Abstract:
A system includes a switching transistor that provides a current source to a load when activated by an input voltage and at least one Zener diode connected in parallel with the load that acts as a shunt regulator. The system may be especially suited for matching the controlled luminance of light emitting diodes to the controlled luminance of incandescent lighting. The system may also be useful for displays where a single master voltage regulator switch or controller controls the different types of lighting. The system may be packaged in a chip scale package for compactness, reduced weight, cost effectiveness, and higher efficiency and reliability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/560,130, filed on Apr. 6, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention generally relates to electronic power regulation and, more particularly, to voltage-controlled lamp dimming and luminance matching of light emitting diodes (LED) to incandescent bulbs.  
         [0003]     While incandescent bulbs are widely known for many lighting applications, there are many uses for which the use of light emitting diodes provides significant advantages due, for example, to the greater durability, cost effectiveness, longer life, reliability, and light generating efficiency along with the lower heat generation and power consumption of LEDs in comparison with incandescent lights. LEDs have been found to be particularly useful, for example, in aircraft cockpits and automobile dashboards for such applications as illuminated switches, lighted control panels, displays, legends, and indicators. Control panels for aircraft and other vehicles often provide a control dimming switch that allows the pilot or driver to manually dim the display, for example, to match night vision conditions or to otherwise adjust the display visibility, e.g., for personal preference.  
         [0004]     For a display having both incandescent and LED illumination, consistent dimming of the entire display from a single controller switch may be achieved if each of the lights has similar brightness characteristics. The luminance, or brightness, level of LEDs is different, however, from that of incandescent lights given the same input voltage or input current. Therefore, to provide consistent dimming from a single control dimming switch requires some form of input power (either voltage or current) compensation among the different types of lighting used for the display. Even for displays in which all incandescent lighting has been replaced by LEDs or for newly designed displays with all LED illumination, it may be desirable for the response of the control dimming switch to mimic that of the familiar incandescent lit display by using some form of compensation to match the luminance characteristics of LEDs to those of incandescent lighting. For example, the unintentional emission of light by LEDs can be a problem, since LEDs—unlike incandescent lamps—have the potential to produce detectable levels of illumination with as little as a few microamperes of current. Since some electronic devices—such as aircraft avionic equipment coupled to aircraft control panel display elements—have inherent current at levels at least that high, the display elements may be unintentionally illuminated. By compensating the input power to LEDs, the low power level characteristics of incandescent light (e.g., requiring a minimum positive power input before illumination is detectable) could be mimicked so that unintentional control panel illumination is avoided.  
         [0005]     As can be seen, there is a need for input power compensation for luminance matching for different types of lighting. There is also a need for consistent dimming of different types of lighting from a single control dimming switch. Moreover, there is a need for luminance compensation for LEDs that avoids unintentional illumination of the LEDs.  
       SUMMARY OF THE INVENTION  
       [0006]     In one embodiment of the present invention, a system includes a switching transistor that provides a current source to a load when activated by an input voltage and at least one Zener diode connected in parallel with the load that acts as a shunt regulator.  
         [0007]     In another embodiment of the present invention, a voltage-controlled power regulation circuit includes a switching transistor activated by an input voltage and a pair of Zener diodes connected (in series) to the emitter of the switching transistor so that the switching transistor provides an output current to a load, and the Zener diodes regulate the current to the load.  
         [0008]     In still another embodiment of the present invention, a voltage-controlled luminance matching circuit includes an input port having a positive input port and a negative input port; an output port having a positive output port and a negative output port; at least one diode transistor, having a base and a collector with the base connected to the collector, and connected between the positive input port and the positive output port. A switching transistor is connected between the positive input port and the positive output port. A Zener diode is connected between the positive input port and the base of the switching transistor. At least one Zener diode is connected at the positive output port so as to be connected between the positive output port and both the negative output port and negative input port.  
         [0009]     In yet another embodiment of the present invention, a chip scale package includes an input port pin; an output port pin; and a switching transistor. The input port pin is connected to apply a first voltage to the collector of the switching transistor when an input voltage V IN  is applied at the input port pin. The input port pin is also connected to provide a base current to the base of the switching transistor when the input voltage V IN  is applied at the input port pin; and the emitter of the switching transistor is connected to the output port pin. At least one Zener diode is connected at the output port pin as a shunt regulator in parallel with a load connected externally to the chip scale package at the output port pin.  
         [0010]     In a further embodiment of the present invention, a method includes operations of: applying an input voltage to a switching transistor connected as a current source to a load; and shunting excess current through a Zener diode connected as a shunt regulator in parallel with the load.  
         [0011]     In a still further embodiment of the present invention, a method for voltage-controlled power regulation includes operations of: applying an input voltage V IN    50  that a first voltage is applied at the collector of a switching transistor; and applying the input voltage V IN  so that a base current is supplied to the base of the switching transistor. The switching transistor then provides an emitter current to a circuit node E. Excess current is shunted from the circuit node E through at least one Zener diode; and current is provided from the circuit node E to a load.  
         [0012]     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a circuit diagram of a transistorized, voltage-controlled dimming circuit in accordance with one embodiment of the present invention;  
         [0014]      FIGS. 2A and 2B  presents side and bottom orthographic views of a chip scale package for a transistorized, voltage-controlled dimming circuit according to one embodiment of the present invention;  
         [0015]      FIG. 3  is a graph showing design, upper, and lower performance curves for LED current relative to input voltage to a transistorized, voltage-controlled dimming circuit in accordance with one embodiment of the present invention;  
         [0016]      FIG. 4  is a normalized graph showing luminance curves for an LED driven by a transistorized, voltage-controlled dimming circuit in accordance with one embodiment of the present invention;  
         [0017]      FIG. 5  is a graph obtained from Spice (Simulation Program with Integrated Circuit Emphasis) model simulation, showing current for three series-connected LEDs relative to the load voltage (V L ) to the LEDs supplied by a transistorized, voltage-controlled dimming circuit in accordance with one embodiment of the present invention; and  
         [0018]      FIG. 6  is a flowchart of a method for voltage-controlled power regulation of an electrical load in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
         [0020]     Broadly, the present invention provides voltage-controlled power regulation for an electrical load such as a specific light source, which may comprise, for example, a light emitting diode (LED) or an array of LEDs connected in series or in any appropriate configuration as desired. One embodiment may be particularly useful for illuminating switches or switch indicators with LEDs in control panels such as automobile dashboards, control panels of power generating stations, or aircraft cockpits. One embodiment may provide input power compensation for luminance matching for different types of lighting. For example, the voltage-controlled power regulation may be suitably configured for luminance matching of LEDs to incandescent lights and may provide consistent dimming of different types of instrument panel lighting from a control dimming switch, operated manually, for example, or from a single master control dimming circuitry operated automatically. One embodiment may be implemented in a chip scale package (CSP) useful for dimming, e.g., changing the light output in response to changes in input voltage, of illuminated control panel displays, illuminated control panel switches, and indicators. In addition, one embodiment may provide luminance compensation for LEDs that avoids unintentional illumination of the LEDs, in effect mimicking the characteristics of incandescent illumination at low power levels.  
         [0021]     One embodiment differs, for example, from prior art dimming and luminance compensation that use only passive circuit elements (e.g., circuit elements with two terminals)—such as resistors and Zener diodes—by using an active circuit element (e.g., circuit elements with three terminals), i.e., a transistor, to effectively provide a current source that can supply a range of output current values at each particular value of output voltage. Because the resistance of a load can vary, for example, with operating temperature, one embodiment can provide better luminance matching since any required current within a range of currents can be supplied by the transistor at the required particular voltage, in contrast to prior art luminance matching using only passive circuit elements—such as Zener diodes and resistors—where changes in load resistance may affect the values both of current and voltage supplied to the load. Thus, the luminance matching provided by one embodiment can be much more accurate and dependable than prior art luminance matching. Moreover, one embodiment uses at least one Zener diode (e.g., Zener diodes D 3   z    134  and D 2   z    136 , see  FIG. 1 ), which may act as a shunt regulator that regulates the applied current (e.g., output current I L    102 , see  FIG. 1 ) into the load, the shunt Zener diode being connected in parallel with the load. Such a circuit configuration is contrary to prior art circuits that connect Zener diodes only in series with the load and prior art circuits that connect only forward biased current diodes—not reverse biased Zener diodes—in parallel with the load.  
         [0022]     Referring now to the figures,  FIG. 1  illustrates system  100  for controlling the output current I L  (or I LOAD )  102  to a load  104  according to the input voltage V IN    106 , in accordance with one embodiment of the present invention. Input voltage V IN    106  may be applied across an input port  108 —which may comprise positive input port  108   a  and negative input port  108   b . Output current I L    102  may be supplied to an output port  110  and may flow from positive output port  110   a  through load  104  into negative output port  110   b . When system  100  is implemented in a chip scale package—such as chip scale package  200  shown in FIGS.  2 A and  2 B—positive input port  108   a  may be provided at a pin  208  through an electrical connection to positive input port  108   a  on a chip inside the chip scale package. Pin  208  may be formed as a solder bump as shown in  FIGS. 2A and 2B . Likewise, negative input port  108   b  may be provided at a pin  209 ; positive output port  110   a  may be provided at a pin  210 ; and negative output port  110   b  may be provided at a pin  211 . Chip scale package  200  may have additional pins  212 , as shown, which may be connected to ground. The pins  208 - 212  may be arranged in a six bump, 0.5 millimeter (mm) pitch (i.e., distance between balls, center-to-center), 0.3 mm (diameter) ball, 2×3 array as shown in  FIG. 2B . A typical package size of chip scale package  200 , for example, may be approximately 0.9 millimeter in height  214  by 1.05 mm in width  216  by 1.5 mm in length  218 .  
         [0023]     Returning to  FIG. 1 , collector current I C    112  of switching transistor Q 1   114  may be supplied via diode D 3   sh    116 , connected between positive input port  108   a  and the collector of switching transistor Q 1   114 , when diode D 3   sh    116  is forward biased (e.g., when a large enough positive voltage is applied across input port  108 ). Diode D 3   sh    116  may be a Schottky diode, for example, with a forward biased voltage drop of 0.3 to 0.5 volts (V).  
         [0024]     Base current I B    124  of switching transistor Q 1   114  may be supplied from circuit node B by Zener diode D 1   z    118 , resistor R 1   120 , and resistor R 2   122 , which may act to bias or switch switching transistor Q 1   114  on or off, depending on the value of input voltage V IN    106 . Zener diode D 1   z    118 , resistor R 1   120 , and resistor R 2   122  may be connected in series as shown from positive input port  108   a  to negative output port  110   b  (circuit node C). The base of switching transistor Q 1   114  may be connected between resistor R 1   120  and resistor R 2   122  at circuit node B. It should be noted that the connection of resistor R 2   122  at negative output port  110   b , as shown in  FIG. 1 , may be identical with circuit node C. It should also be noted that  FIG. 1  follows a standard circuit diagram convention that electrical connection is indicated by a dot where lines cross and absence of a dot indicates that there is no electrical connection where the lines cross. So, for example,  FIG. 1  indicates that positive output port  110   a  is not connected at circuit node B nor at resistor R 2   122 . Zener diode D 1   z    118  may be rated, for example, at V Z =6.6V (Zener reverse bias breakdown voltage), I Z =5 milliamps (mA), and R Z =80 ohms. Resistor R 1   120  may have a value, for example, of 850 ohms, and resistor R 2   122  may have a value, for example, of 12,000 ohms, or 12 K ohms. The node B currents may be current I 1    126 , which may pass through Zener diode D 1   z    118  and resistor R 1   120 ; current I 2    128 , which may pass through resistor R 2   122 ; and switching transistor Q 1  base current I B    124 .  
         [0025]     Emitter current I E    130  of switching transistor Q 1   114  may flow into circuit node E. Circuit node E may be connected to the emitter of switching transistor Q 1   114 , to positive output port  110 , to resistor R 4   132 , and to Zener diode D 3   z    134 , which may connected in series with a second Zener diode D 2   z    136 , for example, to increase the Zener voltage drop between circuit node E and circuit node C. Zener diodes D 3   z    134  and D 2   z    136 , like Zener diode D 1   z    118 , may be rated, for example, at V Z =6.6V, I Z =5 mA, and R Z =80 ohms. Resistor R 4   132  may have a value, for example, of 2.2 K ohms. The node E currents may be Zener diode current I Z    138 , which may pass through Zener diodes D 3   z    134  and D 2   z    136 ; current I 4    140 , which may pass through resistor R 4   132 ; switching transistor Q 1  emitter current I E    130 ; and output current I L    102 .  
         [0026]     Diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142  may be connected in series between positive input port  108   a  and circuit node D, where a terminal one of them, e.g., diode transistor Q 2 , may be connected to resistor R 4   132  and resistor R 5   144 . The base of each of diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142  may be connected to its respective collector so that each of diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142  may operate as a diode. When input voltage V IN    106  is sufficiently large across input port  108  and positive at positive input port  108   a , diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142  may be forward biased so that diode transistor current I Q2    146  may flow with a voltage drop of approximately 0.7 V across each diode transistor. Resistor R 5   144  may be connected at circuit node D to resistor R 4   132  and series-connected diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142 . Resistor R 5   144  may be connected between circuit node D and circuit node C and may provide a path for current I 5    148  to circuit node C. Resistor R 5   144  may have a value, for example, of 100 K ohms. The node D currents may be current I Q2    146 , which may pass through series-connected diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142 ; current I 4    140 , which may pass through resistor R 4   132 ; and current I 5    148 , which may pass through resistor R 5   144 .  
         [0027]     System  100  may also include trimming components used for adjusting the load current, e.g., output current I LOAD    102 , during the chip scale package wafer manufacturing process, which may be use to implement system  100  in a chip scale package—such as chip scale package  200 . Trimming components may include resistor R 3   150 , resistor R 3   a    152 , resistor R 3   b    154 , transistor Q program    156 , and fuse  158 , which may be connected as shown in  FIG. 1 . For example, resistor R 3   150  may have a value of 450 ohms, and resistors R 3   a    152  and R 3   b    154  may each have a value of 3.4 K ohms. For example, transistor Q program    156  may be used during manufacture of chip scale package  200  to selectively either “blow” or not “blow” fuse  158  in order to adjust the parameter values of the trimming components to compensate for variations and manufacturing tolerances of the components and parameters of the chip used to implement system  100  in a chip scale package  200 . Various means for providing and using trimming components may be known in the art.  
         [0028]     For the purpose of explaining the operation and circuit analysis of system  100 , the trimming components may be safely ignored and negative input port  108   b  may be considered as being directly connected at circuit node C. The operating parameters for switching transistor Q 1   114  and diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142  may be chosen—for example, by adjusting the area occupied by each component on the surface of the chip when implementing system  100  as a chip scale package such as chip scale package  102 —so that collector current I C    112  is 10 times diode transistor current I Q2    146 . For the example used to illustrate one embodiment, as illustrated by  FIG. 1 , current I C    112  may be taken nominally to be 5 mA. When the circuit and devices are conducting under normal operating conditions, then base current I B    124  may be calculated as I C /β=5 mA/100 so 
 
I B =0.05 mA  (1) 
 
 where β, having a typical value of about 100, is the current gain parameter of switching transistor Q 1   114 . 
 
         [0029]     The voltage drop from circuit node E to circuit node C, V EC  may be regulated by Zener diodes D 3   z    134  and D 2   z    136 , which may act as a shunt regulator that regulates the applied current (e.g., output current I L    102 ) into the load  104 . For example, with Zener diodes D 3   z    134  and D 2   z    136  each rated at 6.6 V then 
 
V EC =13.2 V  (2). 
 
 The voltage at circuit node D, V D  may be determined from input voltage V IN    106  according to the voltage drop across series connected diode transistors Q 2 , Q 3 , Q 4 , and Q 5   142  when input voltage V IN    106  varies, for example, in a range from about 8.4 V to 28.0 V, so 
 
 V   D   =V   IN −4(0.7)= V   IN −2.8  (3). 
 
 The voltage drop across resistor R 2   122 , V R2  is the sum of the voltage from circuit node B to circuit node E, V BE , and the voltage from circuit node E to circuit node C, V EC , but V BE  may be approximated as the base to emitter voltage drop of switching transistor Q 1   114 , e.g., approximately 0.7 V, so 
 
 V   R2   =V   BE   +V   EC =0.7+13.2=13.9 V  (4). 
 
 Thus, using Ohm&#39;s law to calculate current I 2    128  using the exemplary value of 12 K ohms for resistor R 2   122 , 
 
 I   2   =V   R2   /R 2=13.9/12 K=1.16 mA  (5). 
 
 Current I 1    126  may be calculated by summing the node B currents to zero, so 
 
 I   1   =I   2   +I   B =1.16+0.05=1.21 mA  (6). 
 
 The voltage drop across resistor R 1   120 , V R1  may be calculated from Ohm&#39;s law using the exemplary value of 850 ohms for resistor R 1   120 , 
 
 V   R1   =I   1   ×R   1 =1.21×10 −3 ×850=1.028 V  (7). 
 
 Thus, the voltage at circuit node B, V B  may be determined from input voltage V IN    106  according to the voltage drop across resistor R 1   120  and the voltage drop V Z1  across Zener diode D 1   z    118  using the exemplary value, 6.6 V, of the rated voltage of Zener diode D 1   z    118 , so that 
 
 V   B   =V   IN   −V   R1   −V   Z1   =V   IN −1.028−6.6= V   IN −7.628  (8). 
 
 The voltage at circuit node E, V E  differs from the voltage at circuit node B, V B , by the voltage drop from circuit node B to circuit node E, V BE , thus 
 
 V   E   =V   B   −V   BE   =V   IN −7.628−0.7= V   IN −8.328  (9). 
 
 Equations (3), (8), and (9) show that the voltage at circuit nodes D, B, and E, respectively, may be determined by the amount of the input voltage V IN    106  and not affected (within practical limits) by the parameters, e.g., resistance, of the load  104 . 
 
         [0030]     Continuing with  FIG. 1 , the voltage drop V R4  across resistor R 4   132 , connected between circuit nodes D and E, may be the voltage drop from circuit node D to circuit node E, V DE , which by definition may be V D −V E . Thus, 
 
 V   R4   =V   DE   =V   D   −V   E =( V   IN −2.8)−( V   IN −8.328)=5.528  V   (10). 
 
 Then, using Ohm&#39;s law to calculate current I 4    140 , using the exemplary value of 2.2 K ohms for resistor R 4   132 , 
 
 I   4   =V   R4   /R 4=5.528/2.2 K=2.51 mA  (11). 
 
         [0031]     Using a loop equation (e.g., voltage drops around a closed loop circuit sum to zero) for circuit nodes E, D, and C shows that V EC =V ED +V DC , so 
 
 V   DC   =V   EC   −V   ED   =V   EC −(− V   DE )=13.2+5.528=18.728  V   (12). 
 
 The voltage drop V R5  across resistor R 5   144 , connected between circuit nodes D and C, may be the voltage drop from circuit node D to circuit node C, by definition V DC , so V R5 =V DC =18.728 V. Then, using Ohm&#39;s law to calculate current I 5    148 , using the exemplary value of 100 K ohms for resistor R 5   144 , the value of current I 5    148  may be given approximately as 
 
 I   5   =V   R5   /R 5=18.728/100 K=0.187 mA  (13). 
 
 Diode transistor current I Q2    146  may be calculated by summing the node D currents to zero, so 
 
 I   Q2   =I   4   +I   5 =2.51+0.187=2.697 mA  (14). 
 
 By definition, V DC =V D −V C  so 
 
 V   C   =V   D   −V   DC =( V   IN −2.8)−18.728= V   IN −21.528  (15). 
 
         [0032]     Applying a node equation (e.g., the sum of currents into a node equals the sum of currents out of the node) at circuit node E to the node E currents: Zener diode current I Z    138 ; current I 4    140 ; switching transistor Q 1  emitter current I E    130 ; and output current I L    102  yields I E +I 4 =I Z +I L  so that 
 
 I   L   =I   E   +I   4   −I   Z   (16). 
 
 Equation (16) shows that current (e.g., output current I L    102 ) may be provided to load  104  by switching transistor Q 1   114  while excess current may be shunted around the load, for example, by Zener diodes D 3   z    134  and D 2   z    136 , so that a proper amount of output current I L    102  may be provided to the load depending on the load  104  resistance R L  and input voltage V IN    106 . 
 
         [0033]     For a switching transistor such as Q 1   114 , it is generally known that the emitter current I E  and collector current I C  may be related as I E =I C /α, and that for a transistor having a typical current gain parameter β of about 100, α=100/101, so that I E  is approximately equal I C . For example, with the exemplary nominal value of current I C    112  of 5 mA, and exemplary value of base current I B    124  of 0.05 mA (see Equation (1)) emitter current I E    130  may have an exemplary value of 5.05 mA. Thus, Equation (16) may be rewritten 
 
 I   L   =I   C   +I   4   −I   Z   (17). 
 
 Neglecting the values of the trimming components and assuming that V C =0 (e.g., that V C  equals the voltage at negative input port  108   b  or, equivalently, that input voltage V IN    106  may be applied across positive input port  108   a  and node C), and relating the output current I L    102 , the voltage V L    160 , and the resistance R L  of load  104 , (and using Equations (10) and (3)) then 
 
* I   L   =I   L   =V   L   /R   L   =V   E   /R   L =(− V   R4   +V   D )/ R   L =(−5.528+( V   IN −2.8))/ R   L   (18). 
 
 Assuming, for the sake of example, that R L =540 ohms, then 
 
 I   L =( V   IN −8.328)/540  (19). 
 
 Equation (19) indicates, for example, that output current I L    102  of system  100  may be controlled by the applied input voltage V IN    106 . 
 
         [0034]     An example of operation of one embodiment of a system  100 , which may be implemented in a chip scale package such as chip scale package  200 , is illustrated in  FIG. 3 .  FIG. 3  shows LED current (ILED) versus applied voltage on controlled output current curve  300 . Controlled output current curve  300  may show, for example, values of output current I L    102 , on vertical axis  302 , provided to an LED load  104  for corresponding values of the input voltage V IN    106 , on horizontal axis  304 , applied at the input port  108  of a system  100 . Curve  306  represents a typical specified upper performance limit for the normalized LED current parameter and the curve  308  represents a typical specified lower performance limit for the same parameter. Curves  306  and  308  may be transposed onto  FIG. 3 , for example, to set and illustrate the performance requirement boundaries for normalized ILED versus variable input voltage applied to the CSP chip. The controlled output current curve  300  may be the normalized CSP design performance curve for dimming the LEDs from a variable voltage source—such as system  100 . It should be noted that all three curves  300 ,  306 ,  308  may be plotted on the same coordinate system to graphically demonstrate the boundary requirements for the CSP performance values of controlled output current curve  300  so that the curve  300  of the controlled output current versus input voltage matches the luminance of a light emitting diode (e.g., load  104 ) to a curve of the luminance of an incandescent lamp versus the input voltage (e.g., for the same voltage as input voltage V IN    106 ). Curve  306  may show the allowable upper boundary performance curve, and curve  308  may show the allowable lower boundary performance curve that may indicate, for example, variations in output of system  100  due, for example, to normal manufacturing variations in component values or variations in operating temperature. The normalized controlled output current curve  300  may be composed of two segments, the first beginning at approximately 8.5 Volts direct current (VDC) where the resultant LED current (e.g., output current I L    102 ) may be approximately 2.0 microamps and ending at approximately 18 VDC where the resultant LED current may be approximately 2.2 mA, and the second beginning at approximately 18 VDC where the resultant LED current may be approximately 2.2 mA and ending at approximately 28 VDC where the resultant LED current may be approximately 20±2 mA.  
         [0035]     Another example of operation of an embodiment of a system  100 , which may also be implemented in a chip scale package such as chip scale package  200 , is illustrated in  FIG. 4  by LED normalized luminance curve  400 . The LED normalized luminance curve  400  (also referred to as the “dimmed luminance curve” or the “controlled luminance curve”) may show, for example, the values of normalized luminance on vertical axis  402  corresponding to the values of the normalized input voltage on horizontal axis  404  for an LED light source. For example, the LED light source may be connected as load  104  and the input voltage may be an input voltage V IN    106 , applied across the input port  108  of a system  100 . In general, the LED luminance output is directly proportional to its input current (e.g., output current I L    102  is the input current of LED load  104 ). Curve  406  shows a high limit normalized curve of LED luminance for approximating the normalized luminance of a comparable incandescent light and, similarly, curve  408  shows a low limit normalized curve of LED luminance for approximating the luminance of a comparable incandescent light. The curves  406 ,  408  of  FIG. 4  may be the transposed simulation of the characteristics of an LED. The curves  406  (upper boundary limit) and  408  (lower boundary limit) may be the transposed normalized luminance versus applied voltage. Curves  406  and  408  may be transposed onto  FIG. 4  to set and illustrate the performance requirement boundaries of normalized luminance of LEDs versus the voltage applied to the CSP chip. The LED normalized luminance curve  400  may be the simulated normalized performance curve for dimming the LEDs from a variable voltage source—such as system  100 . It should be noted that all three curves  400 ,  406 ,  408  may be plotted on the same coordinate system to graphically demonstrate the boundary requirements for the nominal performance values of dimmed luminance curve  400  so that the shape of the dimmed luminance curve of the light emitting diode matches the shape of the dimmed luminance curve of an incandescent light when the input voltage is within a specified range of values. Curve  400  thus indicates the normalized luminance matching of an LED to incandescent lighting over a range of input voltages (e.g., input voltage V IN    106 ), for example, between zero and 30 V and, more specifically, within a range between about 8.5 and 28 V.  
         [0036]     A further example of operation of an embodiment of a system  100 , which may also be implemented in a chip scale package such as chip scale package  200 , is illustrated in  FIG. 5  by the controlled LED current curve  500 . The controlled LED current curve  500  shows the Spice (Simulation Program with Integrated Circuit Emphasis) model simulation results where the values of the load current (Iled) are plotted on the vertical axis  502  for corresponding values of the input voltage on horizontal axis  504 . For example, the load current may be supplied as output current I L    102  from a system  100 ; the load may be an LED load  104  connected to system  100 ; and the input voltage may be an input voltage V IN    106  applied at the input port  108  of system  100 . In this example, the load  104  may comprise an array of three LEDs connected in series. As shown, the output current I L    102  to the load  104  array of three LEDs connected in series may be suitable for the LED array when driven by system  100  to approximate the luminance of an incandescent light.  
         [0037]      FIG. 6  illustrates method  600  for voltage-controlled power regulation of an electrical load in accordance with one embodiment of the present invention. Operation  602  may include applying an input voltage such as input voltage V IN    106  to a switching transistor such as switching transistor Q 1   114  in the circuit configuration of system  100 . Switching transistor Q 1   114  may be connected as a current source to a load such as load  104 . In other words, an output current—such as output current I L    102 —may be provided from a circuit node that is maintained at stable voltage relative to the input voltage—such as circuit node E of system  100 , to which the emitter of switching transistor Q 1   114  may be connected. Operation  604  may include shunting excess current through a Zener diode connected as a shunt regulator in parallel with the load. For example, Zener diodes D 3   z    134  and D 2   z    136  may be connected at a circuit node E that supplies the output current I L    102  to the load, and Equation (16) shows that the currents into and out of the node, including the output current I L    102  and the Zener diode current I Z    138 , are balanced to maintain the voltage, e.g., voltage V E , at the circuit node E. Operation  606  may include providing one or more light emitting diodes in the load  104  and matching the luminance of the one or more light emitting diodes to the luminance of an incandescent light for various values of the input voltage as shown, for example, in  FIG. 4 . System  100  may also be used, as at operation  608 , for dimming of one or more light emitting diodes (in a load  104 ) in response to changes in the input voltage (e.g., input voltage V IN    106 ) as illustrated, for example, by  FIGS. 3 and 5 .  
         [0038]     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.