# Patent

## Title
Charge pump for generating arbitrary voltage levels

## Abstract

A charge pump for generating an arbitrary voltage level includes “M” pieces of pump units PUi and “M+1” pieces of first switches Sj. The pump unit PUi includes a first terminal Ni,1 coupled to a reference voltage Vi,1, a second terminal Ni,2 coupled to a reference voltage Vi,2, a third terminal Ni,3, a fourth terminal Ni,4 and at least one capacitor Ci. Ci is charged by Vi,1 and Vi,2 during a first period, and a voltage is provided from Ni,4 by Ci according to a voltage at Ni,3 during a second period. The first switch Sj is adapted for electrically connecting the first terminal and the second terminal during the second period. The first and second terminals of Sk is coupled to Nk−1,4 and Nk,3, respectively. The first terminal of S1 receives the input voltage and the second terminal of SM+1 outputs the output voltage.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 94121792, filed on Jun. 29, 2005. All disclosure of the Taiwan application is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to charge pumps, and particularly to a charge pump for generating arbitrary voltage levels.
2. Description of Related Art
In an electronic circuit, power source voltages of different levels are often required; therefore, a charge pump circuit for generating power source voltages of different levels with the present power source voltage is usually equipped therein.
FIG. 1A is a circuit diagram of a conventional charge pump. FIG. 1B is a timing sequence diagram of control signals for illustrating the charge pump according to FIG. 1A. Referring to FIGS. 1A and 1B together, the charge pump has five transistors; the control signals ph1 and ph2 are opposite in phase, wherein it is Vdd when at high voltage level and it is zero when at low voltage level.
During the charging period (CP) of a capacitor C1, the control signals ph1 and ph2 are respectively at low voltage level and high voltage level. Therefore, the transistors M1 and M3 are turned off while the transistors M0, M2 and M4 are turned on. The voltage at the terminal of the capacitor C1 which is coupled to the control signal ph1 is zero. The power source voltage Vdd charges the capacitor C1 via the transistor M0 until the voltage difference between the two terminals is Vdd−Vt; wherein, Vt is a threshold voltage of the transistors M0 to M4.
During the pumping period (PP) of the capacitor C1, the control signals ph1 and ph2 are respectively at high voltage level and low voltage level, therefore the transistors M1 and M3 are turned on while the transistors M0, M2 and M4 are turned off. The voltage at the terminal of the capacitor C1 which is coupled to the control signal ph1 is Vdd. Because a charge of voltage difference of Vdd−Vt has been stored during the charging period CP, the voltage of another terminal of the capacitor C1 will raise up to 2Vdd−Vt. Also, the voltage of a terminal of a capacitor C2 which is coupled to the control signal ph2 is zero, therefore the voltage 2Vdd−Vt of another terminal of the capacitor C1 charges the capacitor C2 via the transistor M1 until the voltage difference between the two terminals of the capacitor C2 is 2Vdd−2Vt.
Then, by analyzing the voltage variation according to the foregoing method, it can be known that the output voltage Vo from the charge pump is 5×(Vdd−Vt). Similarly, if the charge pump has N transistors, the output voltage is N×(Vdd−Vt). It can be known from the formula of the output voltage Vo that the charge pump can only generate some power source voltages having certain voltage levels.
SUMMARY OF THE INVENTION
In view of the above, an object of the present invention is to provide a charge pump for generating arbitrary voltage levels.
The invention provides a charge pump for generating an output voltage having arbitrary voltage levels according to the input voltage. The charge pump includes “M” pieces of pump units PUi and “M+1” pieces of first switches Sj; wherein, PUi represents the “ith” pump unit, “i” is an integer greater than 0 and smaller than or equal to “M”, “M” is an integer greater than or equal to 1; Sj represents the “jth” first switch, “j” is an integer greater than 0 and smaller than or equal to “M+1”.
The pump unit PUi includes a first terminal Ni,1 coupled to a reference voltage Vi,1, a second terminal Ni,2 coupled to a reference voltage Vi,2, a third terminal Ni,3, a fourth terminal Ni,4 and at least one capacitor Ci. Wherein, Ni,1, Ni,2, Ni,3, and Ni,4 respectively represent the first terminal, the second terminal, the third terminal and the fourth terminal of the PUi; and Ci represents an internal capacitor of the PUi. The pump unit PUi is adapted for charging Ci with Vi,1 and Vi,2 in a first period and providing a voltage to Ni,4 in a second period according to the voltage level of Ni,3.
The first switch Sj includes a first terminal and a second terminal and is adapted for electrically connecting the first terminal and the second terminal during the second period. The first terminal of S1 receives the input voltage and the second terminal of S1 is coupled to N1,3; the first terminal of Sk is coupled to Nk−1,4 and the second terminal of Sk is coupled to Nk,3; the first terminal of SM+1 is coupled to NM,4 and the second terminal of SM+1 outputs the output voltage, wherein “k” is an integer greater than 1 and smaller than or equal to “M”.
The invention employs a multi-stage pump units coupled in series with the adjustable voltages of the input terminal and the pump units; therefore, the charge pump according to the invention can generate arbitrary voltage levels.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a circuit diagram of a conventional charge pump.
FIG. 1B is a timing sequence diagram of control signals illustrating the charge pump according to FIG. 1A.
FIG. 2 is a circuit diagram of a charge pump in accordance with an embodiment of the invention.
FIGS. 3A and 3B are equivalent circuit of the charge pump shown in FIG. 2 respectively during the first and the second periods, wherein the charge pump is for generating a positive voltage.
FIGS. 4A and 4B are equivalent circuit of the charge pump shown in FIG. 2 respectively during the first and the second periods, wherein the charge pump is for generating a negative voltage.



DESCRIPTION OF THE EMBODIMENTS
In order to more clearly illustrate the embodiment of the invention, a charge pump composed of three pump units is exemplified herein; those who are skilled in related art may obtain charge pumps composed of one or more pump units in accordance with the invention.
FIG. 2 is a circuit diagram of a charge pump in accordance with an embodiment of the invention. Referring to FIG. 2, a charge pump 20 includes three pump units PU1, PU2, PU3, four first switches S1, S2, S3, S4 and an output capacitor CL. Each of the pump units PUi includes a first terminal Ni,1, a second terminal Ni,2, a third terminal Ni,3, a fourth terminal Ni,4, two second switches SWi,1, SWi,2 and a capacitor Ci, wherein, “i” is an integer and 0<i≦3. The first terminal Ni,1 is coupled to a reference voltage Vi,1 and a second terminal Ni,2 is coupled to a reference Vi,2.
Only the pump unit PU1 will be illustrated as an example herein; the architecture of other pump units is similar to that of the pump unit PU1 and thus description is omitted. The pump unit PU1 includes a first terminal N1,1 coupled to a reference voltage V1,1, a second terminal N1,2 coupled to a reference voltage V1,2, a third terminal N1,3, a fourth terminal N1,4, a second switch SW1,1, a second switch SW1,2 and a capacitor C1; wherein, the first terminal and the second terminal of the second switch SW1,1 are respectively the first terminal N1,1 and the fourth terminal N1,4 of the pump unit PU1; the first terminal and the second terminal of the second switch SW1,2 are respectively the third terminal N1,3 and the second terminal N1,2 of the pump unit PU1; and the first terminal and the second terminal of the capacitor C1 are respectively coupled to the second terminal of the second switch SW1,1 and the first terminal of the second switch SW1,2.
First switches S1, S2, S3 and S4 are coupled respectively to the corresponding pump units PU1, PU2 and PU3. The first terminal of the first switch S1 is adapted for receiving an input voltage Vi and the second terminal of the first switch S1 is coupled to the third terminal N1,3 of the pump unit PU1. The first terminal and the second terminal of the first switch S2 are respectively coupled to the fourth terminal N1,4 of the pump unit PU1 and the third terminal N2,3 of the pump unit PU2. The first terminal and the second terminal of the first switch S3 are respectively coupled to the fourth terminal N2,4 of the pump unit PU2 and the third terminal N3,3 of the pump unit PU3. The first terminal of the first switch S4 is coupled to the fourth terminal N3,4 of the pump unit PU3 and the second terminal of the first switch S4 outputs the output voltage Vo.
The timing sequence diagram of the control signals ph1 and ph2 of the charge pump 20 shown in FIG. 2 may be the same as that shown in FIG. 1B. Referring to ph1 and ph2 which are opposite in phase, the charge pump 20 makes the second switches SW1,1, SW1,2, SW2,1, SW2,2, SW3,1 and SW3,2 (all of which are controlled by control signal ph1) turn on, and the first switches S1, S2, S3 and S4 (all of which are controlled by control signal ph2) turn off during the first period; and the second switches SW1,1, SW1,2, SW2,1, SW2,2, SW3,1 and SW3,2 turn off, and the first switches S1, S2, S3 and S4 turn on during the second period. Herein, when at high levels the voltages of the control signals ph1 and ph2 are not necessary to be Vdd, and when at low voltage levels the voltages of the control signals ph1 and ph2 are not required to be zero as long as the voltages at high levels can control the switches to be turned on and at low levels the switches are turned off. Furthermore, considering the design of the switches, it is not a must to turn on the switches with a high level and to turn off the switches at a low level. Such switches may be constituted of NMOS transistors, PMOS transistors, transmission gates or a combination of such components.
Moreover, a retaining circuit (e.g. an output capacitor CL) is coupled between the second terminal of the first switch S4 and grounding for retaining the voltage level of output voltage Vo when the first switch S4 is turned off. Herein, other retaining circuit design can also be adopted for obtaining similar performance. As arbitrary voltages, the input voltage Vi and all the reference voltages V1,1, V1,2, V2,1, V2,2, V3,1 and V3,2 can be provided by a power source voltage or a voltage regulator.
FIGS. 3A and 3B are equivalent circuit of the charge pump 20 shown in FIG. 2 respectively during the first and the second periods. Wherein, the charge pump 20 is for generating a positive voltage. To simplify the illustration, the unnecessary components, terminals and nodes are not shown in the figures.
Referring to FIG. 3A, in the first period, the second switches SWi,1 and SWi,2 are turned on, and the first switch Sj is turned off. The equivalent circuit of the charge pump 20 is as shown in FIG. 3A. The charge pump 20 charges the capacitor Ci according to reference voltages Vi,1 and Vi,2. As an example, suppose Vi,1>Vi,2, namely V1,1>V1,2, V2,1>V2,2, and V3,1>V3,2, then the capacitors C1 to C3 are respectively charged to have voltage differences ΔV1 to ΔV3, wherein ΔV1=V1,1−V1,2, ΔV2=V2,1−V2,2 and ΔV3=V3,1−V3,2. Thus, the first period is a charging period.
Referring to FIG. 3B, in the second period, the second switches SWi,1 and SWi,2 are turned off, and the first switches Sj is turned on. The equivalent circuit of the charge pump 20 is as shown in FIG. 3B. Herein, the pump unit PUi makes the capacitor Ci provide voltage to the fourth terminal Ni,4 according to the voltage level of the third terminal Ni,3. For example, the voltage of the fourth terminal N1,4 of the pump unit PU1 is the sum of the voltage Vi of the third terminal N1,3 and the voltage difference ΔV1 of the capacitor C1. Thus, the output voltage is Vo=Vi+ΔV1+ΔV2+ΔV3, wherein ΔV1>0, ΔV2>0, ΔV3>0, and the range of voltage increase can be adjusted via designing Vi and ΔVi. Thus, the second period is a pumping period, and a positive voltage is produced.
In the first embodiment, Vi,1=Vdd and Vi,2=0, namely the terminals of all pump units PU1 to PU3 receive voltage Vdd while the second terminals are grounded, thus ΔV1=ΔV2=ΔV3 and the output voltage is Vo=Vi+3Vdd. However, the aforementioned charge pump 20 is only a special example in accordance with the invention. Practically, the invention may also connect with multiple pump units in series to increase voltage for obtaining an output voltage at arbitrary positive voltage level. For example, if a charge pump according to the invention connects with “M” pieces of pump units in series which needs “M+1” pieces of first switches, the output voltage is Vo=Vi+M×Vdd. Accordingly, the value of “M” and the input voltage Vi can be determined in accordance with the requirement of the output voltage. Generally, the input voltage Vi is lower than the power source voltage Vdd.
In another embodiment, Vi,1=Vi and Vi,2=0, thus ΔVi=Vi. If a charge pump connects with M pieces of pump units in series, the output voltage is Vo=Vi+M×Vi.
FIGS. 4A and 4B are equivalent circuit of the charge pump 20 shown in FIG. 2 respectively during the first and the second periods; wherein, the charge pump 20 is for generating a negative voltage. Referring to FIGS. 4A and 4B, the equivalent circuit is respectively similar to that in FIGS. 3A and 3B. The difference is that the reference voltages Vi,1 are smaller than Vi,2, namely V1,1<V1,2, V2,1<V2,2 and V3,1<V3,2. Hence, the capacitors C1 to C3 are respectively charged to have voltage differences ΔV1 to ΔV3, wherein ΔV1=V1,1−V1,2, ΔV2=V2,1−V2,2 and ΔV3=V3,1−V3,2. However, the polarities of the capacitors C1 to C3 in FIG. 4A are opposite to those of the capacitors C1 to C3 in FIG. 3A. Therefore, as shown in FIG. 4B, in the second period, the output voltage is Vo=Vi+ΔV1+ΔV2+ΔV3, wherein ΔV1<0, ΔV2<0, ΔV3<0, and a negative voltage is produced. In an aspect of the embodiment, Vi,1=0 and Vi,2=Vdd, thus the output voltage Vo=Vi−3Vdd. In another aspect of the embodiment, Vi,1=Vi and Vi,2=0, thus the output voltage Vo=Vi−3Vi. Of course, the invention may also connect with multiple pump units in series to decrease voltage for obtaining an output voltage at arbitrary negative voltage level.
In summary, the charge pump according to the present invention employs a plurality of pump units connected in series incorporating with adjustable input voltage Vi and the adjustable reference voltages of all the pump units. Thus, arbitrary positive/negative voltage levels can be obtained.
It should be noted that specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize that modifications and adaptations of the above-described preferred embodiments of the present invention may be made to meet particular requirements. This disclosure is intended to exemplify the invention without limiting its scope. All modifications that incorporate the invention disclosed in the preferred embodiment are to be construed as coming within the scope of the appended claims or the range of equivalents to which the claims are entitled.

## Claims

1. A charge pump adapted for generating an output voltage having an arbitrary voltage level according to an input voltage and at least one reference voltage, the charge pump comprising:
M pieces of pump units PUi, wherein PUi represents the ith pump unit, “i” being an integer which is greater than 0 and smaller than or equal to M, M being an integer which is greater than or equal to 1, the pump unit PUi comprising:
a first terminal Ni,1, coupled to a reference voltage Vi,1, wherein Ni,1 represents the first terminal of PUi and Vi,1 represents the voltage coupled to Ni,1;
a second terminal Ni,2, coupled to a reference voltage Vi,2, wherein Ni,2 represents the second terminal of PUi and Vi,2 represents the voltage coupled to Ni,2;
a third terminal Ni,3, wherein Ni,3 represents the third terminal of PUi;
a fourth terminal Ni,4, wherein Ni,4 represents the fourth terminal of PUi; and
at least one capacitor Ci, wherein Ci represents an internal capacitor or capacitors of PUi, the pump unit PUi being adapted for charging C1 with Vi,1 and Vi,2 in a first period, and providing a voltage to Ni,4 according to the voltage level of Ni,3 in a second period;

“M+1” pieces of first switches Sj, wherein Sj represents the jth first switch, “j” being an integer which is greater than 0 and smaller than or equal to “M+1”; Sj is adapted for electrically connecting the first terminal and the second terminal thereof during the second period, wherein the first terminal of S1 receives the input voltage and the second terminal of S1 is coupled to N1,3; the first terminal of Sk is coupled to Nk−1,4 and the second terminal of Sk is coupled to Nk,3; the first terminal of SM+1 is coupled to NM,4 and the second terminal of SM+1 outputs the output voltage, wherein “k” is an integer which is greater than 1 and smaller than or equal to “M”; and
a voltage regulator for providing and regulating the reference voltage Vi,1 and Vi,2, wherein, when Vi,1>Vi,2, the output voltage is larger than the input voltage, on the contrary, when Vi,1<Vi,2, the output voltage is smaller than the input voltage, the output voltage being the sum of the input voltage and M pieces of voltage differences ΔVi in which ΔVi represents the ith voltage difference and ΔVi=Vi,1−Vi,2.

2. The charge pump as claimed in claim 1, wherein the first switch Sj is one of a group consisting of NMOS transistors, PMOS transistors, transmission gates or a combination of such components.

3. The charge pump as claimed in claim 1, wherein the pump unit PUi further comprises:
a second switch SWi,h, SWi,h representing the “hth” second switch of the pump unit PUi, wherein “h” is 1 or 2, for electrically conducting the first terminal and the second terminal during the first period, wherein the first terminal of SWi,1 is Ni,1, the second terminal of SWi,1 is coupled to the first terminal of Ci and the first terminal of SWi,2 is coupled to the second terminal of Ci; the second terminal of SWi,2 is Ni,2; the first terminal of Ci is Ni,4; and the second terminal of Ci is Ni,3.

4. The charge pump as claimed in claim 3, wherein the second switch SWi,h is one of a group consisting of NMOS transistors, PMOS transistors, transmission gates or a combination of such components.

5. The charge pump as claimed in claim 1, further comprising a voltage regulator coupled to the first terminal of the first switch S1 for providing and adjusting the input voltage.

6. The charge pump as claimed in claim 1, wherein the input voltage is provided by a power source voltage or a ground voltage.

7. The charge pump as claimed in claim 1, wherein the reference voltages Vi,1 and Vi,2 are provided by a power source voltage or a voltage regulator.

8. The charge pump as claimed in claim 1, further comprising a retaining circuit coupled to the second terminal of the first switch SM+1 for retaining the output voltage.

9. The charge pump as claimed in claim 8, wherein the retaining circuit comprises an output capacitor, the output capacitor being coupled between the second terminal of the first switch SM+1 and a ground.

10. A voltage converting method adapted for generating an output voltage having an arbitrary voltage level according to an input voltage and at least one reference voltage, the voltage converting method comprises:
providing M pieces of reference voltage Vi,1 and M pieces of reference voltage Vi,2, where i being an integer which is greater than 0 and smaller than or equal to M, M being an integer which is greater than or equal to 1;
adjusting the voltage level of the Vi,1 and Vi,2 to result the Vi,1 is greater than the Vi,2 or the Vi,1 is less than the Vi,2;
obtaining M pieces of voltage differences according to the voltage difference between the Vi,1 and Vi,2; and
accumulating said M pieces of voltage differences to produce the output voltage based on the input voltage.

11. The voltage converting method as claimed in claim 10, further comprises:
adjusting the voltage level of the input voltage.

12. The voltage converting method as claimed in claim 10, wherein M pieces of voltage difference are represented by ΔV1 ̃ΔVM, ΔVi is equal to the Vi,1 subtract from the Vi,2.

13. The voltage converting method as claimed in claim 10, wherein the steps of “accumulating said M pieces of voltage difference to produce the output voltage based on the input voltage” comprises:
a. selecting one of M pieces of voltage difference as a specific voltage difference;
b. adding the input voltage and the specific voltage difference; and
c. judging whether all voltage difference are sequentially selected, wherein if all voltage difference are not sequentially selected, one voltage difference among the unselected voltage differences is selected as the specific voltage difference, and repeating steps of b and c; and if all voltage differences have been sequentially selected, making the accumulated input voltage as the output voltage.

14. A charge pump adapted for generating an output voltage having a voltage level corresponding to an input voltage and at least one reference voltage, the charge pump comprising:
M pieces of pump units PUi, wherein PUi represents the ith pump unit, “i” being an integer which is greater than 0 and smaller than or equal to M, M being an integer which is greater than or equal to 1, the pump unit PUi comprising:
a first terminal Ni,1, coupled to a reference voltage Vi,1, wherein Ni,1 represents the first terminal of PUi and Vi,1 represents the voltage coupled to Ni,1;
a second terminal Ni,2, coupled to a reference voltage Vi,2, wherein Ni,2 represents the second terminal of PUi and Vi,2 represents the voltage coupled to Ni,2;
a third terminal Ni,3, wherein Ni,3 represents the third terminal of PUi;
a fourth terminal Ni,4, wherein Ni,4 represents the fourth terminal of PUi; and
at least one capacitor Ci, wherein Ci represents an internal capacitor or capacitors of PUi, the pump unit PUi being adapted for charging Ci with Vi,1 and Vi,2 in a first period, and providing a voltage to Ni,4 according to the voltage level of Ni,3 in a second period;
“M+1” pieces of first switches Sj, wherein Sj represents the jth first switch, “j” being an integer which is greater than 0 and smaller than or equal to “M+1”; Sj is adapted for electrically connecting the first terminal and the second terminal thereof during the second period, wherein the first terminal of S1 receives the input voltage and the second terminal of S1 is coupled to N1,3; the first terminal of Sk is coupled to Nk−1,4 and the second terminal of Sk is coupled to Nk,3; the first terminal of SM+1 is coupled to NM,4 and the second terminal of SM+1 outputs the output voltage, wherein “k” is an integer which is greater than 1 and smaller than or equal to “M”; and
a voltage regulator, for providing and regulating the reference voltage Vi,1 and Vi,2, wherein, when Vi,1>Vi,2, the output voltage is larger than the input voltage, on the contrary, when Vi,1<Vi,2, the output voltage is smaller than the input voltage, the output voltage being the sum of the input voltage and M pieces of voltage differences ΔVi in which Δ Vi represents the ith voltage difference and ΔVi=Vi,1−Vi,2.

15. The charge pump as claimed in claim 14, wherein the first switch Sj is one of a group consisting of NMOS transistors, PMOS transistors, transmission gates or a combination of such components.

16. The charge pump as claimed in claim 1, wherein the pump unit PUi further comprises:
a second switch SWi,h, SWi,h representing the “hth” second switch of the pump unit PUi, wherein “h” is 1 or 2, for electrically conducting the first terminal and the second terminal during the first period, wherein the first terminal of SWi,1 is Ni,1, the second terminal of SWi,1 is coupled to the first terminal of Ci and the first terminal of SWi,2 is coupled to the second terminal of Ci; the second terminal of SWi,2 is Ni,2; the first terminal of Ci is Ni,4; and the second terminal of Ci is Ni,3.

17. The charge pump as claimed in claim 16, wherein the second switch SWi,h is one of a group consisting of NMOS transistors, PMOS transistors, transmission gates or a combination of such components.

18. The charge pump as claimed in claim 14, further comprising a voltage regulator coupled to the first tenninal of the first switch S1 for providing and adjusting the input voltage.

19. The charge pump as claimed in claim 14, wherein the input voltage is provided by a power source voltage or a ground voltage.

20. The charge pump as claimed in claim 14, wherein the reference voltages Vi,1 and Vi,2 are provided by a power source voltage or a voltage regulator.

21. The charge pump as claimed in claim 14, further comprising a retaining circuit coupled to the second terminal of the first switch SM+1 for retaining the output voltage.

22. The charge pump as claimed in claim 21, wherein the retaining circuit comprises an output capacitor, the output capacitor being coupled between the second terminal of the first switch SM+1 and a ground.

23. A voltage converting method adapted for generating an output voltage having a voltage level corresponding to an input voltage and at least one reference voltage, the voltage converting method comprises:
providing M pieces of reference voltage Vi,1 and M pieces of reference voltage Vi,2, where i being an integer which is greater than 0 and smaller than or equal to M, M being an integer which is greater than or equal to 1;
adjusting the voltage level of the Vi,1 and Vi,2 to result the Vi,1 is greater than the Vi,2 or the Vi,1 is less than the Vi,2;
obtaining M pieces of voltage differences according to the voltage difference between the Vi,1 and Vi,2; and
accumulating said M pieces of voltage differences to produce the output voltage based on the input voltage.

24. The voltage converting method as claimed in claim 23, further comprises:
adjusting the voltage level of the input voltage.

25. The voltage converting method as claimed in claim 23, wherein M pieces of voltage difference are represented by ΔV1 ̃ΔVM, ΔVi is equal to the Vi,1 subtract from the Vi,2.

26. The voltage converting method as claimed in claim 23, wherein the steps of “accumulating said M pieces of voltage difference to produce the output voltage based on the input voltage” comprises:
a. selecting one of M pieces of voltage difference as a specific voltage difference;
b. adding the input voltage and the specific voltage difference; and
c. judging whether all voltage difference are sequentially selected, wherein if all voltage difference are not sequentially selected, one voltage difference among the unselected voltage differences is selected as the specific voltage difference, and repeating steps of b and c; and if all voltage differences have been sequentially selected, making the accumulated input voltage as the output voltage.