Abstract:
A series pass transistor voltage regulator is described having a main power source and an auxiliary power source. When the main power source voltage falls to a level the regulator can no longer use, the regulator seamlessly draws power from the auxiliary source. A voltage dropping circuit of two series-coupled diodes allows for a single feedback control amplifier to control the regulator&#39;s pass transistors in series with the main and auxiliary power sources.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/614,961, filed Sep. 30, 2004. 

   TECHNICAL FIELD 
   The present invention relates to power supplies generally and, more particularly, to voltage regulators. 
   BACKGROUND OF THE INVENTION 
   Industry standards have been widely relied upon in the design and manufacture of a number of computer system components and functions. One particular example is computer bus architectures. Generally speaking, computer bus architectures are concerned with the interface and communication between processing, memory and input/output computer system components. One commonly-used bus interface is defined as “Peripheral Component Interconnect” (PCI). At the time it was developed, PCI was a very advanced, high-performance parallel bus standard. More recently, a newer bus standard has been developed to more fully utilize new communication technologies (e.g., packet-based, point-to-point communication). This standard is referred to as “PCI Express”. 
   This newer PCI Express standard defines the auxiliary power (Vaux) to be at the same voltage level as the primary power supply. That is, the auxiliary power is defined as 3.3 Vaux, with the primary supply being +3.3V. The use of the same voltage levels makes it significantly more difficult to design a seamless Vaux switch mechanism for the PCI Express standard. In the prior PCI standard (PCI 2.2 in particular), the primary supply was maintained at a voltage level of 5.0 volts, with Vaux at 3.3 volts. There exists a variety of commercially-available products capable of providing relatively seamless switching between these two different power supplies. The ability to “seamlessly” switch (i.e., abruptly switching sources without any interruption in supplied voltage) to and from Vaux is an important capability for PCI Express add-in cards. 
   Prior art arrangements that are intended for the older PCI 2.2 standard typically add more circuitry, complexity and cost to the power supply by performing the primary/Vaux power supply switching as a stand-alone function on the power input lines. Moreover, existing prior art devices do not allow a voltage offset for the switch between the primary and Vaux power supplies while operating both rails at 3.3 volts. Such an offset is highly desirable in the PCI Express environment to ensure that the plug-in card operates on the correct supply at all times, even if the supplies are at the far limits of their respective tolerance ranges. 
   SUMMARY OF THE INVENTION 
   The need remaining in the prior art is addressed by the present invention, which relates to power supplies generally and, more particularly, to voltage regulators. 
   In accordance with the present invention, a voltage regulator is constructed to include a first pass transistor coupled between a first power supply node and the output, and a second pass transistor coupled between a second power supply node and the output. A differential amplifier is coupled between the output node and a control terminal of the first pass transistor, with a voltage dropping circuit connected between the output of the differential amplifier and a control terminal of the second pass transistor. 
   In one embodiment of the present invention, a closed-loop linear series voltage regulator coupled between the primary supply and the regulated output line is supplemented by a second series drop element disposed between the Vaux power supply and the same regulated output. One or more diodes are coupled between the primary control line and the Vaux control line to provide the desired voltage offset. In a particular arrangement, switching transistors are utilized as the series drop elements. 
   Various embodiments of the present invention may be formed in either bipolar or MOS technology, with additional components used to insure proper operation under a variety of different circumstances. 
   Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, where like numerals represent like elements in different embodiments, 
       FIG. 1  illustrates a first embodiment of the present invention, utilizing a pair of diodes between the primary control line and the Vaux control line; 
       FIG. 2  is an alternative embodiment of the arrangement of  FIG. 1  with additional elements utilized to provide temperature stability; 
       FIG. 3  illustrates yet a further variation of the present invention, incorporating a steering diode to prevent reverse current flow; and 
       FIG. 4  contains an alternative embodiment of the present invention, formed using MOS devices. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a first embodiment of a switchable voltage regulator for PCI Express applications, formed in accordance with the present invention. As shown, a differential amplifier  10  and a first transistor  12  are coupled together to form a conventional closed-loop linear series voltage regulator to provide a substantially constant voltage output Vout along output voltage rail  14 . In one particular embodiment, the Vout level may be approximately 1 volt. As shown, a reference voltage source, denoted Vref is applied as a first input to differential amplifier  10 , with the remaining input being coupled to Vout. The output voltage from amplifier  10  is coupled to the base of first transistor  12  and used, as discussed below, as the control signal for the switching function of the present invention. 
   As shown, the emitter of first transistor  12  is coupled to primary voltage supply rail  16 , where in the case of PCI-Express, the primary voltage is maintained at +3.3 volts. To complete the circuit, the collector of first transistor  12  is coupled to output voltage rail  14 , supplying a voltage level of, for example, of 1 volt. 
   In accordance with the present invention, a second linear drop element, shown as transistor  18  and a pair of diodes  20 ,  22  are added to the voltage regulator to provide the desired Vaux switching function. As shown, second transistor  18  is connected between a Vaux voltage input supply rail  24  and Vout regulated voltage output rail  14 . Diodes  20  and  22  are connected in series between the base of second transistor  18  and the base of first transistor  12 . The voltage drop v d  across each of these diodes (on the order of, for example, 0. 5 volts) is therefore sufficient to provide a voltage drop exceeding the 0.6 volt tolerance difference desired between the primary +3.3 volt source and the 3.3 Vaux source, with sufficient headroom remaining to control the power devices. It is to be understood that the use of a pair of such diodes is exemplary only, with more or less diodes or other voltage-dropping elements used (as necessary) to provide the desired amount of offset for the switching function. 
   As required by the PCI Express standard, if 3.3 Vaux is not present, the arrangement of  FIG. 1  simply runs from primary rail  16 , supplying the +3.3 voltage through first transistor  12  and into the Vout regulated output supply rail  14 . In this mode, without Vaux present, second transistor  18  will be “off”. In accordance with the teachings of the present invention, If/when the voltage along rail  16  drops below the level from which first transistor  12  can be regulated (i.e., “Vref”), the control signal output from amplifier  10  will abruptly drop by the voltage level associated with diodes  20  and  22 , second transistor  18  will turn “on”, and the regulation will begin through second transistor  18  from Vaux supply rail  24 . The provision of the supply voltage from Vaux supply rail  24  still maintains the minimum dropout voltage provided by second transistor  18 . 
   Once the +3.3 voltage level is regained along primary supply rail  16 , the control signal output from amplifier  10  will increase, following the increase along primary supply rail  16  and regulation will seamlessly be resumed again through first transistor  12 , with second transistor  18  simultaneously turning “off”. Thus, in accordance with the teachings of the present invention, the utilization of the switching transistors and diode drops, in combination with the closed-loop voltage regulator, allows for seamless transition between the primary voltage supply and the Vaux power supply. 
     FIG. 2  illustrates an alternative embodiment of the present invention, with a second pair of diodes  26 ,  28  and resistor  30  added to provide for wide temperature stability, while also assuring that the leakage current through second transistor  18  will remain sufficiently low. It is to be understood that the addition of these components is not required in every application of the voltage regulation arrangement of the present invention.  FIG. 3  contains a schematic of yet a further variation of the present invention, in this case adding a steering diode  32  to limit (prevent) reverse current flow from regulated output rail  14  and input rail  16 . In the arrangement as illustrated in  FIG. 3 , diode  32  is coupled between the collector of first transistor  12  and output supply rail  14 . Although diode  32  needs to carry the full current of the primary supply voltage, the drop across this element is not critical since it is compensated for by the normal function of the regulator arrangement. In some applications, if the system voltage levels are such that second transistor  18  may inadvertently be turned “on”, a second diode  34  may be disposed between the collector of second transistor  18  and supply rail  14  to prevent reverse flow through 3.3 Vaux input supply rail  24 . 
   As mentioned above, the voltage regulator of the present invention may be formed of MOS devices in place of bipolar devices.  FIG. 4  illustrates an exemplary embodiment of the present invention formed using MOS devices  40  and  42  as first and second transistors, respectively. To mitigate the potential current flow from the regulated output through the MOSFET substrate (reverse) diode, both steering diodes  32  and  34  should be used. Remaining elements  20 ,  22 ,  26 ,  28  and  30  function in a similar manner as discussed above in association with  FIGS. 1-3 . Inasmuch as the threshold voltages associated with MOS devices may be different than those for bipolar devices, the particular diodes selected for use in this embodiment (in terms of voltage drop) may be different than those preferred for use with the bipolar embodiment of  FIGS. 1-3 . 
   It is to be understood that various modifications may be made to the present invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specification embodiments disclosed in the specification and claims. Rather, the scope of the present invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.