Patent Publication Number: US-7710195-B2

Title: Two stage voltage boost circuit with precharge circuit preventing leakage, IC and design structure

Description:
This application is related to U.S. Ser. No. 12/031,729, filed Feb. 15, 2008, currently pending and is related to US patent application having U.S. Ser. No. 12/031,725, filed Feb. 15, 2008, and currently pending. All related US applications referenced above have common inventors and are assigned to the same assignee. 
   BACKGROUND OF THE INVENTION 
   The disclosure relates generally to voltage boost circuits. 
   A voltage boost circuit or charge pump is an electronic circuit that uses capacitors for energy storage to create a higher voltage power source. One challenge with charge pumps is that when creating a higher voltage power source, such as in a three times a supply voltage (3× Vdd) charge pump, voltages may be generated in excess of the oxide-stress limit of a field effect transistor (FET), i.e., a stress limit of the gate oxide thickness. Previous approaches have used FET devices with an oxide stress limit greater than the output voltage of the pumping system. This situation forces inclusion of a thicker, and typically lower performance FET in a technology menu which adds cost and complexity such as additional mask steps and extra characterization requirements. For example, a “medium” gate oxide thickness FET may have a gate oxide of approximately 22 Angstroms (Å), while a thicker gate oxide, lower performance FET may require a gate oxide of approximately 52 Å. Gate voltage controllers for generating a safe gate drive level below an excessive stress level of the oxide have been implemented, but they are limited in terms of the amount of boost permissible and require a precision current source for calibration. 
   BRIEF SUMMARY OF THE INVENTION 
   A two stage voltage boost circuit, IC and design structure are disclosed for boosting a supply voltage using gate control circuitry to reduce gate oxide stress, thus allowing lower voltage level FETs to be used. The voltage boost circuit may include a first stage for boosting the supply voltage to a first boosted voltage and a second stage for boosting the first boosted voltage to a second boosted voltage. Each stage may include a passgate and a gate control circuit for generating an on-state gate voltage level for the respective passgate adjusted to reduce gate oxide voltage stress on the passgate. The circuit may also include a precharge circuit for coupling a voltage on a high node of the second stage to a gate node of a precharge transistor thereof for disabling the precharge transistor and preventing leakage back to a power supply voltage. 
   A first aspect of the disclosure provides a voltage boost circuit for boosting a supply voltage, the voltage boost circuit comprising: a first stage for boosting the supply voltage to a first boosted voltage, the first stage including a first voltage boost capacitor with a low node and a high node, the high node having an output of the first boosted voltage; a first passgate coupled to the high node of the first stage; a first gate control circuit for generating an on-state gate voltage level for the first passgate adjusted to reduce gate oxide voltage stress on the passgate; a second stage for boosting the first boosted voltage to a second boosted voltage, the second stage including a second voltage boost capacitor with a low node coupled to the high node of the first stage and a high node; a second passgate coupled to the high node of the second stage for transferring the second boosted voltage to an output node; a second gate control circuit for generating an on-state gate voltage level for the second passgate adjusted to reduce gate oxide voltage stress on the second passgate; and a precharge circuit for coupling a voltage on the high node of the second stage to a gate node of a precharge transistor thereof for disabling the precharge transistor and preventing leakage back to a power supply voltage, the precharge circuit including a stack of transistors configured to prevent voltage stress on a transistor during a precharge phase of operation. 
   A second aspect of the disclosure provides an integrated circuit (IC) designed to substantially operate at a supply voltage, the IC comprising: circuitry requiring a boosted voltage relative to the supply voltage; and a voltage boost circuit including: a first stage for boosting the supply voltage to a first boosted voltage, the first stage including a first voltage boost capacitor with a low node and a high node, the high node having an output of the first boosted voltage, a first passgate coupled to the high node of the first stage, a first gate control circuit for generating an on-state gate voltage level for the first passgate adjusted to reduce gate oxide voltage stress on the passgate, a second stage for boosting the first boosted voltage to a second boosted voltage, the second stage including a second voltage boost capacitor with a low node coupled to the high node of the first stage and a high node, wherein the second boosted voltage is the boosted voltage required by the circuitry, a second passgate coupled to the high node of the second stage for transferring the second boosted voltage to an output node, a second gate control circuit for generating an on-state gate voltage level for the second passgate adjusted to reduce gate oxide voltage stress on the second passgate, and a precharge circuit for coupling a voltage on the high node of the second stage to a gate node of a precharge transistor thereof for disabling the precharge transistor and preventing leakage back to a power supply voltage, the precharge circuit including a stack of transistors configured to prevent voltage stress on a transistor during a precharge phase of operation. 
   A third aspect of the disclosure provides a design structure embodied in a machine readable medium for designing, manufacturing, or testing an integrated circuit, the design structure comprising: an integrated circuit (IC) designed to substantially operate at a supply voltage, the IC including: circuitry requiring a boosted voltage relative to the supply voltage; and a voltage boost circuit including: a first stage for boosting the supply voltage to a first boosted voltage, the first stage including a first voltage boost capacitor with a low node and a high node, the high node having an output of the first boosted voltage, a first passgate coupled to the high node of the first stage, a first gate control circuit for generating an on-state gate voltage level for the first passgate adjusted to reduce gate oxide voltage stress on the passgate, a second stage for boosting the first boosted voltage to a second boosted voltage, the second stage including a second voltage boost capacitor with a low node coupled to the high node of the first stage and a high node, a second passgate coupled to the high node of the second stage for transferring the second boosted voltage to an output node, a second gate control circuit for generating an on-state gate voltage level for the second passgate adjusted to reduce gate oxide voltage stress on the second passgate, and a precharge circuit for coupling a voltage on the high node of the second stage to a gate node of a precharge transistor thereof for disabling the precharge transistor and preventing leakage back to a power supply voltage, the precharge circuit including a stack of transistors configured to prevent voltage stress on a transistor during a precharge phase of operation. 
   The illustrative aspects of the present disclosure are designed to solve the problems herein described and/or other problems not discussed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of this disclosure will be more readily understood from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings that depict various embodiments of the disclosure, in which: 
       FIG. 1  shows a schematic diagram of embodiments of a two stage voltage boost circuit according to the disclosure. 
       FIG. 2  shows a waveform plot showing inputs to the voltage boost circuit of  FIG. 1 . 
       FIG. 3  shows a flow diagram of a design process used in semiconductor design, manufacturing, and/or test. 
   

   It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows one embodiment of a voltage boost circuit  100  according to the disclosure. Voltage boost circuit  100  may be used in an integrated circuit (IC)  102  designed to substantially operate at a supply voltage Vdd. IC  102 , however, includes circuitry  104  requiring a boosted voltage relative to supply voltage Vdd. Voltage boost circuit  100  presents a two stage voltage pump circuit that generates an output, boosted voltage (on output node VPP) approximately 3 times a supply voltage 3Vdd. For example, output, boosted voltage of approximately 2.8 Volts (V) for a supply voltage Vdd of approximately 1V may be obtained using voltage boost circuit  100 . However, as will be described herein, in contrast to conventional voltage boost circuits capable of attaining a 3Vdd boosted voltage, voltage boost circuit  100  may use lower voltage limit field effect transistors (FET) without lower performance, thicker gate oxide FETs. For example, voltage boost circuit  100  may use approximately 22 Angstrom (Å) gate oxide thickness FETs that can only withstand a gate oxide stress voltage of approximately 1.7V. As described herein, adjustment of the FET overdrive levels has been made so boosted voltages can be generated with FETs having lower voltage limits. 
   In one embodiment, voltage boost circuit  100  includes a first stage  110  and a second stage  112 . Each stage  110 ,  112 , respectively, includes a voltage boost capacitor C 1 , C 2  having a low node L 1 , L 2  and a high node V 1 , V 2 . Respectively, each stage  110 ,  112  also includes a restore circuit  120 ,  122  connecting ground-level to its low node L 1 , L 2 , and a precharge circuit  130 ,  132  connected to each high node V 1 , V 2 . Output passgates T 32 , T 31  are also connected to each high node V 1 , V 2 , respectively, with a gate control circuit  140 ,  142  connected to each passgate T 32 , T 31  providing an on-state gate voltage level adjusted to reduce oxide stress. Voltage boost circuit  100  may also include a timing signal circuit  150 , which may include any now known or later developed circuitry for generating timing signals AA, BB, G 1  and G 2 , described in greater detail herein. Voltage boost circuit  100  also includes an n-well node NW_VPP which is biased at a voltage greater than output voltage VPP by a power supply, not shown. 
     FIG. 2  shows a 2-phase clock diagram of non-overlapping inputs AA and BB, non-overlapping inputs G 1  and G 2 , capacitor nodes L 1 , V 1 , L 2 , V 2  and output, boosted voltage VPP. In one embodiment, non-overlapping inputs AA and BB may be approximately supply voltage level inputs, e.g., approximately 1.1V, and non-overlapping inputs G 1  and G 2  may be approximately at an output boosted voltage VPP. Output boosted voltage VPP is set at a voltage stress limit of devices used in a particular technology. For example, output boosted voltage VPP may be approximately 1.7V for an oxide thickness of approximately 22 Angstroms. 
   Referring to  FIGS. 1 and 2  collectively, in a precharge phase, input BB drives a restore device  120  (i.e., transistor T 40 ) of first stage  110  to restore low node L 1  of first stage  110  to ground (˜0V). Contemporaneously, input G 1  charges high node V 1  of first stage  110  to a supply voltage Vdd by precharge circuit  130  (i.e., FET T 42 ), and the gate of first stage passgate T 32  is held in an off state at output boosted voltage VPP level by gate control circuit  140  (i.e., FET T 36 ) controlled by input G 2 . Here, output boosted voltage VPP may be approximately twice supply voltage Vdd, e.g., approximately 1.7V. Second stage  112  is also held in precharge with low node L 2  connected to ground (˜0V) by restore circuit  122  (i.e., FETs T 33  and T 34 ) controlled by input G 1 , and second stage high node V 2  is pre-charged to supply voltage Vdd by precharge circuit  132  (i.e., FET T 45 ) controlled by input G 1 . Passgate T 31  is held in an off state by gate control circuit  142  by input G 2 . 
   In a boost or transfer phase, input AA is driven high by timing circuit  150  which drives first stage low node L 1  to supply voltage Vdd through FET T 41 . High node V 1  increases to almost twice the supply voltage to 2Vdd. Conventionally, the twice supply voltage 2Vdd present on first stage high node V 1  would exceed an oxide stress voltage limit of first stage passgate T 32 , e.g., of approximately 1.7V. This situation may occur, for example, when passgate T 32  includes a 22 Å thick gate oxide. To address this situation, however, gate control circuit  140  includes a stress control current source FET T 11  which provides a predetermined amount of current into resistor R 1  enabled by FETs T 37  and T 35 , which produces an intermediate gate voltage (G 2 BUF) of approximately 300 millivolts (mV). (Stress control current source FET T 11  is disconnected from supply voltage Vdd by FET T 35  in the precharge phase by signal AABUF, which is an inversion of input AA). Accordingly, with a gate voltage level of first stage passgate T 32  held no lower than 300 mV, a first boosted voltage level on first stage high node V 1  of approximately 2Vdd (e.g., ˜1.7V) can be passed without exceeding the oxide stress voltage limit of first stage passgate T 32 . Hence, first stage passgate T 32  may include lower voltage limit FETs, e.g., 22 Å gate oxide thickness FET, which reduces the additional cost and complexity of adding in a thicker gate oxide, lower performance FET for first stage passgate T 32 . 
   With passgate T 32  in its on stage, first boosted voltage on first stage high node V 1  is transferred to second stage low node L 2 . Second stage high node V 2  then increases to a second boosted voltage of approximately 3Vdd (e.g., ˜2.8V). Conventionally, a 3Vdd gate voltage present on second stage high node V 2  would pose another gate oxide stress voltage level issue for passgate T 31 . However, gate control circuit  142  provides a low level voltage (e.g., ˜1.1V) by action of current source T 12  and resistor R 2  activated by FETs T 10  and T 38 , which are controlled by input AABUF and G 2  respectively. Hence, an on-state gate to source voltage level of second passgate T 31  can be set to an output boosted voltage VPP at a maximum supply voltage thereof. (The stress an FET experiences is proportional to the source/drain voltage minus its gate voltage. To operate in a safe region this gate-source voltage Vgs, gate-drain voltage Vgd must be held below the oxide stress limit which, for example, may be 1.7V for a 22 Å device. In an embodiment with an output voltage at the drain of output passgate T 31  of 2.8V, the gate voltage may be 1.1V or above to stay below the oxide stress limit. In a second embodiment, when the passgate T 31  voltage may be 2.95V at a higher supply voltage Vdd level, the gate may be at 1.25V or higher. The resistor and current source are designed such that the oxide stress limit is not exceeded at the maximum power supply voltage.) Second boosted voltage of approximately thrice the supply voltage 3Vdd on high node V 2  is transferred to output node VPP by second stage passgate T 31 . 
   In one embodiment, this output boosted voltage may be approximately 2.95V with a supply voltage Vdd of 1.1V. Second stage passgate T 31 , however, receives an on-state gate voltage level (e.g., ˜1.25V) which does not exceed a gate oxide voltage stress limit of a low voltage FET, which reduces the additional cost and complexity of adding in a thicker gate oxide, lower performance FET for second stage passgate T 31 . Second stage passgate T 31  is turned off by activation of FET T 8  with timing signal G 2 . 
   During the boost phase, precharge circuit  132  (i.e., FET T 45 ) needs to be off to prevent output charge from bleeding back through to supply voltage Vdd. Conventional practice is to use output boosted voltage VPP, but this voltage is regulated to a voltage level less than high node V 2  of second stage  112 , while output boosted voltage VPP may be regulated to approximately 1.7V. Consequently, high node V 2  of second stage  112  can rise to approximately 2.9V, causing part of the charge to pass through FET T 45  of precharge circuit  132  and into a supply voltage Vdd node. Since output boosted voltage VPP is regulated to a lower voltage than high node V 2  of second stage  112 , it is insufficient to use output boosted voltage VPP to turn off precharge circuit  132 . In order to address this situation, precharge circuit  132  couples high node V 2  (indicated as G 1 BUF 2 ) of second stage  112  to a gate node of a precharge transistor (FET) T 45  to disable the precharge transistor T 45  and prevent leakage back to power supply voltage Vdd. Precharge circuit  132  may include a stack of transistors T 43 , T 44 , T 9  configured to prevent voltage stress on a transistor during a precharge phase of operation. In contrast, in the precharge phase, precharge circuit  132  connects G 1 BUF 2  to ground (˜0V), through FETs T 44  and T 43  when timing signal G 1  goes high. 
   In the boost phase of operation, when timing signal G 1  is at ground, FET T 43  is isolated from output node VPP (e.g., ˜2.95V) on node G 1 BUF 2  by FET T 44 , which has supply voltage Vdd level on its gate. Similarly, the gate of FET T 34  of restore circuit  122  is shielded from boosted voltages by shielding FET T 33 . As a result, precharge circuit  132  can swing between ground (˜0V) and output node NW_VPP (3Vdd) without reliability concerns. Hence, precharge circuit  132  can be completely turned off in the boost phase, preventing leakage through precharge circuit  132  transistor T 45 . The output voltage VPP may be regulated by a voltage regulator which may include a voltage comparator circuit which will determine the level of output voltage VPP relative to some reference level. The voltage regulator may regulate the 2-stage voltage boost circuit  100  by adjustment of the control signals AA, BB, G 1  and G 2  by changing their frequency or, by temporarily halting their cycling altogether. Although voltage boost circuit  100  is capable of producing an output voltage of almost 3 times its supply voltage, i.e., 3V for a Vdd supply of 1 Volt, a typical regulated voltage may be approximately 1.7 Volts. 
   A two stage voltage boost circuit  100  as described herein uses voltage levels other than ground, supply voltage Vdd and output, boosted voltage (3Vdd) VPP to control the on and off levels of passgates T 31 , T 32 . 
   It is understood that while particular illustrative electronic parameter levels (e.g., voltages, frequency, resistance, etc.) have been presented herein, the values presented are not limiting of the claimed disclosure since those with ordinary skill in the art will recognize that variations in the particular dimensions and structure of voltage boost circuit  100  may readily provide different electronic parameter levels. 
   Voltage boost circuit  100  ( FIG. 1 ) as described above may be part of the design for an integrated circuit chip. The chip design is created in a graphical computer programming language, and coded as a set of instructions on machine readable removable or hard media (e.g., residing on a graphical design system (GDS) storage medium).  FIG. 3  shows a block diagram of an exemplary design flow  900  used for example, in semiconductor design, manufacturing, and/or test. Design flow  900  may vary depending on the type of IC being designed. For example, a design flow  900  for building an application specific IC (ASIC) may differ from a design flow  900  for designing a standard component. Design structure  920  is preferably an input to a design process  910  and may come from an IP provider, a core developer, or other design company or may be generated by the operator of the design flow, or from other sources. Design structure  920  comprises an embodiment of the disclosure as shown in  FIG. 1  in the form of schematics or HDL, a hardware-description language (e.g., Verilog, VHDL, C, etc.). Design structure  920  may be contained on one or more machine readable medium. For example, design structure  920  may be a text file or a graphical representation of an embodiment of the disclosure as shown in  FIG. 1 . Design process  910  preferably synthesizes (or translates) an embodiment of the disclosure as shown in  FIG. 1  into a netlist  980 , where netlist  980  is, for example, a list of wires, transistors, logic gates, control circuits, I/O, models, etc. that describes the connections to other elements and circuits in an integrated circuit design and recorded on at least one of machine readable medium. This may be an iterative process in which netlist  980  is re-synthesized one or more times depending on design specifications and parameters for the circuit. 
   Design process  910  may include using a variety of inputs; for example, inputs from library elements  930  which may house a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a supply manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications  940 , characterization data  950 , verification data  960 , design rules  970 , and test data files  985  (which may include test patterns and other testing information). Design process  910  may further include, for example, standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. One of ordinary skill in the art of integrated circuit design can appreciate the extent of possible electronic design automation tools and applications used in design process  910  without deviating from the scope and spirit of the disclosure. The design structure of the disclosure is not limited to any specific design flow. 
   Design process  910  preferably translates an embodiment of the disclosure as shown in  FIG. 1 , along with any additional integrated circuit design or data (if applicable), into a second design structure  990 . Design structure  990  resides on a storage medium in a data format used for the exchange of layout data of integrated circuits (e.g. information stored in a GDSII (GDS2), GL1, OASIS, or any other suitable format for storing such design structures). Design structure  990  may comprise information such as, for example, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a semiconductor manufacturer to produce an embodiment of the disclosure as shown in  FIG. 1 . Design structure  990  may then proceed to a stage  995  where, for example, design structure  990 : proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc. 
   The structure as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
   The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
   The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.