# Patent

## Title
High-voltage selecting circuit which can generate an output voltage without a voltage drop

## Abstract

A high-voltage selecting circuit generates an output voltage with no voltage drop by means of an auxiliary NMOS transistor turning on the corresponding selecting PMOS transistor of the high-voltage selecting circuit when the voltage levels of a first input voltage and a second input voltage are equal. In addition, when one of the first input voltage and the second input voltage is higher than the other one, the high-voltage selecting circuit avoids the leakage current by means of an auxiliary PMOS transistor turning off the corresponding selecting PMOS transistor of the high-voltage selecting circuit. In this way, the high-voltage selecting circuit can correctly generate the output voltage according to the first input voltage and the second input voltage, and avoid the leakage current at the same time.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a high-voltage selecting circuit, and more particularly, to a high-voltage selecting circuit without a voltage drop.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional high-voltage selecting circuit 100. The high-voltage selecting circuit 100 includes selecting P-channel Metal Oxide Semiconductor (PMOS) transistors QPS1 and QPS2. The selecting PMOS transistor QPS1 includes a source, a drain, a gate, and a well. The source of the selecting PMOS transistor QPS1 receives an input voltage VIN1. The gate of the selecting PMOS transistor QPS1 receives an input voltage VIN2. The well of the selecting PMOS transistor QPS1 is coupled to the drain of the selecting PMOS transistor QPS1. The selecting PMOS transistor QPS2 includes a source, a drain, a gate, and a well. The source of the selecting PMOS transistor QPS2 receives the input voltage VIN2. The gate of the selecting PMOS transistor QPS2 receives the input voltage VIN1. The well of the selecting PMOS transistor QPS2 is coupled to the drain of the selecting PMOS transistor QPS2. The drains of the selecting PMOS transistors QPS1 and QPS2 are coupled together for outputting an output voltage VOUT.
The high-voltage selecting circuit 100 is utilized for receiving input voltages VIN1 and VIN2, and selecting the higher one of the input voltages VIN1 and VIN2 to generate an output voltage VOUT. For instance, it is assumed that the voltage level of the input voltage VIN1 is fixed and is equal to the voltage level of a supply voltage VDD, which is about 3.3V. When the input voltage VIN2 is about 6 ̃7.5V (that is, the input voltage VIN2 is higher than the input voltage VIN1), the selecting PMOS transistor QPS1 is turned off and the selecting PMOS transistor QPS2 is turned on. Thus, the voltage level of the voltage on the drain of the selecting PMOS transistor QPS2 is equal to the voltage level of the voltage on the source of the selecting PMOS transistor QPS2. In this way, the voltage level of the output voltage VOUT is equal to that of the input voltage VIN2, which means the high-voltage selecting circuit 100 selects the higher input voltage VIN2 (6 ̃7.5V) as the output voltage VOUT. When the input voltage VIN2 is about 0V (that is, the input voltage VIN1 is higher than the input voltage VIN2), the selecting PMOS transistor QPS2 is turned off and the selecting PMOS transistor QPS1 is turned on. In this way, the voltage level of the voltage on the drain of the selecting PMOS transistor QPS1 is equal to the voltage level of the voltage on the source of the selecting PMOS transistor QPS1. Therefore, the voltage level of the output voltage VOUT is equal to that of the input voltage VIN1, which means the high-voltage selecting circuit 100 selects the higher input voltage VIN1 (3.3V) as the output voltage VOUT.
However, when the input voltage VIN2 is 3.3V (that is, the voltage level of the input voltage VIN2 is equal to that of the input voltage VIN1, as shown in FIG. 2), both the selecting PMOS transistors QPS1 and QPS2 are turned off. Meanwhile, the parasitic diode DSP1 of the selecting PMOS transistor QPS1 and the parasitic diode DSP2 of the selecting PMOS transistor QPS2 are turned on, so that the voltage level of the output voltage VOUT is equal to that of the input voltage VIN1 (or VIN2) deducting the forward voltage VFW1 (or VFW2) of the parasitic diodes DSP1 (DSP2). For instance, when the forward voltages VFW1 and VFW2 are 0.7V, the output voltage VOUT is 2.6V. However, since both the input voltages VIN1 and VIN2 are 3.3V, the output voltage VOUT should be 3.3V according to the requirement. In other words, the conventional high-voltage selecting circuit 100 generates the output voltage VOUT with a voltage drop.
More particularly, when the voltage level of the input voltage VIN2 is equal to that of the input voltage VIN1, the output voltage VOUT (2.6V) of the conventional high-voltage selecting circuit 100 is not at the required voltage level. If the output voltage VOUT (2.6V) is inputted to an inverter 101 coupled between a supply voltage source VDD (3.3V) and a ground (0V), as shown in FIG. 3, both the PMOS transistor QINVP of the inverter 101 and the NMOS transistor QINVN are turned on. In this way, a large leakage current ILEAK is generated from the supply voltage source VDD to the ground, causing inconvenience to the users.
SUMMARY OF THE INVENTION
The present invention provides a high-voltage selecting circuit. The high-voltage selecting circuit comprises a first selecting P-channel Metal Oxide Semiconductor (PMOS) transistor, a second selecting PMOS transistor, and a first auxiliary N-channel Metal Oxide Semiconductor (NMOS) transistor. The first selecting PMOS transistor comprises a source, a drain, and a gate. The source of the first selecting PMOS transistor is utilized for receiving a first input voltage. The second selecting PMOS transistor comprises a source, a drain, and a gate. The source of the second selecting PMOS transistor is utilized for receiving a second input voltage. The drain of the second selecting PMOS transistor is coupled to the drain of the first selecting PMOS transistor. The gate of the second selecting PMOS transistor is coupled to the source of the first selecting PMOS transistor. The first auxiliary NMOS transistor comprises a source, a drain, and a gate. The source of the first auxiliary NMOS transistor is coupled to the gate of the first selecting PMOS transistor. The drain of the first auxiliary NMOS transistor is utilized for receiving the second input voltage. The gate of the first auxiliary NMOS transistor is utilized for receiving the first input voltage.
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 a conventional high-voltage selecting circuit.
FIG. 2 is a diagram illustrating the conventional high-voltage selecting circuit incorrectly generates the output voltage.
FIG. 3 is a diagram illustrating the output voltage of the conventional high-voltage selecting circuit inputted to an inverter.
FIG. 4 is a diagram illustrating a high-voltage selecting circuit according to a first embodiment of the present invention.
FIG. 5 is a diagram illustrating a high-voltage selecting circuit according to a second embodiment of the present invention.
FIG. 6 is a diagram illustrating a leakage current of the high-voltage selecting circuit of FIG. 4.
FIG. 7 is a diagram illustrating a high-voltage selecting circuit according to a third embodiment of the present invention.
FIG. 8 is a diagram illustrating a high-voltage selecting circuit according to a fourth embodiment of the present invention.
FIG. 9 is a diagram illustrating a memory module of the present invention.



DETAILED DESCRIPTION
Please refer to FIG. 4. FIG. 4 is a diagram illustrating a high-voltage selecting circuit 300 according to a first embodiment of the present invention. The high-voltage selecting circuit 300 includes selecting PMOS transistors QPS1 and QPS2, and an auxiliary NMOS transistor QNA1. Each of the selecting PMOS transistors QPS1 and QPS2, and the auxiliary NMOS transistor QNA1 includes a source, a drain, a gate, and a well. The source of the selecting PMOS transistor QPS1 receives an input voltage VIN1. The gate of the selecting PMOS transistor QPS1 is coupled to the source of the auxiliary NMOS transistor QNA1. The drain of the selecting PMOS transistor QPS1 is coupled to the drain of the selecting PMOS transistor QPS2 for outputting the output voltage VOUT. The well of the selecting PMOS transistor QPS1 is coupled to the drain of the selecting PMOS transistor QPS1. The source of the selecting PMOS transistor QPS2 receives an input voltage VIN2. The gate of the selecting PMOS transistor QPS2 receives the input voltage VIN1. The well of the selecting PMOS transistor QPS2 is coupled to the drain of the selecting PMOS transistor QPS2. The gate of the auxiliary NMOS transistor QNA1 receives the input voltage VIN1. The drain of the auxiliary NMOS transistor QNA1 receives the input voltage VIN2. The well of the auxiliary NMOS transistor QNA1 is coupled to a ground.
The voltage level of the input voltage VIN1 is assumed to be fixed and equal to the voltage level of a supply voltage VDD as an example for illustrating the operational principle of the high-voltage selecting circuit 300, wherein the supply voltage VDD is about 3.3V.
When the input voltage VIN2 is higher than the input voltage VIN1 (for example, the input voltage VIN2 is about 6 ̃7.5V), the selecting PMOS transistor QPS2 is turned on. Therefore, the voltage level of the voltage on the drain of the selecting PMOS transistor QPS2 is pulled up by the selecting PMOS transistor QPS2 to be equal to that of the voltage on the source of the selecting PMOS transistor QPS2. In this way, the voltage level of the output voltage VOUT is equal to that of the input voltage VIN2 (6 ̃7.5V), which means the high-voltage selecting circuit 300 selects the higher input voltage VIN2 (6 ̃7.5V) as the output voltage VOUT.
When the input voltage VIN1 is higher than the input voltage VIN2 (for example, the input voltage VIN2 is about 0V), the selecting PMOS transistor QPS2 is turned off, and the auxiliary NMOS transistor QNA1 is turned on so that the voltage level of the gate voltage VG—PS1 of the selecting PMOS transistor QPS1 is pulled by the auxiliary NMOS transistor QNA1 to be equal to the input voltage VIN2 (0V). As a result, the selecting PMOS transistor QPS1 is turned on, so that the voltage level of the voltage on the drain of the selecting PMOS transistor QPS1 is equal to the voltage level of the voltage on the source of the selecting PMOS transistor QPS1. In this way, the voltage level of the output voltage VOUT is equal to that of the input voltage VIN1 (3.3V). That is, when the input voltage VIN1 (3.3V) is higher than the input voltage VIN2 (0V), the high-voltage selecting circuit 300 selects the higher input voltage VIN1 (3.3V) as the output voltage VOUT.
When the voltage level of the input voltage VIN1 is equal to that of the input voltage VIN2 (for example, the input voltage VIN2 is 3.3V), the auxiliary NMOS transistor QNA1 is turned on, so that the voltage level of the gate voltage VG—PS1 of the selecting PMOS transistor QPS1 is pulled by the auxiliary NMOS transistor QNA1 to be equal to the voltage level of the gate voltage (VIN1) of the auxiliary NMOS transistor QNA1 deducting the threshold voltage VTH—NA1 of the auxiliary NMOS transistor QNA1. Since the well of the auxiliary NMOS transistor QNA1 is coupled to the ground, the body effect of the auxiliary NMOS transistor QNA1 is enhanced, so that the magnitude of the threshold voltage VTH—NA1 of the auxiliary NMOS transistor QNA1 is larger than the magnitude of the threshold voltage VTH—PS1 of the selecting PMOS transistor QPS1. Hence, the difference between the voltage level of the input voltage VIN1 (3.3V) and the voltage level of the gate voltage VG—PS1 (3.3−VTH—NA1) is larger than the threshold voltage VTH—PS1 of the selecting PMOS transistor QPS1, so that the selecting PMOS transistor QPS1 is turned on. In this way, the voltage level of the voltage on the drain of the selecting PMOS transistor QPS1 is pulled by the selecting PMOS transistor QPS1 to be equal to that of the voltage on the source of the selecting PMOS transistor QPS1, so that the voltage level of the output voltage VOUT is equal to that of the input voltage VIN1 (3.3V). Therefore, compared with the conventional high-voltage selecting circuit 100, when the voltage level of the input voltage VIN2 is equal to that of the input voltage ViN1, the high-voltage selecting circuit 300 correctly generates the output voltage VOUT by means of the auxiliary NMOS transistor QNA1 reducing the gate voltage VG—PS1 to turn on the selecting PMOS transistor QPS1.
Please refer to FIG. 5. FIG. 5 is a diagram illustrating a high-voltage selecting circuit 400 according to a second embodiment of the present invention. Compared with the high-voltage selecting circuit 300, the high-voltage selecting circuit 400 further includes an auxiliary PMOS transistor QPA1. The auxiliary PMOS transistor QPA1 includes a source, a gate, a drain, and a well. The source of the auxiliary PMOS transistor QPA1 receives the input voltage VIN2. The gate of the auxiliary PMOS transistor QPA1 receives the input voltage ViN1. The drain of the auxiliary PMOS transistor QPA1 is coupled to the gate of the selecting PMOS transistor QPS1.
In the high-voltage selecting circuit 300 shown in FIG. 4, when the input voltage VIN2 is higher than the input voltage VIN1 (for example, the input voltage VIN2 is about 6 ̃7.5V), the voltage level of the gate voltage VG—PS1 is pulled by the auxiliary transistor QNA1 to be equal to the voltage level of the gate voltage (VIN1) of the auxiliary NMOS transistor QNA1 deducting the threshold voltage VTH-NA1 of the auxiliary NMOS transistor QNA1. Therefore, the selecting PMOS transistor QPS1 is turned on. In this way, since both the selecting PMOS transistors QPS1 and QPS2 are turned on, a leakage current ILEAK may flow from the input voltage VIN2 to VIN1, as shown in FIG. 6. However, in the high-voltage selecting circuit 400, when the input voltage VIN2 (6 ̃7.5V) is higher than the input voltage VIN1 (3.3V), the auxiliary PMOS transistor QPA1 is turned on and pulls the voltage level of the gate voltage VG—PS1 to be equal to that of the input voltage VIN2 (6 ̃7.5V). In this way, the selecting PMOS transistor QPS1 is turned off, so that the leakage current ILEAK is avoided in the high-voltage selecting circuit 400.
Please refer to FIG. 7. FIG. 7 is a diagram illustrating a high-voltage selecting circuit 600 according to a third embodiment of the present invention. Compared with the high-voltage selecting circuit 400, the high-voltage selecting circuit 600 further includes an auxiliary NMOS transistor QNA2. The auxiliary NMOS transistor QNA2 includes a source, a drain, a gate, a well. The source of the auxiliary NMOS transistor QNA2 is coupled to the gate of the selecting PMOS transistor QPS2. The gate of the auxiliary NMOS transistor QNA2 receives the input voltage VIN2. The drain of the auxiliary NMOS transistor QNA2 receives the input voltage VIN1. The well of the auxiliary NMOS transistor QNA2 is coupled to the ground for enhancing the body effect, so that the magnitude of the threshold voltage VTH—NA2 of the auxiliary NMOS transistor Q2 is larger than the magnitude of the threshold voltage VTH—PS2 of the selecting PMOS transistor QPS2. Consequently, when the voltage level of the input voltage VIN1 is equal to that of the input voltage VIN2, the gate voltage VG—PS2 is controlled to be low enough so that the selecting PMOS transistor QPS2 is turned on. In other words, when the voltage level of the input voltage VIN1 is equal to that of the input voltage VIN2, both the selecting PMOS transistors QPS1 are QPS2 are turned on, so that the speed of the output voltage VOUT rising to the required voltage level (3.3V) increases, reducing the response time of the high-voltage selecting circuit 600.
Please refer to FIG. 8. FIG. 8 is a diagram illustrating a high-voltage selecting circuit 700 according to a fourth embodiment of the present invention. Compared with the high-voltage selecting circuit 600, the high-voltage selecting circuit 700 further includes an auxiliary PMOS transistor QPA2. The auxiliary PMOS transistor QPA2 includes a source, a gate, a drain, and a well. The source of the auxiliary PMOS transistor QPA2 receives the input voltage VIN1. The gate of the auxiliary PMOS transistor QPA2 receives the input voltage VIN2. The drain of the auxiliary PMOS transistor QPA2 is coupled to the gate of the selecting PMOS transistor QPS2. In the high-voltage selecting circuit 600, if the input voltage VIN1 is about 6 ̃7.5V and the input voltage VIN2 is 3.3V, the selecting PMOS transistor QPS2 is turned on by the auxiliary NMOS transistor QNA2. Thus, a leakage current ILEAK may flow from the input voltage VIN1 to VIN2. However, in the high-voltage selecting circuit 700, when the input voltage VIN1 is about 6 ̃7.5V and the input voltage VIN2 is 3.3V, the auxiliary PMOS transistor QPA2 is turned on, so that the gate voltage VG—PS2 is pulled to be 6 ̃7.5. In this way, the selecting PMOS transistor QPS2 is turned off so that the leakage current ILEAK is avoided. In other words, no matter the input voltage VIN1 is higher, lower, or equal to the input voltage VIN2, the high-voltage selecting circuit 700 can correctly generate the output voltage VOUT and avoid the leakage current ILEAK at the same time.
The high-voltage selecting circuit of the present invention can be utilized in various applications. For instance, the high-voltage selecting circuit provides the output voltage as a supply voltage to an external load. In addition, the high-voltage selecting circuit of the present invention can be applied in a memory module. Please refer to FIG. 9. FIG. 9 is a diagram illustrating a memory module 800 of the present invention. The memory module 800 includes a high-voltage selecting circuit 810, a memory cell 820, and a memory control circuit 830. The structure and the operational principle of the high-voltage selecting circuit 810 is similar to those of the high-voltage selecting circuit 300, 400, 600, or 700, and thus will not be repeated again for brevity. The memory cell 820 enters a program mode or a read mode according to the output voltage VOUT of the high-voltage selecting circuit 810. For instance, when the output voltage VOUT is 3.3V, the memory cell 820 operates in the read mode. Meanwhile, the memory cell 820 is not allowed to be written data. When the output voltage VOUT is 6 ̃7.5V, the memory cell 820 operates in the program mode. Meanwhile, the memory cell 820 is allowed to be written data. In this way, the memory control circuit 830 can control the voltage levels of the input voltages VIN1 and VIN2 according to the output voltage VOUT so as to control the memory cell 820 entering the read mode or the program mode.
In conclusion, the high-voltage selecting circuit provided by the present invention generates the output voltage without a voltage drop by means of the auxiliary NMOS transistor turning on the corresponding selecting PMOS transistor when the voltage levels of the first input voltage and the second input voltage are equal. In addition, when one of the first and the second input voltages is higher than the other one, the high-voltage selecting circuit of the present invention avoids the leakage current by means of the auxiliary PMOS transistor turning off the corresponding selecting PMOS transistor, causing a great convenience.
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.

## Claims

1. A high-voltage selecting circuit comprising:
a first selecting P-channel Metal Oxide Semiconductor (PMOS) transistor comprising:
a source for receiving a first input voltage;
a drain; and
a gate;

a second selecting PMOS transistor, comprising:
a source for receiving a second input voltage;
a drain coupled to the drain of the first selecting PMOS transistor; and
a gate coupled to the source of the first selecting PMOS transistor; and

a first auxiliary N-channel Metal Oxide Semiconductor (NMOS) transistor, comprising:
a source coupled to the gate of the first selecting PMOS transistor;
a drain for receiving the second input voltage; and
a gate for receiving the first input voltage.

2. The high-voltage selecting circuit of claim 1, wherein a magnitude of a threshold voltage of the first selecting PMOS transistor is smaller than a magnitude of a threshold voltage of the first auxiliary NMOS transistor.

3. The high-voltage selecting circuit of claim 1, wherein the first selecting PMOS transistor further comprises a well coupled to the drain of the first selecting PMOS transistor; the second selecting PMOS transistor further comprises a well coupled to the drain of the second selecting PMOS transistor; and the first auxiliary NMOS transistor further comprises a well coupled to ground.

4. The high-voltage selecting circuit of claim 1, wherein the first input voltage having a fixed voltage level.

5. The high-voltage selecting circuit of claim 4, wherein the first input voltage is about 3.3V and the second input voltage is about 0V, about 3.3V, or about 6 ̃7.5V.

6. The high-voltage selecting circuit of claim 1, further comprising:
a first auxiliary PMOS transistor, comprising:
a source for receiving the second input voltage;
a drain coupled to the gate of the first selecting PMOS transistor; and
a gate for receiving the first input voltage.

7. The high-voltage selecting circuit of claim 6, wherein the first auxiliary PMOS transistor further comprises a well coupled to the source of the first auxiliary PMOS transistor.

8. The high-voltage selecting circuit of claim 6, further comprising:
a second auxiliary NMOS transistor, comprising:
a source coupled to the gate of the second selecting PMOS transistor;
a drain for receiving the first input voltage; and
a gate for receiving the second input voltage.

9. The high-voltage selecting circuit of claim 8, wherein a magnitude of a threshold voltage of the second selecting PMOS transistor is smaller than a magnitude of a threshold voltage of the second auxiliary NMOS transistor.

10. The high-voltage selecting circuit of claim 8, wherein the second auxiliary NMOS transistor further comprises a well coupled to ground.

11. The high-voltage selecting circuit of claim 8, further comprising:
a second auxiliary PMOS transistor, comprising:
a source for receiving the first input voltage;
a drain coupled to the gate of the second selecting PMOS transistor; and
a gate for receiving the second input voltage.

12. The high-voltage selecting circuit of claim 11, wherein the second auxiliary PMOS transistor further comprises a well coupled to the source of the second auxiliary PMOS transistor.

13. A memory module, comprising:
the high-voltage selecting circuit of claim 1;
a memory cell for entering a program mode or a read mode according to an output voltage of the high-voltage selecting circuit; and
a memory control circuit for controlling a voltage level of the first input voltage and a voltage level of the second input voltage according to the output voltage of the high-voltage selecting circuit so as to control the memory cell.