Patent Publication Number: US-11652476-B2

Title: Pad-tracking circuit design to prevent leakage current during power ramp up or ramp down of output buffer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of U.S. Provisional Application No. 63/142,517, filed on Jan. 28, 2021, which is included herein by reference in its entirety. 
    
    
     BACKGROUND 
     In conventional general purpose input/output (GPIO) designs, an output buffer and an input buffer are connected to the same pad, and the GPIO can be operate in an output mode, an input mode or a fail-safe mode. Regarding three modes of the GPIO, when the GPIO operates in the output mode, the output buffer is enabled so that a signal can be outputted to another device via the pad; when the GPIO operates in the input mode, the output buffer is disabled; and when the GPIO operates in the fail-safe mode, the output buffer is disabled while no supply voltage is applied to the output buffer, and a high voltage is provided to the pad. In the above design, however, the output buffer may have leakage current if the supply voltage ramps up or ramp down, that is the output buffer may have leakage when a switching operation between the input mode and the fail-safe mode. 
     SUMMARY 
     It is therefore an objective of the present invention to provide an output buffer design, which can prevent leakage current during power ramp up and power ramp down of the output buffer, to solve the above-mentioned problems. 
     According to one embodiment of the present invention, an output buffer comprising a first transistor, a second transistor and a pad-tracking circuit is disclosed. The first transistor is coupled between a supply voltage and an output node, wherein the output node is coupled to a pad. The second transistor is coupled between the output node and a reference voltage. The pad-tracking circuit is coupled to the control circuit and the first transistor, and is configured to generate a gate control signal to a gate electrode of the first transistor. The output buffer is selectively operated in an input mode and a fail-safe mode, when the output buffer operates in the input mode, the pad-tracking circuit generates the gate control signal to disable the first transistor, and the second transistor is also disabled; and when the output buffer operates in the fail-safe mode, the first transistor is powered down, and the pad-tracking circuit generates the gate control signal to the gate electrode of the first transistor according to a voltage of the pad. When the output buffer switches between the input mode and the fail-safe mode and the supply voltage of the first transistor ramps up or ramps down, the pad-tracking circuit generates the gate control signal to the gate electrode of the first transistor according to the voltage of the pad. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating an output buffer operating in an output mode according to one embodiment of the present invention. 
         FIG.  2    is a diagram illustrating the output buffer operating in an input mode according to one embodiment of the present invention. 
         FIG.  3    is a diagram illustrating the output buffer operating in a fail-safe mode according to one embodiment of the present invention. 
         FIG.  4    shows a conventional output buffer that a voltage at the pad has an IR drop when switching between the input mode and the fail-safe mode. 
         FIG.  5    shows that no leakage current occurs in the embodiment of the present invention. 
         FIG.  6    is a diagram illustrating the pad-tracking circuit according to one embodiment of the present invention. 
         FIG.  7    is a diagram illustrating the first multiplexer and the second multiplexer when the output buffer operates in the input mode according to one embodiment of the present invention. 
         FIG.  8    is a diagram illustrating the first multiplexer and the second multiplexer when the output buffer operates in the fail-safe mode according to one embodiment of the present invention. 
         FIG.  9    is a diagram illustrating an output buffer according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. The terms “couple” and “couples” are intended to mean either an indirect or a direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
       FIG.  1    is a diagram illustrating an output buffer  100  operating in an output mode according to one embodiment of the present invention. As shown in  FIG.  1   , the output buffer  100  comprises a control circuit  110 , a pad-tracking circuit  120 , a P-type transistor MP 1  and an N-type transistor MN 1 , wherein the output buffer  100  is supplied by a supply voltage VDIO 1  and a reference voltage VSS, and the supply voltage VDIO 1  may be equal to a fixed high voltage VDDQ, and the reference voltage VSS may be equal to 0V. In addition, the output buffer  100  is coupled to a pad  104 , wherein the pad  104  is selectively coupled to a specific voltage provider  102 , wherein the specific voltage provider  100  is configured to selectively provide a supply voltage VDIO 2  to the pad  104  via a resistor R. 
     When the output buffer  100  operates in the output mode, the output buffer  100  is configured to output a high voltage (i.e., logical value “1”) or a low voltage (i.e., logical value “0”) to the pad  104 , and the specific voltage provider  102  is disabled. Specifically, the control circuit  110  receives two control signals VC_I and VC_E, wherein the control signal VC_E is used to indicate if the output buffer  100  operates in the output mode, and the control signal VC_I is used to control the output buffer  100  to output the high voltage or the low voltage. In this embodiment, when the control signal VC_E is equal to “1”, the control circuit  110  determines the output mode, and if the control signal VC_I is equal to “0”, the control circuit  110  controls the pad-tracking circuit  120  to generate a gate control signal PG 1  having high voltage such as VDDQ to a gate of the transistor MP 1 , and to generate a bulk control signal PB 1  having high voltage such as VDDQ to a bulk of the transistor MP 1 , to disable transistor MP 1 ; and the control circuit  110  further generates a gate control signal NG 1  having high voltage such as VDDQ to a gate of the transistor MN 1  to enable the transistor MN 1 . At this time, the pad  104  has a low voltage such as 0V. On the other hand, if the control signal VC_I is equal to “1”, the control circuit  110  controls the pad-tracking circuit  120  to generate the gate control signal PG 1  having low voltage such as 0V to the gate of the transistor MP 1 , and to generate the bulk control signal PB 1  having high voltage such as VDDQ to the bulk of the transistor MP 1 , to enable transistor MP 1 ; and the control circuit  110  further generates the gate control signal NG 1  having low voltage such as 0V to the gate of the transistor MN 1  to disable the transistor MN 1 . At this time, the pad  104  has a high voltage such as VDDQ. 
       FIG.  2    is a diagram illustrating the output buffer  100  operating in an input mode according to one embodiment of the present invention. As shown in  FIG.  2   , when the output buffer  100  operates in the input mode, the output buffer  100  and the specific voltage provider  102  are disabled, and an input buffer (not shown) is configured to receive signal at the pad  104 . Specifically, the control circuit  110  receives two control signals VC_I and VC_E, wherein the control signal VC_E is used to indicate if the output buffer  100  operates in the input mode. In this embodiment, when the control signal VC_E is equal to “0”, the control circuit  110  determines the output mode, and under the condition that a voltage VPAD at the pad  104  has lower voltage level such as 0V, regardless of whether the control signal VC_I is equal to 0 or 1, the control circuit  110  controls the pad-tracking circuit  120  to generate the gate control signal PG 1  having high voltage such as VDDQ to the gate of the transistor MP 1 , and to generate the bulk control signal PB 1  having high voltage such as VDDQ to the bulk of the transistor MP 1 , to disable transistor MP 1 ; and the control circuit  110  further generates the gate control signal NG 1  having low voltage such as 0V to the gate of the transistor MN 1  to disable the transistor MN 1 . At this time, the output buffer  100  does not provide any signal to the pad  104 . 
       FIG.  3    is a diagram illustrating the output buffer  100  operating in a fail-safe mode according to one embodiment of the present invention. As shown in  FIG.  3   , when the output buffer  100  operates in the fail-safe mode, the control circuit  110  and the transistor MP 1  of the output buffer  100  are powered down (i.e., the supply voltage VDIO 1  becomes 0V), the specific voltage provider  102  is enabled to provide high voltage to the pad  104  (in this embodiment, the pad  104  has voltage VPAD which is much close to supply voltage VDIO 2 , and the supply voltage VDIO 2  may be equal to or higher than VDDQ), and the pad-tracking circuit  120  can be supplied by the supply voltage VDIO 2  to generate the gate control signal PG 1  and the bulk control signal PB 1  based on the voltage VPAD at the pad  104 . Specifically, when the output buffer  100  operates in the fail-safe mode, the pad-tracking circuit  120  generates the gate control signal PG 1  and the bulk control signal PB 1  having the voltage VPAD to the transistor MP 1 , to disable the transistor MP 1  to prevent leakage current. In addition, in this embodiment, VDDQ is provided by a voltage source, and VDDQ always has a high voltage level (fixed high voltage level) no matter if the output buffer  100  operates in the output mode and the input mode. 
     In the conventional art, when the output buffer  100  operates in the input mode, the control circuit  110  always controls the pad-tracking circuit  120  to generate the gate control signal PG 1  having VDDQ to disable the transistor MP 1 . However, when the output buffer  100  switches between the input mode and the fail-safe mode, that is the supply voltage VDIO 1  increases from 0V to VDDQ or decreases from VDDQ to 0V, the transistor MP 1  may be enabled within a period of time, thus causing leakage current. Specifically, referring to  FIG.  4   , when the supply voltage VDIO 1  ramps up or ramps down, the gate control signal PG 1  and the bulk control signal PB 1  may be dropped and the transistor MP 1  may be enabled so that a leakage current flowing from the pad  104  to the source electrode of the transistor MP 1 , causing the voltage VPAD has an IR drop. In order to solve this problem, the pad-tracking circuit  120  is configured to always provide the voltage VPAD at the pad  104  as the gate control signal PG 1  and the bulk control signal PB 1  when the supply voltage VDIO 1  ramps up or ramps down, and there will be no leakage current between the pad  104  and the supply voltage VDIO 1 , as shown in  FIG.  5   . 
       FIG.  6    is a diagram illustrating the pad-tracking circuit  120  according to one embodiment of the present invention. As shown in  FIG.  6   , the pad-tracking circuit  120  comprises a first multiplexer  610  and a second multiplexer  620 , wherein the first multiplexer  610  is configured to receive a plurality of input signals such as VDDQ and VPAD, and select one of the input signals as the gate control signal PG 1  according to a first selection signal SEL 1 ; and the second multiplexer  620  is configured to receive a plurality of input signals such as VDDQ and VPAD, and select one of the input signals as the bulk control signal PB 1  according to a second selection signal SEL 2 . In this embodiment, when the supply voltage VDIO 1  is equal to 0V and the output buffer  100  operates in the fail-safe mode, the first multiplexer  610  outputs the voltage VPAD at the pad  104  as the gate control signal PG 1 , and the second multiplexer  620  outputs the voltage VPAD at the pad  104  as the bulk control signal PB 1 . That is, if VPAD is a low voltage such as 0V, the gate control signal PG 1  and the bulk control signal PB 1  are equal to 0V; and if VPAD is a high voltage such as VDIO 2 /VDDQ, the gate control signal PG 1  and the bulk control signal PB 1  are equal to VDDQ. In addition, when the supply voltage VDIO 1  is equal to VDDQ and the output buffer  100  operates in the input mode, if VPAD is a low voltage such as 0V, the first multiplexer  610  outputs VDDQ at the pad  104  as the gate control signal PG 1 , and the second multiplexer  620  outputs VDDQ at the pad  104  as the bulk control signal PB 1 ; and if VPAD is a high voltage such as VDIO 2 /VDDQ, the first multiplexer  610  outputs VPAD at the pad  104  as the gate control signal PG 1 , and the second multiplexer  620  outputs VPAD at the pad  104  as the bulk control signal PB 1 . In addition, in the input mode, the first selection signal SEL 1  and the second selection signal SEL 2  can be any signal capable of indicating the level of VPAD. 
     In the embodiment shown in  FIG.  6   , no matter what mode it is (input mode or fail-safe mode), when the pad  104  receives a high voltage such as VDIO 2  from the specific voltage provider  102 , the pad-tracking circuit  120  always provides the voltage VPAD at the pad  104  as the gate control signal PG 1  and the bulk control signal PB 1 , to prevent the leakage current of the transistor MP 1  when the supply voltage VDIO 1  ramps up or ramps down. 
       FIG.  7    is a diagram illustrating the first multiplexer  610  and the second multiplexer  620  when the output buffer  100  operates in the input mode according to one embodiment of the present invention. As shown in  FIG.  7   , the first multiplexer  610  comprises P-type transistors MP 2 -MP 4  and N-type transistors MN 2 -MN 3 , wherein the transistors MP 2  and MN 2  serve as a transmission gate to selectively output VDDQ as the gate control signal PG 1 , the transistors MP 3  and MN 3  are coupled between VPAD and VSS, and the transistor MP 4  is used to selectively outputs VPAD as the gate control signal PG 1 . In the first multiplexer  610 , the transistors MN 2 , MP 3  and MN 3  are controlled by an enable signal EN that is equal to 0V, the transistor MP 2  is controlled by an output signal of the transistors MP 3  and MN 3 . In addition, the second multiplexer  620  comprises P-type transistors MP 5 -MP 7  and an N-type transistor MN 4 , wherein the transistors MP 5  and MN 4  are coupled between the supply voltage VDIO 1  and VSS, the transistors MP 6  and MP 7  are coupled between the supply voltage VDIO 1  and VPAD. In the second multiplexer  620 , the transistors MP 5 , MN 4  and MP 6  are controlled by the voltage VPAD, the transistor MP 7  is controlled by the output signal of the transistors MP 5  and MN 4 , the output signal of the transistors MP 5  and MN 4  are also used to control the transistor MP 4  of the first multiplexer  610 , and an output signal of the transistors MP 6  and MP 7  serves as the bulk control signal PB 1 . 
     In the operation of the first multiplexer  610  and the second multiplexer  620  shown in  FIG.  7   , the enable signal EN is equal to 0V, and when the voltage VPAD has a high voltage level such as VDDQ, the transistors MP 2 , MN 2 , MN 3 , MP 5 , MP 6  are disabled while the transistors MP 3 , MP 4 , MN 4  and MP 7  are enabled, so that the gate control signal PG 1  and the bulk control signal PB 1  are equal to VPAD. In addition, when the voltage VPAD has a low voltage level such as 0V, the transistors MN 2 , MN 3 , MP 4 , MN 4  and MP 7  are disabled while the transistors MP 2 , MP 3 , MP 5  and MP 6  are enabled, so that the gate control signal PG 1  and the bulk control signal PB 1  are equal to VDDQ. 
       FIG.  8    is a diagram illustrating the first multiplexer  610  and the second multiplexer  620  when the output buffer  100  operates in the fail-safe mode according to one embodiment of the present invention. As shown in  FIG.  8   , regardless of whether the voltage VPAD has high voltage level or low voltage level, the gate control signal PG 1  and the bulk control signal PB 1  are equal to VPAD. 
     It is noted that the embodiment shown in  FIGS.  6 - 8    are for illustration without a limitation of the present invention. In other embodiments, as long as the pad-tracking circuit  120  can always provide the voltage VPAD at the pad  104  as the gate control signal PG 1  and the bulk control signal PB 1  when the supply voltage VDIO 1  ramps up and ramps down, the pad-tracking circuit  120  may have other circuit designs. 
     In the above embodiment shown in  FIG.  1 - 3   , the output buffer  100  comprises only one P-type transistor MP 1  and only one N-type transistor MN 1  serving as an output stage, however, in other embodiment, the output stage may have more than one P-type transistor and/or more than one N-type transistor. Taking  FIG.  9    as an example, the output stage comprises P-type transistors MP 1 -MPN and N-type transistors MN 1 -MNM connected in cascode, and the P-type transistors MP 1 -MPN and N-type transistors MN 1 -MNM are controlled by control signals PG 1 -PGM and NG 1 -NGM, respectively. 
     Briefly summarized, in the output buffer of the present invention, when the output buffer switches between the input mode and the fail-safe mode, the pad-tracking circuit always provides the gate control signal that is substantially equal to the voltage of the pad to control the P-type transistor when the supply voltage ramps up or ramps down. Therefore, there will be no leakage current flowing through the P-type transistor when the supply voltage of the P-type transistor ramps up or ramps down. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.