This invention pertains generally to the field of charge pumps and more particularly to techniques for cancelling threshold voltages of diodes in the pump.
Charge pumps use a switching process to provide a DC output voltage larger or lower than its DC input voltage. In general, a charge pump will have a capacitor coupled to switches between an input and an output. During one clock half cycle, the charging half cycle, the capacitor couples in parallel to the input so as to charge up to the input voltage. During a second clock cycle, the transfer half cycle, the charged capacitor couples in series with the input voltage so as to provide an output voltage twice the level of the input voltage. This process is illustrated in
Charge pumps are used in many contexts. For example, they are used as peripheral circuits on flash and other non-volatile memories to generate many of the needed operating voltages, such as programming or erase voltages, from a lower power supply voltage. A number of charge pump designs, such as conventional Dickson-type pumps, are know in the art. But given the common reliance upon charge pumps, there is an on going need for improvements in pump design, particularly with respect to trying to reduce the amount of layout area and the efficiency of pumps.
A charge pump circuit for generating an output voltage is described. The charge pump includes an output generation section and a threshold voltage cancelation section, where these sections have the same structure including a first branch, which receives a first clock signal and provides a first output, and a second branch, which receives a second clock signal and provides a second output. The first and second clock signals are non-overlapping. The charge pump circuit also includes first and second transistors, where the first and second outputs of the output generation stage are respectively connected through the first and second transistors to provide the output voltage of the charge pump, and where the first and second outputs of the threshold voltage cancellation stage are respectively connected to the control gate the first and second transistors.
Various aspects, advantages, features and embodiments of the present invention are included in the following description of exemplary examples thereof, which description should be taken in conjunction with the accompanying drawings. All patents, patent applications, articles, other publications, documents and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of terms between any of the incorporated publications, documents or things and the present application, those of the present application shall prevail.
The various aspects and features of the present invention may be better understood by examining the following figures, in which:
a is a simplified circuit diagram of the charging half cycle in a generic charge pump.
b is a simplified circuit diagram of the transfer half cycle in a generic charge pump.
The techniques presented here are widely applicable to various charge pump designs for the use of cancelling the threshold voltages of the switches (typically implemented as diodes in the prior art) used to prevent the backflow of charge after pump stages. In the following, the description will primarily be based on an exemplary embodiment using a voltage doubler-type of circuit, but the concepts can also be applied to other pump designs.
More information on prior art charge pumps, such Dickson type pumps and charge pumps generally, can be found, for example, in “Charge Pump Circuit Design” by Pan and Samaddar, McGraw-Hill, 2006, or “Charge Pumps: An Overview”, Pylarinos and Rogers, Department of Electrical and Computer Engineering University of Toronto, available on the webpage “www.eecg.toronto.edu/˜kphang/ece1371/chargepumps.pdf”. Further information on various other charge pump aspects and designs can be found in U.S. Pat. Nos. 5,436,587; 6,370,075; 6,556,465; 6,760,262; 6,922,096; and 7,135,910; and applications Ser. No. 10/842,910 filed on May 10, 2004; Ser. No. 11/295,906 filed on Dec. 6, 2005; Ser. No. 11/303,387 filed on Dec. 16, 2005; Ser. No. 11/497,465 filed on Jul. 31, 2006; Ser. No. 11/523,875 filed on Sep. 19, 2006; Ser. Nos. 11/845,903 and 11/845,939, both filed Aug. 28, 2007; and Ser. Nos. 11/955,221 and 11/995,237, both filed on Dec. 12, 2007.
Although the transistors in
Various methods are known to overcome this voltage drops. For example, the number of stages in each branch can be increased to just pump the voltage up higher and the later stages can be used to cancel the threshold voltages. Another example could be a four phase Vt cancellation scheme. However, these prior cancelation techniques have limitations of one sort or another. For example, increases in the number of stages results in increases for both the required layout area and power consumption. Further, as each subsequent transistor in the series is subjected to higher voltages, their respective voltage drops become higher and the incremental gain in each stage correspondingly diminishes. In a four phase Vt cancellation scheme, the clock skews used can be difficult to control due to mismatch and routings.
Instead, the techniques presented here cancel the threshold voltage by introducing a threshold voltage cancellation section that has the same structure as the main section of the charge pump that supplies the output. In the main section, rather than use the transistors connected as diodes, the threshold voltage cancellation stage uses the outputs from the section of the main section that it is mirroring to control the transistors. This will be illustrated using an exemplary embodiment based on a voltage doubler type of charge pump, which has been found to particular for use as an efficient low voltage output charge pump, where, in this example, the goal is to generate a target output of 4 volts from an input voltage of 2.5 volts.
More specifically, with an input voltage of Vcc=2.5 volts, to generate a 4 volt output supply able to deliver 2 mA output current, with minimum input current Icc and area requirements and good power efficiency is challenging. Normally, the sort of Dickson pump of
To prevent the charge from flowing back from the output into the pump, the nodes N1 and N2 are respectively connected to the output through transistors 421 and 423. In a typical prior art arrangement, these two transistors would be connected as diodes, having their control gates connected to also receive the voltages on N1 and N2, respectively. However, this would result in the sort of voltage drops described above. Instead, a threshold voltage cancellation section, as shown on the left side of
The Vt cancellation section has the same structure and the output section and mirrors its function. A first branch includes transistor 411 and capacitor 415 and a second branch includes transistor 413 and capacitor 417, with the control gates of the transistor in each branch cross-coupled to the output node of the other branch. The output of each branch of the threshold cancellation stage is used to drive the output transistor of the corresponding branch in the output section: the node N11 of the cancellation section is used for the control gate voltage of transistor 421 and the node N22 of the cancellation section is used for the control gate voltage of transistor 423. Since the capacitors in the cancellation section are clocked the same as the same element that they mirror in the output section, when the node N1 of the output section is high, the node N11 in the cancellation section will also be high, so that transistor 421 is on and the output voltage passed; N1 and N11 will similarly be low at the same time, so that 421 is turned off to prevent the back flow of charge. The nodes N2, N22 and transistor 423 function similarly.
Although described here for a pump design based on a voltage doubler, this sort of arrangement for the cancellation of threshold can be used charge pump types. More generally, when used with other designs, in addition to the output section, which will be formed with the same architecture as usual, there will also be a voltage threshold cancellation section formed with the same structure. In the main output section, the transistors typically connected as diodes to charge from back flowing will now have their control gates connected to be set to a voltage from the mirrored node in the voltage cancellation section. For example, going back to FIG. 3A, taking the shown Dickson pump as the output section, a voltage cancellation stage of the same structure (less transistors 309 and 319, which take the role of 421 and 423 in
It should be noted that although the output section and the cancellation section have the same structure, the various mirrored elements of the circuits need not have the same size since the elements of the output stage need to drive the load of the charge pump, whereas those of the cancellation are only driving some control gates. Returning to the exemplary embodiment, the transistors 401 and 403 and capacitors 405 and 407 need provide sufficient output for the application (e.g., 4 volts and 2 mA). In contrast, the transistors 411 and 413 and capacitors 415 and 417 need only provide sufficient output for the control gate voltage of transistors 421 and 423. For example, if the transistors in the cancellation stages need only be sized a tenth or twentieth that of the elements they mirror in the output stage.
Compared with a typical prior art design based upon a Dickson pump, the exemplary embodiment of
The scheme presented here also has a number of other advantages. Unlike other Vt cancellation techniques, there is no requirement of the main, output section's stages to cancel the Vts, as the cancellation section handles this. There is also no reliance on complex clock phases or skews since the two pump sections operate in phase with each other. Additionally, the use of identical structures for the two sections results in a better and easier layout matching and clock skew matching of the pump clocks. In particular, the simple design and simple layout requirements are a distinct practical advantage.
Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. Consequently, various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as encompassed by the following claims.