P type MOSFETs have the bulk silicon connected to an external +2.5 volt power source. The N type MOSFETs have the bulk silicon connected to ground. The N type MOSFET N1 is a low threshold voltage transistor. The gate of transistor N1 is connected to an internal +2.1 volt source. The input voltage (IN) is a logic signal having logic 1, here represented by +2.1 volts or a logic 0 state, here represented by ground potential. Here, the level-shifter also provides an inversion in the logic state of the input signal as well as shifting the input signal logic 1 state from +2.1 volts to a higher voltage output signal logic 1 state, here +2.5 volts. Thus, in operation, when the input voltage is logic 0, transistors N1, and P2 are “on” and transistors P1 and N2 are “off”, thereby providing a logic 1, here a +2.5 volt level, at the output OUT. Thus, the input logic 1 condition of a +2.1 volt input signal has been shifted to a +2.5 volt output logic 1. On the other hand, when the input voltage IN is logic 1 (i.e., here +2.1 volts), transistors N1, and P2 are “off” and transistors P1 and N2 are “on”, thereby providing ground potential (i.e., an output logic 0) at the output OUT.
SUMMARY OF THE INVENTION
In accordance with the present invention, level-shifting circuitry is provided having a level-shifting section responsive to an input logic signal. The input logic signal has a first voltage level representative of a first logic state or a second voltage level representative of a second logic state. The level-shifting section provides an output logic signal at an output terminal having a third voltage level representative of the first logic state of the input logic signal. The level-shifting circuitry also includes an enable/disable section responsive to an enable/disable signal for placing the output terminal at a relatively high output impedance condition independent of the logic state of the input signal during a disable mode.
In one embodiment, the level-shifting section includes: an input transistor having a control electrode, a first electrode coupled to the input logic signal, and a second electrode. An output pair of serially coupled complementary type transistors is provided. A first one of the pair of transistors has a first electrode coupled to a source of the third voltage level through a first switching transistor and a control electrode coupled to the second electrode of the input transistor. (It should be noted that in the case of a FET, the terms first and second electrode refer to source and drain electrodes, it being understood that while each transistor has a source and drain electrode, the terms may be used interchangeable. Further, in the case of a FET, the term control electrode refers to the gate electrode). A junction between the output pair of transistors provides an output terminal for the level-shifting circuitry. The junction provides the output terminal. A control electrode of the second one of the pair of transistors is connected to the first electrode of the input transistor. The second one of the pair of transistors has a second electrode coupled to the second voltage level through a second switching transistor. The first and second switching transistors are fed by the enable/disable signal.
In one embodiment, the level-shifting section includes an additional transistor. The additional transistor has a control electrode connected to the junction, a first electrode coupled to the source of the third voltage level through the first switching transistor and a second electrode connected to the second electrode of the input transistor. In one embodiment, the input transistor and the additional transistor are of opposite conductivity type.
In one embodiment, the enable/disable circuit includes an inverter fed by the enable/disable signal, such inverter having an output coupled to the control electrode of the first switching transistor.
In one embodiment, the inverter is powered by a source of the first voltage level and the enable/disable signal operates between the first and second voltages. In such embodiment, the inverter comprises a level shifter for shifting the level of the enable/disable signal from the first voltage level to the third voltage level and for feeding such third voltage level to the control electrode of the first switching transistor to placing the first switching transistor to a non-conducting condition during the disable mode.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.



DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram of a level-shifting circuit according to the PRIOR ART;
FIG. 2 is a schematic diagram of a level-shifting circuit according to the invention; and
FIG. 3 is a schematic diagram of a level-shifting circuit according to the invention.



Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
Referring now to FIG. 2, level-shifting circuitry 10 is shown to include: a level-shifting section 12 responsive to an input logic signal IN and an enable/disable section 14 fed by an enable/disable signal ENABLE. The input logic signal IN has a first voltage level, here +2.1 volts representative of a first input logic state, here logic 1, and a second voltage level, here ground, representative of a second input logic state, here logic 0.
The level-shifting section 10 provides an output logic signal at output terminal OUT. The output logic signal at the output terminal OUT has a third voltage level, here +2.5 volts representative of the first output logic state, here logic 1, or the second voltage level, here ground, representative of the second output logic state, here logic 0. Here, during the enable mode (to be described hereinafter), in response to the input logic signal IN having an input logic 1 state (i.e., +2.1 volts), the output logic signal at output terminal OUT will be the ground thereby representing an output logic 0 state. On the other hand, during the enable mode, in response to the input logic signal IN having an input logic 0 state, here ground, the output logic signal at the output terminal OUT will be the third voltage level +2.5 volts thereby representing an output logic 1 state. During a disable mode, the enable/disable section 14 places the output terminal OUT at a relatively high impedance condition independent of the logic state of the input logic signal IN. By high impedance it is meant that the substantially little, or no current, passes though the output terminal OUT.
More particularly, the level-shifting section 12 includes an input transistor N1 having a control, here gate, electrode coupled to a +2.1 volt supply, a first electrode coupled to the input logic signal IN, and a second electrode. An output pair of serially coupled complementary type transistors, i.e., P type MOSFET P2 and N type MOSFET N2, is provided. A first one of the pair of transistors P2 has a first electrode coupled to a source, not shown, of the third voltage level (+2.5 volts) through a first switching transistor P type MOSFET P3 and a control electrode coupled to the second electrode of the input transistor N1. A junction 16 between the output pair of transistors P2, N2 provides the output terminal OUT for the level-shifting circuitry 10. A control electrode of the second one of the pair of transistors N2 is connected to the first electrode of the input transistor N1. The second one of the pair of transistors N2 has a second electrode coupled to the second voltage level, here ground, through a second switching transistor N3. The first and second switching transistors P-type MOSFET P3 and N-Type MOSFET N3 are fed by the enable/disable signal ENABLE, the transistor P3 being coupled to the enable/disable signal ENABLE via an inverter 18, as shown. The level-shifting section 12 includes an additional transistor P type MOSFET P1. The additional transistor P1 has a control electrode connected to the junction 16, a first electrode coupled to the source of the third voltage level +2.5 through the first switching transistor P3 and a second electrode connected to the second electrode of the input transistor N1. The input transistor N1 and the additional transistor P1 are of opposite conductivity type. The enable/disable circuit 14 includes an inverter 18 fed by the enable/disable signal ENABLE. The inverter 18 having an output coupled to the control electrode of the first switching transistor P3. The inverter 18 is powered by a source, not shown, of the third voltage level, +2.5 volts. The control electrode of the input transistor N1 is coupled to the source of the first voltage level +2.1.
In operation, during the disable mode, the enable/disable signal ENABLE is logic 0, here a voltage at ground. In the disable mode, the output of the inverter 18 is at the third voltage i.e., +2.5 volts. Such +2.5 volts turns transistors P3 and N3 “off” thereby placing the output terminal OUT in a high impedance condition independent of the logic state of the input logic signal IN.
During the enable mode, the enable/disable signal ENABLE is logic 1, here the third voltage level, +2.5 volts. The output of inverter 18 is at ground. Thus, transistors N3 and P3 are “on” independent of the logic state of the input logic signal IN. In such condition, in response to the input logic signal IN having an input logic 0 state, here ground, the output logic signal at the output terminal OUT will be +2.5 volts thereby representing output logic 1 state.
More particularly, in the enable mode, the enable/disable signal ENABLE is a logic 1, here the third voltage level, +2.5 volts. In such enable mode, if the input logic signal IN is a logic 0, here ground, transistors N1 and P2 and turn “on” and transistors N2 and P1 turn “off” thereby producing the third voltage level, +2.5 volts, at the junction 16. The third voltage level (+2.5 volts), here output logic 1, is coupled to the output terminal OUT because transistor N2 is “off”. On the other hand, if during the enable mode the input logic signal IN is logic 1, here +2.1 volts, transistors N1, P2 turn “off” while transistors N2 and P1 turn “on” thereby driving junction 16 to ground. This ground, i.e., output logic 0 state appears at the output terminal OUT because transistor P2 is “off”.
It is noted that the operation described above, the inverter 18 is powered by the source of the third voltage level +2.5 volts and thus, the enable/disable signal ENABLE must be at +2.5 volts during the enable mode. Here, the source of the +2.5 volts is an external voltage source. In some applications it might be desirable to use the internal source of the first voltage level, +2.1 volts for generating the enable mode condition of the enable/disable signal ENABLE. In such application, an alternative level-shifting circuitry is shown in FIG. 3.
Thus, referring to FIG. 3, the level-shifting circuitry 10′ is shown where the inverter 18 of FIG. 2 is replaced with an inverter which also provides level-shifting. More particularly, the inverter 18 of FIG. 2 is provided by the level-shifting circuitry 9 described above in connection with FIG. 1. The inverter/level shifter 9 includes a level shifter thus shifts the level of the enable/disable signal ENABLE from the first voltage level +2.1 to the third voltage level +2.5 and feeds such third voltage level +2.5 to the control electrode of the first switching transistor P3 to placing the first switching transistor P3 to a non-conducting (i.e., “off”) condition during the disable mode. More particularly, the inverter/level-shifter 9 is here the level-shifting circuitry 9 shown and described above in connection with FIG. 1, here however, the level-shifting circuitry of FIG. 1 is fed by the enable/disable signal ENABLE and not the input logic signal IN, as in FIG. 1. Thus, in the disable mode, here again the output terminal is at a high output impedance independent of the logic state of the input logic signal IN.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

## Claims

1. Level shifting circuitry, comprising:
a level-shifting section responsive to an input logic signal, such input logic signal having a first voltage level representative of a first logic state or a second voltage level representative of a second logic state, such level-shifting section providing an output logic signal at an output terminal thereof having a third voltage level representative of the first logic state of the input logic signal; and a fourth voltage level representative of the second logic state of the input signal; 
an enable/disable section coupled to the level shifting section, the enable/disable section being responsive to an enable/disable signal, the enable/disable section placing the output terminal at a relatively high output impedance condition independent of the logic state of the input logic signal during a disable mode; 
wherein the level-shifting circuitry includes: 
an input transistor having a control electrode, a first electrode coupled to the input logic signal, and a second electrode; 
a first switching transistor; 
a second switching transistor; 
an output pair of serially coupled complementary type transistors, a first one of the pair of transistors having a first electrode coupled to a source of the third voltage level through the first switching transistor and a control electrode coupled to the second electrode of the input transistor, a junction between the output pair of transistors providing the output terminal for the level-shifting circuitry, a control electrode of the second one of the pair of transistors being connected to the second electrode of the input transistor, the second one of the pair of transistors having a second electrode coupled to the fourth voltage level through the second switching transistor; and 
wherein the first and second switching transistors are fed by the enable/disable signal.

2. The level shifting circuitry recited in claim 1 wherein the enable/disable section includes an inverter, and wherein such inverter is fed by the enable/disable signal, such inverter having an output coupled to the control electrode of the first switching transistor.

3. The level shifting circuitry recited in claims 2 wherein the inverter is powered by a voltage source held at the first voltage level.

4. The level shifting circuitry recited in claim 3 wherein the control electrode of the input transistor is coupled to the source of the first voltage level.

5. The level shifting circuitry recited in claim 1 and further comprising an additional transistor having a control electrode coupled to the junction, a first electrode coupled to the source of the third voltage level through the first switching transistor and a second electrode coupled to the second electrode of the input transistor.

6. The level shifting circuitry recited in claim 5 wherein the input transistor and the additional transistor are of opposite conductivity type.

7. Level shifting circuitry comprising:
a level-shifting section responsive to an input logic signal, such input logic sia having a first voltage level representative of a first logic state or a second voltage level representative of a second logic state, such level-shifting section providing an output logic signal at an output terminal thereof having a third voltage level representative of the first logic state of the input logic signal; 
an enable/disable section coupled to the output terminal, the enable/disable section being responsive to an enable/disable signal, the enable/disable section placing the output terminal at a relatively high output impedance condition independent of the logic state of the input logic signal during a disable mode, the enable/disable section including a first switching transistor and a second switching transistor; 
wherein the level-shifting section includes: 
an input transistor having a control electrode, a first electrode coupled to the input logic signal, and a second electrode; 
an output pair of serially coupled complementary type transistors, a first one of the pair of transistors having a first electrode coupled to a source of the third voltage level through the first switching transistor and a control electrode coupled to the second electrode of the input transistor, a junction between the output pair of transistors providing the output terminal for the level-shifting circuitry, a control electrode of the second one of the pair of transistors being connected to the second electrode of the input transistor, the second one of the pair of transistors having a second electrode coupled to the second voltage level though the second switching transistor; 
wherein the first and second switching transistors are fed by the enable/disable signal; 
wherein the enable/disable section includes an inverter, and wherein such inverter is fed by the enable/disable signal, such inverter having an output coupled to the control electrode of the first switching transistor; 
wherein the inverter is powered by a source of the first voltage level; and 
wherein the inverter comprises a level shifter for shifting the level of the enable/disable signal from the first voltage level to the third voltage level and for feeding such third voltage level to the control electrode of the first switching transistor to place the first switching transistor to a non-conducting condition during the disable mode.

8. A level shifting circuit comprising:
an input node to receive an input signal, the input signal varying between a first voltage level and a second voltage level; 
a first n-channel transistor having a first source/drain a second n-cannel transistor having a first source/drain region coupled to the second source/drain region of the first n-channel transistor, a second source/drain region coupled to a second voltage level reference node and a gate coupled to a first enable signal node; 
a first p-channel transistor having a first source/drain region coupled to the first source/drain region of the first n-channel transistor, a second source/drain region and a gate coupled to the input node; 
a second p-channel transistor having a first source/drain region coupled to the second source/drain region of the fast p-channel transistor, a second source/drain region coupled to a third reference node and a gate coupled to a second enable signal node, the third reference node carrying a third voltage level, the third voltage level being different than the first voltage level; and 
a third n-channel transistor having a first source/drain region coupled to the input node, a second source/drain region coupled to the gate of the first p-channel transistor, and a gate coupled to a first voltage level reference node.

9. The circuit of claim 8 and further comprising a third p-channel transistor having a first source/drain region coupled to the gate of the first p-channel transistor, a second source/drain region coupled to the second source/drain region of the first p-channel transistor having a first source/drain region coupled to the gate of the first p-channel transistor, a second source/drain region coupled to the second source/drain region of the first p-channel transistor, and a gate coupled to the first source/drain region of the first p-channel transistor.

10. The circuit of claim 9 and further comprising an inverter having an input and an output, the input coupled to the first enable signal node and the output coupled to the second enable signal node.

11. The circuit of claim 10 wherein the inverter comprises:
a fourth n-channel transistor with a first source/drain region coupled to the second enable signal node, a second source/drain region coupled to the first voltage level reference node, and a gate coupled to the first enable signal node; 
a fourth p-channel transistor with a first source/drain region coupled to the second enable signal node, a second source/drain region coupled to the third reference voltage node, and a gate coupled to the first enable signal node.

12. The circuit of claim 11 wherein the inverter further comprises:
a fifth n-channel transistor with a first source drain region coupled to the first enable signal node, a second source/drain region coupled to gate of the fourth p-channel transistor, and a gate coupled to the first voltage level reference node; and 
a fifth p-channel transistor with a first source/drain region coupled to the gate of the fourth p-channel transistor, a second source/drain region coupled to the third reference voltage node, and a gate coupled to first source/drain region of the fourth p-channel transistor.

13. The circuit of claim 8 wherein a signal carried at the first enable signal node is an inverted version of a signal carried at the second enable signal node.

14. A level shifting circuit comprising:
a second reference voltage node carrying a voltage at a second voltage level, the second voltage level being different than a first voltage level; 
a third reference voltage node carrying a voltage at a third voltage level, the third voltage level being different that the first voltage level and the second voltage level; 
an input node to receive an input signal, the input signal varying between the first voltage level and the second voltage level; 
a first n-channel transistor having a first source/drain region, a second source/drain region and a gate, the gate being coupled to the input node; 
a second n-channel transistor having a first source/drain region coupled to the second source/drain region of the first n-channel transistor, a second source/drain region coupled to the second voltage level reference node and a gate coupled to a first enable signal node; 
a first p-channel transistor having a first source/drain region coupled to the first source/drain region of the first n-channel transistor, a second source/drain region and a gate coupled to the input node; 
a second p-channel transistor having a first source/drain region coupled to the second source/drain region of the first p-channel transistor, a second source/drain region coupled to the third reference node and a gate coupled to a second enable signal node; and 
an inverter coupled between the first enable signal node and the second enable signal node, the inverter including a level shifting circuit.

15. The circuit of claim 14 wherein the inverter includes an input coupled to the first enable signal node and an output coupled to the second enable signal node.

16. The circuit of claim 14 wherein the third voltage level is greater than the first voltage level.

17. The circuit of claim 14 wherein the third voltage level is 2.5 volts and the first voltage level is 2.1 volts.

18. A level shifting circuit comprising:
a level-shifting section responsive to an input logic signal, the input logic signal varying between a first voltage level and a second voltage level, the level-shifting section providing an output logic signal at an output terminal thereof, the output logic signal varying between the first voltage level and a third voltage level, the third voltage level being different than the second voltage level; 
a first reference voltage node carrying a voltage at the first voltage level; 
a third reference voltage node carrying a voltage at the third voltage level; 
an enable/disable section including a first portion coupled between the level shifting section and the first reference voltage node and a second portion coupled between the level shifting section and the third reference voltage node, the enable/disable section being responsive to an enable/disable signal, the enable/disable section causing the output terminal to be placed at a relatively high output impedance condition independent of the logic state of the input logic signal in response to a disable mode indication from the enable/disable signal, wherein the first portion of the enable/disable section comprises a first switch between the level shifting section and the first reference voltage node and wherein the second portion of the enable/disable section includes a second switch coupled between the level shifting second and the third reference voltage node; and 
an inverter coupled between a control terminal of the first switch and a control terminal of the second switch, wherein the inverter is coupled to the third reference voltage node and wherein the inverter includes a level shifter coupled to the third reference voltage node and to a voltage node at the first voltage level.

19. The circuit of claim 18 wherein the first voltage level and the third voltage level are representative of a first logic state and wherein the first voltage level and the second voltage level are representative of a second logic state.

20. The circuit of claim 18 wherein the level-shifting section comprises:
a first transistor with a current path coupled between the third reference voltage node and the output terminal; and 
a second transistor with a current path coupled between the output to and the first reference voltage node.

21. The circuit of claim 20 wherein, in the enable/disable section, the first switch comprises a third transistor with a current path coupled in series the current path of the first transistor, the current path of the third transistor coupled between the third reference voltage node and the first transistor; and the second switch comprises a fourth transistor with a current path coupled in series the current path of the second transistor, the current path of the fourth transistor coupled between the first reference voltage node and the second transistor.

22. The circuit of claim 21 wherein both the third and fourth transistors are rendered conductive when the enable/disable signal indicates hat the level shifting circuit is in an enable mode and wherein both the third and fourth transistors are rendered non-conductive when the enable/disable signal indicates that the level shifting circuit is in the disable mode.

23. The circuit of claim 18 wherein the inverter includes an input coupled to the control terminal of the first switch and an output coupled to the control terminal of the second switch.

24. The circuit of claim 18 wherein the first and second switches comprise MOS transistors.