Patent Publication Number: US-8970190-B2

Title: Using low voltage regulator to supply power to a source-biased power domain

Description:
RELATED PATENT APPLICATION 
     This application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 61/451,202; filed Mar. 10, 2011; entitled “Using Ultra-Low Power Voltage Regulator to Supply Power to a Source-Biased Power Domain,” by James Muha, Tim Wilson, D C Sessions and Yong Yuenyongsgool; which is hereby incorporated by reference herein for all purposes. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to voltage regulators, and, more particularly, to using a low voltage regulator to significantly reduce standby, sleep mode current draw in source-biased power domains of an integrated circuit device. 
     BACKGROUND 
     An integrated circuit device may electrically alter the threshold voltage of its NMOS transistors by raising the Vss power rail voltage above the bulk (e.g., well, tub, or substrate) voltage of the integrated circuit substrate (sometimes referred to as a “virtual ground”). This technique is commonly used to reduce the power consumption of the integrated circuit device due to sub-threshold leakage. Generally, the integrated circuit device will have two or more independent voltage domains to service respective core logic circuits that have signal paths therebetween; some of these voltage domains may operate on the virtual ground, and other voltage domains may operate on true ground. 
     Separate voltage supplies may be used to connect to N-MOS and P-MOS bulk regions in multiple well CMOS technologies. Modification of these voltages with respect to the primary power and ground supplies is called well-biasing. These supplies can be modulated to provide a back-bias voltage which causes an increase in the MOS device threshold voltage, Vth, thereby reducing the sub-threshold leakage. Back-bias tap cells have a basic function to provide access to the wells and/or substrate independent of the source connected transistors therein. Back bias tap cells provide power for wells of always-on cells while power is gated for retention of flip-flops states, power gates with buffers and always-on buffers. They also provide well access such that back biasing can be used for leakage optimization. 
     One way to dramatically lower the current of an integrated circuit device in a sleep state is to raise the ground rail voltage used by standard cells above the substrate voltage, commonly referred to as back-biasing. This reduces leakage current. Another way to reduce current while in a sleep state is to utilize a low voltage regulator since a loosely regulated, lower voltage is sufficient to maintain the logic cell states. This reduces bias current of not only the voltage regulator but of supporting macro cells like a band gap voltage reference. The aforementioned two techniques cannot be combined since the low voltage regulator does not provide a high enough voltage to maintain adequate noise margin when standard cells are in a back-biased state. A normal voltage regulator must be used to maintain adequate noise margin. 
     One problem with implementing source back-biasing is that the effective voltage across the biased circuits decreases due to the ground (common source) voltage rising which in turn reduces the reliability of the biased circuits. For example, in a source-biased power domain in 180 nanometer technology, the ground rail, called virtual ground, is raised to approximately 0.6 volts, so it is necessary to supply 1.8 volts to the power rail to allow for 1.2 volts of noise margin. Presently, that requires that the main voltage regulator be in operation since the output voltage of a low voltage regulator in 180 nanometer technology, for example, is only 1.2 volts, leaving just 0.6 volts of noise margin which is insufficient. 
     SUMMARY 
     Therefore it would be desirable for source back-biased circuits to retain the same effective voltage for noise margin when being powered by a low voltage regulator as when these circuits are not being back-biased. 
     According to an embodiment, a low voltage regulator coupled to source back-biased capable power domains may comprise: a low voltage regulator having a common thereof coupled to a virtual ground of at least one power domain in an integrated circuit die that is capable of being back-biased, an input coupled to a supply voltage, and an output coupled to and supplying a regulated voltage to transistors in the at least one power domain; and a true ground is coupled to a substrate of the integrated circuit die, wherein when the virtual ground is back-biased relative to the true ground sufficient to reduce leakage current to an acceptable level in a given process technology, the output voltage of the low voltage regulator rises with the virtual ground voltage so as to maintain substantially the same voltage to the transistors in the at least one power domain during back-biasing thereof. 
     According to a further embodiment, the regulated voltage from the low voltage regulator is approximately the normal operating voltage for logic minus an offset voltage at the virtual ground sufficient to reduce the leakage current to the acceptable level in the given process technology. According to a further embodiment, the regulated voltage from the low voltage regulator is approximately 1.2 volts for 180 nanometer process technology. According to a further embodiment, the at least one power domain is back-biased with a ground offset voltage relative to the true ground sufficient to reduce leakage current to an acceptable level in the given process technology. 
     According to a further embodiment, the ground offset voltage is about 0.6 volts for 180 nanometer process technology. According to a further embodiment, the true ground is at substantially zero (0) volts. According to a further embodiment, bias current of the low voltage regulator is about 100 nanoamperes which is typical for 180 nanometer process technology. According to a further embodiment, the substrate is a p-substrate having holes as majority carriers. According to a further embodiment, the virtual ground is coupled to sources of n-mos transistors fabricated in the p-substrate. According to a further embodiment, the low voltage regulator is used to power the at least one power domain during back-biasing thereof. 
     According to another embodiment, a method for powering a source back-biased capable power domain with a low voltage regulator may comprise the steps of: providing a low voltage regulator having a common thereof coupled to a virtual ground of at least one power domain in an integrated circuit die that is capable of being back-biased, an input coupled to a supply voltage, and an output coupled to and supplying a regulated voltage to transistors in the at least one power domain; coupling a true ground to a substrate of the integrated circuit die; and back-biasing the virtual ground relative to the true ground sufficient to reduce leakage current to an acceptable level in a given process technology, wherein the output voltage of the low voltage regulator rises with the virtual ground voltage so as to maintain substantially the same voltage to the transistors in the at least one power domain during the step of back-biasing thereof. 
     According to a further embodiment of the method, the regulated voltage from the low voltage regulator is approximately the normal operating voltage for logic minus an offset voltage at the virtual ground sufficient to reduce the leakage current to the acceptable level in the given process technology. According to a further embodiment of the method, during the step of back-biasing the virtual ground voltage there is a ground offset voltage sufficient to reduce leakage current to an acceptable level in the given process technology. According to a further embodiment of the method, the true ground is at substantially zero (0) volts. 
     According to a further embodiment of the method, bias current of the low voltage regulator is about 100 nanoamperes which is typical for 180 nanometer process technology. According to a further embodiment of the method, the substrate is a p-substrate having holes as majority carriers. According to a further embodiment of the method, the virtual ground comprises the step of coupling sources of n-mos transistors fabricated in the p-substrate to the virtual ground. According to a further embodiment of the method, the step of powering the at least one power domain during back-biasing thereof is done with the low voltage regulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a schematic elevational view of a portion of an integrated circuit device showing separate substrate and source common (ground) connections that are used to source back-bias transistors in the integrated circuit device, according to a specific example embodiment of this disclosure; 
         FIG. 2  illustrates a greatly simplified schematic diagram of a standard voltage regulator; 
         FIG. 3  illustrates a greatly simplified schematic diagram of a low voltage regulator; 
         FIG. 4  illustrates a greatly simplified schematic diagram of a low voltage regulator, modified according to a specific example embodiment of this disclosure; 
         FIG. 5  illustrates a schematic diagram of a low voltage regulator for source-biased power domains, according to a specific example embodiment of this disclosure; and 
         FIG. 6  illustrates a schematic block diagram of an integrated voltage regulator comprising switchable main and low voltage regulators, according to a specific example embodiment of this disclosure. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
     DETAILED DESCRIPTION 
     If the common (ground) of a low voltage regulator is connected to a virtual ground of the integrated circuit die, the regulated output voltage from the low voltage regulator is raised by approximately the same amount that the back-biased virtual ground voltage is raised. Therefore, the output of the low voltage regulator will be approximately the normal operating voltage for logic minus the ground offset voltage. For example, in 180 nanometer process technology, this voltage level is approximately 1.8 volts which is about 1.2 volts above a 0.6 volt virtual ground. 
     Since the bias current of the main voltage regulator is in the one to two microampere range while the bias current of the low voltage regulator may be 100 nanoamperes for typical 180 nanometer process technology. Therefore significant power savings may be realized without sacrificing adequate noise margin for standard cells by replacing the main voltage regulator with a low voltage regulator, modifying the integrated circuit design such that transistors that previously were connected to true ground are now connected to virtual ground, and substrate taps are connected to true ground. Several microamperes of current may thereby be eliminated in a sleep or deep sleep state while maintaining adequate noise margin. Additionally, the bias current of a band gap voltage reference can be eliminated, thereby saving several more microamperes. 
     Referring now to the drawings, the details of a specific example embodiment is schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     Referring to  FIG. 1 , depicted is a schematic elevational view of a portion of an integrated circuit device showing separate substrate and source common (ground) connections that are used to source back-bias transistors in the integrated circuit device, according to a specific example embodiment of this disclosure. An integrated circuit die may comprise a p-substrate  102  having n-mos and p-mos transistors formed therein. A typical n-mos transistor comprises an n+ source  106 , a gate  108  and an n+ drain  110 . A typical p-mos transistor comprises p+ drain  112 , a gate  114  and a p+ source  116 . The p-mos transistor is fabricated in an n-well  120  formed in the p-substrate  102 . An n+ tap  122  is formed in the n-well  120  and is coupled to V DD  and the p+ source  116  with a metal connection  118 . A p+ tap  104  separate from the n+ source  106  of the n-mos transistor couples the p-substrate  102  to a true ground  128 , TGND, connection, and the n+ source  106  is therefore independently connected to a virtual ground  130 , VGND, connection. Insulating oxides are not shown for illustrative clarity. 
     Referring to  FIG. 2 , depicted is a greatly simplified schematic diagram of a standard voltage regulator. A standard (main) voltage regulator  232  has a common rail connected to the same true ground, TGND, connection  128  that is also coupled to the p+ (substrate) ties  104 . The regulated output voltage of the regulator  232  has to be the normal operating voltage for logic in a given process technology, for example, in 180 nanometer process technology, this voltage level is approximately 1.8 volts to maintain logic circuits that have been source back-biased to reduce current therein. The voltage regulator  232  uses significant current for its own operation, thus limiting battery life. 
     Referring to  FIG. 3 , depicted is a greatly simplified schematic diagram of a low voltage regulator. A low voltage regulator  334  has a common rail connected to the same true ground, TGND, connection  128  that is also coupled to the p+ (substrate) ties  104 . The low voltage regulator  334  has only an output voltage that is insufficient to maintain logic circuits that have been source back-biased to reduce current therein. 
     Referring to  FIG. 4 , depicted is a greatly simplified schematic diagram of a low voltage regulator, modified according to a specific example embodiment of this disclosure. The low voltage regulator  436  has a common rail connected to the virtual ground, VGND, connection  130  that is only coupled to the n+ source  106 . The low voltage regulator  436  has an output voltage that is substantially the normal operating voltage for the logic minus the ground offset voltage, e.g., 1.2 volts for 180 nanometer process technology. Since the output voltage of the low voltage regulator  436  is referenced to the virtual ground, VGND, connection  130  and not the true ground, TGND, connection  128 , it can maintain an output voltage providing substantially the normal operating voltage referenced to true ground, TGND, with reference to the n+ source  106 , thereby maintaining logic circuits that have been source back-biased to reduce current therein. 
     Referring to  FIG. 5 , depicted is a schematic diagram of a low voltage regulator for source-biased power domains, according to the teachings of this disclosure. There are two GND inputs to the voltage regulator, “true ground” called TGND and “virtual ground” called VGND. The TGND connection  128  is connected to only substrate ties to keep the substrate as close to zero (0) volts as possible. The VGND connection  130  is connected to various transistor drains, gates, or sources, as dictated by the voltage regulator design, the design of which is not covered herein. The output voltage VOUT is relative to VGND since the circuitry of the regulator  436  connects only to VGND and not TGND. Thus as VGND rises above zero (0) volts the output voltage, VOUT, will rise similarly. 
     Referring to  FIG. 6 , depicted is a schematic block diagram of an integrated voltage regulator comprising switchable main and low voltage regulators, according to a specific example embodiment of this disclosure. An integrated voltage regulator  640  may comprise the main voltage regulator  232  and the low voltage regulator  436  previously described hereinabove, and voltage steering switches  642  and  644 , e.g., field effect transistor (FET) switches. A direct current voltage (power) source  646 , e.g.,  3  volt supply, battery, etc., is coupled to the voltage steering switch  642  and supplies either the main voltage regulator  232  or the low voltage regulator  436  when in normal operation or low power back-biased standby, respectively. The other voltage steering switch  644  couples either the main voltage regulator  232  or the low voltage regulator  436  to VDD for the integrated circuit transistors when in normal operation or low power back-biased standby, respectively. It is contemplated and within the scope of this disclosure that the main voltage regulator  232 , the low voltage regulator  436 , and the voltage steering switches  642  and  644  may be separate or integrated into the integrated voltage regulator  640 . 
     While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.