Low-swing CMOS input circuit

• US 7,969,191 B2
• Filed: 02/02/2009
• Issued: 06/28/2011
• Est. Priority Date: 02/06/2008
• Status: Active Grant

• Current Assignee: Synopsys Incorporated
• Original Assignee: Synopsys Incorporated
• Inventors: Nedalgi, Dharmaray M
• Primary Examiner(s): Barnie; Rexford N
• Assistant Examiner(s): Tran; Jany
• Application Number:
  US12/866,734
• Publication Number:
  US 20110006810A1
• Time in Patent Office: 876 Days
• Field of Search: 326/31, 326/34, 326/68, 326 70- 72, 326/76, 326 80- 83, 326 86- 87
• US Class Current: 326/81
• CPC Class Codes: H03K 19/0013 in field effect transistor circuits
• CPC Class Codes: H03K 19/018521 of complementary type, e.g. CMOS

Abstract

The invention relates to a CMOS input circuit for receiving low-swing input signals, which is an alternative to the CMOS input circuits as known from the prior art. The CMOS input circuit according to the invention comprises a leveling circuit (LC) that is constructed for arranging, under control of a voltage associated with an output voltage of a CMOS input stage (Inv1), a leveling transistor (M3) which is located in a supply path of the CMOS input stage (Inv1), (i) as a forward-biased diode-connected transistor for regulating the voltage on a source of the CMOS input stage (Inv1) for reducing the gate-source voltage of a switching transistor (M1, M2) in the CMOS input stage (Inv1), when an input voltage of the CMOS input circuit assumes a level associated with a first logical level causing the switching transistor (M1, M2) to be switched off, and (ii) as a conductive path when the input voltage assumes a level associated with a second logical level causing the switching transistor (M1, M2) to be switched on. The invention also relates to an Input-Output circuit, an electronic circuit and a semiconductor device comprising such CMOS input circuit. The invention provides an alternative to known CMOS input circuit that make use of a diode-connected transistor that is short-circuited in case of one of the input voltage levels. An advantageous embodiment of the invention incorporates a positive feedback mechanism that makes the circuit more suitable for low supply voltages.

Claims

1. A CMOS input circuit comprising:

a first power terminal (VDD, VSS);

CMOS input stage (Inv1) comprising a first switching transistor (M1, M2), the first switching transistor (M1, M2) comprising;

a first gate for receiving an input voltage (Vin),

a first drain for supplying an output voltage (Vout1), and

a first source, wherein a first main current path (CP) is arranged between the first source and the first drain; and

a leveling circuit (LC) comprising a leveling transistor (M3) having a leveling circuit main current path (MCP) and a gate electrode (GE), the leveling circuit (LC) being arranged for receiving the input voltage (Vin) and a second voltage associated with the output voltage (Vout1), the leveling circuit main current path (MCP) of the leveling transistor (M3) being electrically connected between the first main current path (CP) of the first switching transistor (M1, M2) and the first power terminal (VDD, VSS), and wherein

the first source is connected to a junction of the first main current path (CP) of the first switching transistor (M1, M2) and the leveling circuit main current path (MCP) of the leveling transistor (M3), and

the leveling circuit (LC) is constructed for arranging, under control of the second voltage, the leveling transistor (M3)

(i) as a forward-biased diode-connected transistor for regulating a voltage (V1) on the first source, for reducing a gate-source voltage of the first switching transistor (M1, M2) when the input voltage (Vin) assumes a level associated with a first logical level causing the first switching transistor (M1, M2) to be switched off, and

(ii) as a conductive path when the input voltage (Vin) assumes a level associated with a second logical level causing the first switching transistor (M1, M2) to be switched on.

2. The CMOS input circuit of claim 1, wherein the leveling circuit (LC) comprises:

a first switching element (M4) coupled between a drain and the gate electrode (GE) of the leveling transistor (M3), and

a second switching element (M5) coupled between the gate electrode (GE) of the leveling transistor (M3) and an input for receiving the input voltage (Vin),

and wherein

the first switching element (M4) is closed and the second switching element (M5) is opened when the input voltage (Vin) assumes the level associated with the first logical level, and

the first switching element (M4) is opened and the second switching element (M5) is closed when the input voltage (Vin) assumes the level associated with the second logical level,

and wherein

the first switching element (M4) and the second switching element (M5) are controlled by the second voltage.

3. The CMOS input circuit of claim 1, wherein the first power terminal is a power supply terminal (VDD) carrying a non-zero power supply voltage.

4. The CMOS input circuit of claim 1, wherein the leveling transistor (M3) is a p-type field effect transistor.

5. The CMOS input circuit of claim 1, wherein the first power terminal is a ground terminal (VSS).

6. The CMOS input circuit of claim 1, wherein the leveling transistor (M3) is an n-type field effect transistor.

7. The CMOS input circuit of claim 1, wherein

the leveling transistor (M3) is configurable as a forward-biased diode-connected transistor by electrically connecting the gate electrode (GE) to the first source and simultaneously electrically disconnecting the gate electrode (GE) from an input of the CMOS input stage (Inv1), and

the leveling transistor (M3) is configurable as a conductive path by electrically connecting the gate electrode (GE) to the input of the CMOS input stage (Inv1) and simultaneously electrically disconnecting the gate electrode (GE) from the first source.

8. The CMOS input circuit of claim 2, wherein

the leveling circuit (LC) comprises a control circuit (CC) having a control output for generating a control signal (V2) for the leveling transistor (M3), wherein the control circuit (CC) comprises:

the first switching element (M4) being connected between the control output and the drain of the leveling transistor (M3), and

the second switching element (M5) being connected between the control output and the first gate, wherein

the first switching element (M4) and the second switching element (M5) are controlled by the second voltage associated with the output voltage (Vout1).

9. The CMOS input circuit of claim 8, wherein the control circuit (CC) is constructed for both controlling the first switching element (M4) and the second switching element (M5) by the second voltage, and wherein the first switching element (M4) and the second switching element (M5) are arranged as MOS transistors of opposite conductivity types.

10. The CMOS input circuit of claim 9, wherein the control circuit is constructed for directly controlling the first and the second switching element (M4, M5) by the output voltage (Vout1).

11. The CMOS input circuit of claim 1, wherein

the CMOS input stage (Inv1) further comprises a second switching transistor (M2, M1), the second switching transistor (M2, M1) comprising:

a second gate for receiving the input voltage (Vin),

a second drain for supplying the output voltage (Vout1), and

a second source, wherein a second main current path of the second switching transistor (M2, M1) is arranged between the second source and the second drain and in series with the first main current path,

the CMOS input circuit further comprising;

a second power terminal (VSS, VDD), and

a second leveling circuit comprising a second leveling transistor having a second main current path and a second gate electrode, the second leveling circuit being arranged for receiving the input voltage (Vin) and a third voltage associated with the output voltage (Vout1), the second main current path being electrically connected between the second main current path (CP) of the second switching transistor (M2, M1) and the second power terminal (VSS, VDD), and wherein the second source is connected to a junction of the second main current path and the second main current path, and wherein

the second leveling circuit is constructed for arranging, under control of the third voltage, the second leveling transistor

(i) as a forward-biased diode-connected transistor for regulating a voltage on the second source for reducing a gate source voltage of the second switching transistor (M2, M1) when the input voltage (Vin) assumes a level associated with the second logical level causing the second switching transistor (M2, M1) to be switched off, and

(ii) as a conductive path when the input voltage (Vin) assumes a level associated with the first logical level causing the second switching transistor (M2, M1) to be switched on.

12. The CMOS input circuit of claim 1, the CMOS input circuit further comprising:

a further leveling circuit comprising a further leveling transistor having a further main current path and a further gate electrode, the further leveling circuit being arranged for receiving the input voltage (Vin) and a further voltage associated with the output voltage (Vout1), the further main current path of the further leveling transistor being connected in between the leveling circuit main current path (MCP) of the leveling transistor (LC) and the first power terminal (VDD, VSS), and

an internal node connected to the junction between the leveling circuit main current path (MCP) and the further main current path of the further leveling transistor, and wherein the further leveling circuit is constructed for arranging, under control of the third voltage, the further leveling transistor

(i) as a forward-biased diode-connected transistor for regulating a voltage on the internal node, and thereby indirectly regulating a voltage on the first source for reducing a gate-source voltage of the first switching transistor (M1, M2) when the input voltage (Vin) assumes a level associated with the first logical level causing the first switching transistor (M1, M2) to be switched off, and

(ii) as a conductive path when the input voltage (Vin) assumes a level associated with the second logical level causing the first switching transistor (M1, M2) to be switched on.

13. The CMOS input circuit of claim 1, wherein the CMOS input stage (Inv1) is selected from a group comprising:

an inverter, a buffer, and a logic gate.

14. An Input-Output circuit comprising the CMOS input circuit of claim 1.

15. An electronic circuit comprising the CMOS input circuit of claim 1.

16. A semiconductor device comprising the circuit of claim 1.