Abstract:
A charge pump has two inputs, each for an input clock signal, and an output for the output of a pumped output potential. Two pumping capacitors are connected to the inputs. Second electrodes of the pumping capacitors are in each case connected via a first circuit module to a supply potential (ground) and via a second circuit module to the output. Also present is a controllable short-circuiting element, the controllable path of which is disposed between the second electrodes of the two pumping capacitors.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/DE00/01715, filed May 26, 2000, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a charge pump. 
     Various charge pumps are described in U.S. Pat. Nos. 4,740,715, 5,126,590, 5,202,588 and 5,343,088. The basic operating principle of a charge pump is now described. 
     A charge pump has two pumping capacitors to each of which an input clock signal is fed at an electrode. Electrodes of the pumping capacitors remote from the input clock signals are connected to ground via transistors and are connected to an output of the charge pump via other transistors. The transistors are p-channel type transistors. The charge pump feeds a load, which has a load capacitance. The control terminals of the transistors are connected to different control signals. 
     With each clock pulse edge of the input clock signals there is a charge reversal of the electrodes of the pumping capacitors remote from the inputs. As this happens, one electrode must be pumped from the value of the output potential to a positive value and the other electrode must be pumped from 0V to a negative value. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a charge pump that overcomes the above-mentioned disadvantages of the prior art devices of this general type, with which a more negative output potential can be produced. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a charge pump. The charge pump contains two inputs, each for receiving an input clock signal; an output for outputting a pumped output potential; a first circuit module; a second circuit module connected to the output; a supply potential terminal connected to the first circuit module; and two pumping capacitors including a first pumping capacitor and a second pumping capacitor each having a first electrode and a second electrode. The first electrode of each of the pumping capacitors is connected to one of the inputs, and the second electrode of the pumping capacitors is connected to both the first circuit module and the second circuit module. The second electrode in each case is coupled through the first circuit module to the supply potential terminal and through the second circuit module to the output. A controllable short-circuiting element is provided and has a control terminal and a controllable path with a first end connected to the second electrode of the first pumping capacitor and a second end connected to the second electrode of the second pumping capacitor. A first switching element is provided and has a first terminal connected to the second electrode of the first pumping capacitor, a second terminal connected to the control terminal of the controllable short-circuiting element, and a control terminal. A second switching element has a first terminal connected to the second electrode of the second pumping capacitor, a second terminal connected to the control terminal of the controllable short-circuiting element, and a control terminal. 
     The charge pump according to the invention has a controllable short-circuiting element, the controllable path of which is disposed between the two electrodes of the two capacitors that are remote from the inputs of the charge pump. By the short-circuiting element it is possible in an advantageous way to carry out a charge equalization between the two electrodes of the two pumping capacitors at any desired points in time. 
     It is particularly favorable if the short-circuiting element is driven by its control terminal in such a way that, immediately before clock pulse edges of the input clock signals, it is conducting, and brings about the charge equalization, and is subsequently blocked again before the clock pulse edges occur. During every pumping period, the potentials of the second electrodes of the pumping capacitors are initially equalized via the short-circuiting element to their arithmetic mean value, before their level is changed by the pumping of the input clock signals. This results in an increase in the absolute amount of the peak values of the potentials at the second electrodes. For this reason, an output potential of a greater absolute amount than without the short-circuiting according to the invention is achieved. The short-circuiting has the effect that a change in potential already takes place to a certain extent at the second electrodes in the direction of the subsequent increase or decrease in potential induced by the input clock signals. As a result, potentials of a greater absolute value are subsequently produced at the second electrodes by the excursion of the input clock signals. 
     According to a development, the first and second circuit modules are non-conducting whenever the short-circuiting element is conducting. This prevents the supply potential and the pumped output potential from being influenced during the short-circuiting of the second electrodes of the pumping capacitors. 
     It is favorable if the control signal is periodic. This has the result that every pumping cycle takes place in the way according to the invention. 
     The first and second switching elements may contain, for example, switching elements such as transistors or diodes for example. 
     According to a development, the charge pump has a first switching element, via which the control terminal of the short-circuiting element is connected to the second electrode of the first pumping capacitor, and a second switching element, via which the control terminal of the short-circuiting element is connected to the second electrode of the second pumping capacitor. The two switching elements make it possible to adapt the potential at the control terminal of the short-circuiting element to the potential of one of the two second electrodes at desired points in time. 
     In accordance with an added feature of the invention, a third switching element is connected between the control terminal of the first switching element and the supply potential terminal, and through the third switching element the control terminal of the first switching element is coupled to the supply potential terminal. A fourth switching element is connected between the control terminal of the first switching element and the second electrode of the second capacitor, and through the fourth switching element the control terminal of the first switching element is coupled to the second electrode of the second pumping capacitor. A fifth switching element is connected between the control terminal of the second switching element and the second electrode of the first pumping capacitor, and through the fifth switching element the control terminal of the second switching element is coupled to the second electrode of the first pumping capacitor. A sixth switching element is connected between the control terminal of the second switching element and the supply potential terminal, and through the sixth switching element the control terminal of the second switching element is coupled to the supply potential terminal. 
     In accordance with an additional feature of the invention, the third switching element has a control terminal connected to the control terminal of the controllable short-circuiting element. The sixth switching element has a control terminal connected to the control terminal of the controllable short-circuiting element. The fourth switching element has a control terminal connected to the control terminal of the second switching element. The fifth switching element has a control terminal connected to the control terminal of the first switching element. 
     In accordance with a concomitant feature of the invention, a third pumping capacitor has a first electrode connected to the control terminal of the first switching element and a second electrode for receiving a first pumping signal. A fourth pumping capacitor has a first electrode connected to the control terminal of the controllable short-circuiting element and a second electrode for receiving a second pumping signal. A fifth capacitor has a first electrode connected to the control terminal of the second switching element and a second electrode receiving a third pumping signal. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a charge pump, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
    
    
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional charge pump; 
     FIG. 2 is a timing diagram showing signal profiles for the charge pump shown in FIG. 1; 
     FIG. 3 is a circuit diagram of a first exemplary embodiment of the charge pump according to the invention; 
     FIG. 4 is a timing diagram showing signal profiles for the charge pump shown in FIG. 3; 
     FIG. 5 is a circuit diagram of a second exemplary embodiment of the charge pump according to the invention; 
     FIG. 6 is a circuit diagram of a detail of a further exemplary embodiment of the charge pump; and 
     FIG. 7 is a timing diagram of signal profiles for the charge pump shown in FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a charge pump with two pumping capacitors CpumpA, CpumpB, to each of which an input clock signal A 1 , B 1  is fed at an electrode. Electrodes VA 1 , VB 1  of the pumping capacitors remote from the input clock signals A 1 , B 1  are connected to ground via transistors T 1 , T 4  and are connected to an output of the charge pump via other transistors T 2 , T 3 . The transistors T 1  to T 4  are of the p-channel type. The charge pump feeds a load, which has a load capacitance C L . The control terminals of the transistors T 1  to T 4  are connected to different control signals DISA 1 , DISB 1 , A 2 , B 2 . 
     FIG. 2 shows the profiles of the input clock signals A 1 , B 1  and of the control signals DISA 1 , DISB 1 , A 2 , B 2  and the profile of the potentials at the switching nodes VA 1 , VA 2  and the profile of the pumped output signal VPUMP at the output of the charge pump. With each clock pulse edge of the input clock signals A 1 , B 1  there is a charge reversal of the electrodes of the pumping capacitors CpumpA, CpumpB remote from the inputs. As this happens, one electrode must be pumped from the value of the output potential to a positive value and the other electrode must be pumped from 0V to a negative value. 
     The first exemplary embodiment of the charge pump according to the invention is shown in FIG.  3  and has the components already explained with reference to FIG.  1 . In this case, the first electrodes of the pumping capacitors CpumpA, CpumpB are connected to inputs of the charge pump via which the input clock signals A 1 , B 1  are fed. The second electrodes VA 1 , VB 1  of the pumping capacitors are connected via the p-channel transistors T 1 , T 4  to ground and via the p-channel transistors T 2 , T 3  to the output of the charge pump, at which a pumped output potential VPUMP is produced. Via the output, the charge pump feeds the capacitive load C L . The control terminals of the transistors T 1  to T 4  are fed the control signals DISA 1 , DISB 1 , A 2 , B 2 . In addition, the second electrodes of the pumping capacitors CpumpA, CpumpB are connected to one another via a short-circuiting element S in the form of a further p-channel transistor. The control terminal of the short-circuiting element S is fed a control signal chan. 
     FIG. 4 shows for the exemplary embodiment shown in FIG. 3 exemplary profiles of the input clock signals A 1 , B 1 , of the control signals DISA 1 , DISB 1 , A 2 , B 2 , chan, of the potentials at the second electrodes VA 1 , VB 1  of the pumping capacitors CpumpA, CpumpB and of the pumped output potential VPUMP at the output of the charge pump. It can be seen that, when the short-circuiting element S is conducting (low level of the control signal chan), the transistors T 1  to T 4  are turned off. The short-circuiting element S is briefly switched to conduct before each change in level of the mutually inverse input clock signals A 1 , B 1 , so that a charge equalization takes place between the second electrodes VA 1 , VB 1  of the pumping capacitors. 
     The charge pump of the exemplary embodiment serves for producing a negative pumped output potential VPUMP. Therefore, before the low potential occurs at the control terminal of the short-circuiting element S, the potentials at the second electrodes VA 1 , VB 1  alternately assume ground potential (0V) and the value of the output potential VPUMP, respectively. The reason for this is that the second electrode VA 1  of the one pumping capacitor CpumpA is always conductively connected to ground via the corresponding transistor T 1  whenever the second electrode VB 1  of the other pumping capacitor CpumpB is conductively connected to the output of the charge pump via the corresponding transistor T 3 , and vice versa. Once the potentials of the second electrodes VA 1 , VB 1  have assumed 0V and the value of the output potential VPUMP, respectively, the corresponding transistors are turned off, so that subsequently all four transistors T 1  to T 4  are turned off. Therefore, in contrast with the input clock signals A 1 , B 1 , the periodic control signals DISA 1 , DISB 1 , A 2 , B 2  are unsymmetrical clocks. During the time period before the next clock pulse edge of the input clock signals A 1 , B 1 , during which the four transistors T 1  to T 4  are turned off, the short-circuiting element S is switched to conduct via the control signal chan. The charge equalization which then takes place between the second electrodes VA 1 , VB 1  of the pumping capacitors has the effect that a potential which corresponds to the arithmetic mean value between the current value of the output potential VPUMP and 0V occurs at the two electrodes. At the latest when the next edge of the input clock signals A 1 , B 1  occurs, the short-circuiting element S is blocked again, so that the potentials of the second electrodes VA 1 , VB 1  are pumped by the input clock signals to opposite maximum and minimum values, respectively. 
     Since FIG. 4 represents a time segment during which the output potential VPUMP has not yet assumed its final negative value, the positive and negative peaks of the potentials at the second electrodes VA 1 , VB 1  shift in the negative direction with every half-period of the input clock signals A 1 , B 1 . The same applies to the mean value occurring during the conducting phase of the short-circuiting element S. 
     FIG. 5 shows a second exemplary embodiment of the charge pump according to the invention. FIG. 5 differs from the exemplary embodiment from FIG. 3 in that the p-channel transistors T 1  to T 4  are replaced by diodes D 1  to D 4 . This dispenses with the control signals DISA 1 , DISB 1 , A 2 , B 2 . The input clock signals A 1 , B 1  and the control signal chan of the short-circuiting element S also have the profile shown in FIG. 4 for the exemplary embodiment represented in FIG.  5 . The potentials at the second electrodes VA 1 , VB 1  of the pumping capacitors CpumpA, CpumpB and of the output potential are also similar to those represented in FIG.  4 . However, the maximum values (peaks) of the potentials at the second electrodes VA 1 , VB 1  are reduced in absolute terms by the value of the inception voltages of the diodes D 1  to D 4 . 
     FIG. 6 shows a detail of a further exemplary embodiment of the charge pump according to the invention. The further exemplary embodiment has the components represented in FIG.  3  and additionally those shown in FIG.  6 . FIG. 6 shows the short-circuiting element S from FIG. 3 between the two electrodes VA 1 , VB 1  of the two pumping capacitors CpumpA, CpumpB. The control terminal of the short-circuiting element S is connected via a fifth transistor T 5  to the second electrode VA 1  of the first pumping capacitor CpumpA and via a sixth transistor T 6  to the second electrode VB 1  of the second pumping capacitor CpumpB. 
     The subcircuit shown in FIG. 6 is fed three pumping signals A 1 cha 1 , cha, B 1 cha 1 . The first pumping signal A 1 cha 1  is connected via a third pumping capacitor CA 1  to the control terminal of the fifth transistor T 5 . The second pumping signal cha is connected via a fourth pumping capacitor Ccha to the control terminal of the short-circuiting element S. The third pumping signal B 1 cha 1  is connected via a fifth pumping capacitor CB 1  to the control terminal of the sixth transistor T 6 . 
     Furthermore, the control terminal of the fifth transistor T 5  is connected through a seventh transistor T 7  to ground and through an eighth transistor T 8  to the second electrode VB 1  of the second pumping capacitor CpumpB. The control terminal of the sixth transistor T 6  is connected through a ninth transistor T 9  to the second electrode VA 1  of the first pumping capacitor CpumpA and through a tenth transistor T 10  to ground. The transistors T 5  to T 10  are p-channel transistors. 
     A control terminal of the seventh transistor T 7  and of the tenth transistor T 10  are connected to the control terminal of the short-circuiting element S. A control terminal of the eighth transistor T 8  is connected to the control terminal of the sixth transistor T 6  and a control terminal of the ninth transistor T 9  is connected to the control terminal of the fifth transistor T 5 . 
     FIG. 7 shows profiles of the signals depicted in FIG.  6 . It can be seen that the fifth transistor T 5  and the sixth transistor T 6  are turned off as long as the short-circuiting element S is conducting (chan=low level). As soon as one of the second electrodes VA 1 , VB 1  is pumped by the corresponding input clock signal A 1 , B 1  to a positive value (positive edge of these signals), the transistor T 5  or T 6  connected to the second electrode is turned on. Consequently, the potential chan at the control terminal of the short-circuiting element S subsequently follows the profile of the potential at the second electrode VA 1 , VB 1  conductively connected to it. This produces the positive peaks of the signal chan. In this way it is ensured that the short-circuiting element S, which is a p-channel transistor, is reliably blocked. This is because at its control terminal there is then always a potential chan that is at least as high as its source potential. 
     The seventh transistor T 7  and the tenth transistor T 10  ensure that the fifth transistor T 5  and the sixth transistor T 6 , respectively, are reliably turned off, while the short-circuiting element S is conducting. In this case (chan=low level), ground is applied via the seventh transistor T 7  and the tenth transistor T 10  to the control terminals of the fifth transistor T 5  and sixth transistor T 6 . Consequently, the gate-source voltage of the last-mentioned transistors is then positive, so that they are reliably turned off. 
     The eighth transistor T 8  serves the purpose of reliably turning off the fifth transistor T 5  when the short-circuiting element S is blocked (chan=high level) and the sixth transistor T 6  is turned on (B 1 cha=low level). If the eighth transistor T 8  is turned on, the potential of the second electrode VB 1  of the second pumping capacitor CpumpB is applied to the control terminal of the fifth transistor T 5 . Since the potential chan at the gate of the short-circuiting element S is at the same time conductively connected to the second electrode VB 1  of the second pumping capacitor CpumpB, the gate-source voltage of the fifth transistor T 5  is then equal to 0. It is consequently reliably turned off. 
     The ninth transistor T 9  serves the purpose in an analogous way of turning off the sixth transistor T 6 , while the short-circuiting element S is blocked and the fifth transistor T 5  is turned on. 
     Consequently, while the fifth transistor T 5  and the sixth transistor T 6  serve the purpose of reliably blocking the short-circuiting element S during its blocking phase, the transistors T 7  to T 10  serve the purpose of reliably turning off the fifth transistor T 5  or the sixth transistor T 6  during their different off phases. In this way, the charge pump of the exemplary embodiment avoids undesired influencing of the potentials of the second electrodes VA 1 , VB 1  of the pumping capacitors CpumpA, CpumpB, because disturbing leakage currents are avoided.