Abstract:
An apparatus and method for reducing variations in a supply voltage signal. The voltage signal which powers a circuit is regulated by adding or removing a series of redundant loads to the circuit. The redundant loads are normally not connected to the circuit. However, when one of the loads of the circuit is switched out of the circuit, one or more of the redundant loads are switched into the circuit, and then removed gradually from the circuit. When one of the loads of the circuit is to be switched into the circuit, one or more of the redundant loads are switched into the circuit first, then switched out of the circuit when the load is switched in. Thus, the voltage supply sees almost the same load during a turn-on and turn-off transition period, and variations in the voltage signal are reduced.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electrical circuits, in particular, an apparatus for controlling variations in a voltage signal supplied to an electrical circuit. 
     DESCRIPTION OF THE RELATED ART 
     Most electrical circuits produced today are fabricated on a single substrate known as an integrated circuit (IC) chip. These IC chips are often interconnected via further substrates, such as printed circuit boards. The IC chips are typically connected to each other by metal traces formed on the surface of the circuit boards. The IC chips may include active devices such as logic circuitry or memory cells. The plurality of IC chips on the circuit board are typically controlled by a microprocessor or central processing unit (CPU) which may or may not be disposed on the board. The circuit board also usually includes a clock generator and a voltage source, both of which may alternatively be located off circuit board. The clock generator and the voltage source produce signals which are applied to the IC chips to make them operate. The IC chips along with the CPU and other associated discrete circuit elements form a circuit device for effecting a particular task. 
     While the circuit device is operating, various IC chips may be active at different times. Thus, some chips are “on” while others are “off.” Typically, a single voltage source provides power to all the IC chips on a given circuit board. In order to conserve power, the CPU often turns “off” IC chips which are not presently needed. Typically, the IC chip is switched “off” by stopping a clock signal (produced by the clock generator) to the device, rather than removing the voltage signal supplied by the voltage source. When these chips (i.e. those switched “off”) are needed again by the device, they are switched back “on” by the CPU, by reapplication of the clock signal. Each time an IC chip is turned “on” or “off” by the CPU, the voltage source must compensate for the increasing and decreasing load. In order to accomplish this, the voltage source includes a feedback loop which serves to keep the voltage signal at a constant level no matter what the load. However, the constant adding and removing of loads from the voltage source causes variations in the voltage signal during the transition time between “on” and “off” states. These variations in the voltage signal can lead to malfunctions in the circuit device. 
     FIG. 1 shows an exemplary circuit device  10 . The device  10  includes a substrate  15  with IC chips  20 ,  25 ,  30 , and  35  attached thereto. The circuit device  10  also includes a CPU  40  for controlling the operations of the chips  20 - 35 . The chips  20 - 35  are connected to the CPU  40  and to the other chips by metal traces (not shown) formed on the substrate  15 . When the CPU  40  determines that one of the chips  20 - 35  is not presently needed for operation of the circuit device  10 , the CPU issues a command which removes the clock signal from the particular chip. When the particular chip is again required by the circuit device  10 , the CPU issues a command which returns the clock signal to the particular device. When a device is turned “on” or “off” by the CPU  40 , a large variation in the supply voltage occurs as shown in FIGS.  2 ( a ) and  2 ( b ), explained in detail below. 
     FIG.  2 ( a ) shows a typical voltage signal during the “power down” (i.e. turning “off”) of one of the chips  20 - 35  of the circuit device  10 . Note that at the time the particular chip (e.g. chip  20 ) is powered down, time T 0  in the figure, the voltage supply signal V dd  experiences a rise due to the decreased load. This higher than normal supply voltage can cause significant damage to the other chips (e.g. chips  25 - 35 ) of the circuit device  10 . FIG.  2 ( b ) shows a typical voltage signal during the “power up” (i.e. turning “on”) of one of the chips  20 - 35  of the circuit device  10 . Note that at the time the particular chip (e.g. chip  20 ) is powered up, time T 0  in the figure, the voltage supply signal V dd  experiences a fall due to the increased load. This variation in the voltage signal can cause various problems for the circuit device  10 . In particular, each circuit device  10  has a threshold voltage V T  which protects the device from undervoltage conditions. If the supply voltage for the circuit device  10  falls below this value V T , the entire device will reset. Often, a variation due to the addition of a load causes the supply voltage signal V dd  to dip below the V T  level as shown in the figure. This causes the circuit device  10  to reset even though no reset was intended. 
     In order to solve the above problems, capacitors are often connected externally of the circuit device  10 . These capacitors are typically large capacitance elements which significantly reduce the variations in the supply voltage signal. However, in many applications, there is insufficient room to add large external capacitors. For instance, in a pacemaker system, the only elements which may be used are a device and a battery. Therefore, there is currently a need for an apparatus for controlling variations in a voltage supply signal which can integrated with a device. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus for reducing variations in a supply voltage signal of a circuit including an actual load. The variations are reduced by adding at least one redundant load to the circuit. An electrical signal for operating the different devices of the circuit is applied to both the redundant load(s) and actual load at different times during the operation of the circuit. The electrical signal is applied to the redundant load(s) when the actual load is not operating and is removed when the actual load is operating. The substitution of redundant load(s) for the actual load is performed by a controller. The adding and removing of loads from the electrical signal causes variations in the supply voltage for the circuit to be minimized. 
    
    
     The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional chip substrate with devices placed thereon. 
     FIG.  2 ( a ) is a graph showing the supply voltage of a conventional chip during power up of one of its devices. 
     FIG.  2 ( b ) is a graph showing the supply voltage of a conventional chip during power down of one of its devices. 
     FIG. 3 shows a chip according to the present invention. 
     FIG. 4 is a schematic diagram showing the circuit of the present invention during a power up phase. 
     FIG. 5 is a schematic diagram showing the circuit of the present invention during a power down phase. 
     FIGS.  6 ( a )-( f ) are a series of timing diagrams for the switches shown in FIG.  4 . 
     FIGS.  6 ( g )-( l ) are a series of timing diagrams for the switches shown in FIG.  5 . 
     FIG.  7 ( a ) is a graph showing the supply voltage of a chip according to the present invention during power up of one of its devices. 
     FIG.  7 ( b ) is a graph showing the supply voltage of a chip according to the present invention during power down of one of its devices. 
     FIG.  8 ( a ) is a schematic diagram showing a first embodiment of the switches shown in FIGS.  5 ( a )- 5 ( b ). 
     FIG.  8 ( b ) is a schematic diagram showing a second embodiment of the switches shown in FIGS.  5 ( a )- 5 ( b ). 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 3, there is shown an exemplary circuit device  100 . The circuit device  100  includes devices  120 ,  125 ,  130 , and  135 , a CPU  140 , a clock generator  150 , and a plurality of redundant loads  160 - 164  arranged on a substrate  115  (e.g. circuit board or silicon substrate). The CPU  140  is coupled to the devices  120 - 135 , the clock generator  150  and the redundant loads  160 - 164 . The clock generator  150  provides a clock signal to all the devices  120 - 135 . The CPU  140  controls the operations of the devices  120 - 135  as they are required by the circuit device  100 . In addition to a ground signal (not shown), each device  120 - 135  is supplied a voltage signal from a voltage supply (not shown) which provides power for the device. The CPU turns the various devices  120 - 135  “on” and “off” by adding or removing the clock signal (produced by the clock generator  150 ), respectively. 
     In the exemplary embodiment, devices  120 - 135  are integrated circuits, or parts of an integrated circuit, however, one or all of these elements may alternatively be any other circuit element which operates with a clock signal. Further, in the exemplary embodiment the redundant loads  160 - 164  are formed as metal traces on the surface of the device substrate  115 , however, the redundant loads may be formed as discrete circuit elements (e.g. resistors, capacitors, inductors), or in any other manner known to those skilled in the art. Additionally, although in the exemplary embodiment there are a plurality of redundant loads  160 - 164 , there may alternately be only one redundant load, as long as the redundant load has an impedance greater than or less than the impedance of the actual load (i.e. the impedance of one of devices  120 - 135 ) by approximately 50%. The process for choosing impedance values for the redundant load or loads is explained below with reference to FIGS. 4 and 5. The CPU  140  controls the activation of devices  120 - 135  and redundant loads  160 - 164  through a plurality of switches S 0 -S 5  (see FIGS. 4,  5 ). The CPU  140  controls the devices  120 - 135  and the loads  160 - 164  by either an internal program, or by a program communicated to the CPU from a source remote from the device  100 . One aspect of the CPU&#39;s function is to manage the devices  120 - 135 , so that they are only active (i.e. supplied with a clock signal) when they are being used. Thus, the CPU  140  “powers up” or “powers down” the devices  120 - 135  based on the requirements of the system in which the device  100  is utilized. The “power up” or “power down” of each device  120 - 135  is accomplished by adding or removing a clock signal from the respective device, as explained below. The removing (i.e. stopping) or adding (i.e. resuming) of the clock signal is accomplished through a circuit  200  coupled between the CPU  140  and the clock generator  150 , and each of the devices  120 - 135  (See FIGS.  4 - 5 ). 
     FIGS. 4 and 5 are schematic diagrams showing the operation of an exemplary circuit  200  according to the present invention. The circuit  200  is coupled between the CPU  140 , the clock generator  150 , the redundant loads  160 - 164 , and each of the devices  120 - 135 . Each device  120 - 135  may include a separate circuit  200  which has redundant loads  160 - 164  matched to the actual current drawn by the respective device, or the devices  120 - 135  may share a circuit  200  and set of redundant loads  160 - 164 . In the exemplary embodiment, where the redundant loads  160 - 164  are connected in parallel, the redundant loads  160 - 164  preferably have impedance values greater than the impedance of the actual load (i.e. the impedance of one of the devices  120 - 135 ). It is necessary to make the impedance of the redundant loads greater than the impedance of the actual load because the parallel coupling of the loads  160 - 164  causes the clock to see a lower impedance then each of the loads would have individually because of impedance division effects. Since resistance is directly proportional to impedance, as resistance is raised, so is impedance and vice versa. For example, if the actual device  120 - 135  were a 100 ohm (Ω) resistor with a specified impedance, the parallel-coupled redundant loads may each comprise 500Ω resistors with associated impedance values. If the redundant loads  160 - 164  were alternatively connected in series with one another (not shown), the loads would preferably have a smaller impedance than the actual load (i.e. the impedance of one of the devices  120 - 135 ). For example, if the actual device  120 - 135  were a 100Ω resistor with a specified impedance, the redundant loads  160 - 164  may each comprise a 20Ω resistors with associated impedances. The redundant loads  160 - 164  also preferably represent incremental portions of the actual load drawn by the device  120 - 135  (e.g. 20Ω is ⅕ of the 100Ω actual load in the series coupled example above). It is important that the loads  160 - 164  be incremental portions of the actual load so that the impedance on the clock line can be gradually raised or lowered to reach the actual impedance value. 
     The circuit  200  operates to add or remove the set of redundant loads  160 - 164  from a clock line  151  coupled to the redundant loads and the actual load (i.e. the load drawn by one of the devices  120 - 135 ). The adding and removing of the actual load and the redundant loads from the clock line directly affects the supply voltage for the circuit device  100 . In particular, each time a load (actual or redundant) is removed from the clock line  151 , the particular load is removed from the supply voltage, and the supply voltage experiences a rise due to the respective decrease in total load. Each time a load (actual or redundant) is added to the clock line  151 , the particular load is added to the supply voltage, and the supply voltage experiences a fall due to the respective increase in total load. Thus, when the redundant loads  160 - 164  are added and removed, the variation in the supply voltage signal are less because the redundant loads  160 - 164  each draw less power than the actual load. As explained above, as few as one redundant load may be utilized without departing from the scope of the invention. In the case where one redundant load is used, the impedance value of the redundant load would preferably be approximately 50% or 150% of the impedance of the actual load. In this way, the impedance of the circuit can be raised or lowered to the actual impedance value (i.e. the impedance of one of the devices  120 - 135 ) from either a higher impedance (e.g. 150% redundant load) or a lower impedance (50% redundant load), and thus variations in the voltage signal are diminished. FIG. 4 shows the operation of the circuit during a “power up” phase, and FIG. 5 shows the operation of the circuit during a “power down” phase. 
     The circuit  200  couples a clock signal generated by the clock generator  150  and propagated on clock line  151  to each of the devices  120 - 135  of the device  100 . Each of the devices  120 - 135  are connected to the clock line  151  and a set of redundant loads  160 - 164  by switches S 0 -S 5 , respectively. The switches S 0 -S 5  receive control signals from the CPU  140  (see FIG. 3) which turn the switches on and off. 
     In operation, when a device  120 - 135 , for example device  120 , receives a “power up” signal from the CPU, switches S 1 -S 5  are closed sequentially beginning with switch S 1 . Switch S 5  is activated (i.e. closed) just prior to the time when the actual power up of the device  120 - 135  is scheduled to take place. The closing of these switches S 1 -S 5  brings the redundant loads  160 - 164  into the circuit sequentially, thus raising the load on the voltage supply incrementally. When all the switches S 1 -S 5  are closed, the load on the voltage supply equals the load of the device  120 . Switches S 1 -S 5  are then opened simultaneously with switch S 0  being closed. Thus, the redundant loads  160 - 164  are replaced by the actual load (i.e. the load drawn by device  120 ) and the voltage supply sees only small variations. FIG.  6 ( a )-( f ) show the timing diagrams for switches S 1 -S 5  during the “power up” phase. 
     The “power down” phase works just the opposite of the “power up” phase. When the CPU  140  receives a command indicating the need to power down a device  120 - 135 , for example device  120 , the CPU opens switch S 0  and simultaneously closes switches S 1 -S 5 . When all the switches S 1 -S 5  are closed, a redundant load equal to the actual load of device  120  is introduced into the circuit  200 . Thus, the voltage supply sees almost no variation due to the substitution of an almost identical load (i.e. loads  160 - 164 ) for the actual load. The redundant loads  160 - 164  are then removed one-by-one by opening switches S 1 -S 5 , beginning with switch S 1 . When the last switch S 5  is opened, the redundant loads  160 - 164  are effectively removed from the circuit. Since the redundant loads are removed sequentially, the voltage supply does not see a large variation. FIG.  6 ( g )-( l ) shows the timing diagrams for switches S 0 -S 5  during the “power down” phase. 
     FIGS.  7 ( a ) and  7 ( b ) show the resulting voltage signal V dd  in the exemplary embodiment. FIG.  7 ( a ) shows the voltage signal V dd  during a “power down” phase, and FIG.  7 ( b ) shows the voltage signal during a “power up” phase. It should be noted that, as opposed to the voltage signal shown in FIGS.  2 ( a ) and  2 ( b ), the signal experiences a plurality of small variations rather than one large variation. These smaller variations cause less problems than the large variations inherent in conventional circuits. In particular, the smaller variations during the “power up” phase prevent the voltage signal from dropping below the threshold voltage V T  and resetting the circuit device. 
     The switches S 1 -S 5  can be fabricated in various ways. FIGS.  8 ( a ) and  8 ( b ) show first and second exemplary embodiments of the switches S 1 -S 5 . FIG.  8 ( a ) shows a first exemplary embodiment wherein the switches are transistor switches  210 , such as a bipolar junction transistors. The transistors  210  are biased by a control signal to its base “B” which turns the switch on and off. FIG.  8 ( b ) shows a second exemplary embodiment wherein the switches are tristateable buffers  220 . The buffers  220  include a control signal line V c  which controls the on and off states of the switch. In both the first and second embodiments, the control signals are supplied to the switches by either the CPU  140  or a separate controller (not shown). 
     Thus, the exemplary embodiment of the present invention operates to decrease variations in a voltage supply signal by introducing and removing loads in a gradual manner. By introducing a plurality of redundant loads to a circuit before an actual load is added, and by removing a plurality of redundant loads before an actual load is removed, variations that occur in the voltage supply signal due to load changes can be significantly minimized to the point where they have little or no effect on the operation of the circuit. 
     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.