Abstract:
Provided is an electrical circuit that provides a boosting circuit that all in one provides a regulated step-up voltage to a non-linear device such as an array of light emitting diodes (LEDs) used in a liquid crystal display (LCD). The unique placement of the current sensing circuit within the boosting circuitry eliminates the need for a separate current regulating circuit, thus minimizing the circuitry needed to provide a constant back lighting LED array of constant luminosity.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the field of DC to DC converters used with non-linear devices such as light emitting diodes (LED) in back lighting for liquid crystal displays (LCD). 
     BACKGROUND INFORMATION 
     Many crystal displays (LCDs) take advantage of light emitting diode (LED) back lighting technology. The LEDs are used to provide a back lighting source for the LCDs so that the displays may be more efficiently viewed. However, illuminosity of LEDs is sensitive to current fluctuations and is directly dependant upon the current flowing through the LEDs. Therefore, back lighting LED circuitry for LCDs must regulate the current flow through the LED to ensure a constant current during all operating conditions. thus providing a constant lighting source for the LCDs. 
     LEDs utilized for back lighting purposes are typically aligned in an array. FIG. 1 illustrates a block diagram of a typical circuit utilizing a LED array in the prior art. LED  101  is placed in series with a predetermined number of equivalent LED  101 s. This multiple LED arrangement provides a stack  105  of four LEDs aligned in series. Those skilled in the art will understand the stack is not limited to four LEDs and that more or less than four LEDs may be used to achieve similar results. 
     Subsequent stacks  106  and  107  are placed in parallel with stack  105 . Stacks  106  and  107  each have the same LED arrangement as stack  105 , each stack consisting of the same number and type of LEDs. The parallel arrangement of stacks  105 ,  106  and  107  provides an LED array  110  for the back lighting of the LCD (not shown). 
     Power requirements of LEDs encourage the LEDs to be stacked in series, with each stack then placed in parallel with other stacks as shown in FIG. 1 in circuit  110 . As diodes, LEDs require some voltage to forward bias the LED and permit proper operation of the light emitting aspect of the diode. Typical LEDs might require as much as 1.2 volts or more to forward bias the diode depending on the diode used. Thus, when four LEDs are connected in series as a LED stack 4.8 volts may be required to forward bias the stack. Additional circuit losses might force the voltage requirements of the stack to be as high as 8 volts to forward bias the LED stack. This level typically exceeds the standard voltage used in the display circuitry requiring a step-up or boosting circuitry to provide the additional voltage. Many in the art have solved this problem by using a DC-DC converter to step-up or boost the voltage available to the diode stacks. Thus, voltage source  125  is input to a DC-DC boosting converter  120  which typically provides a boosted voltage across the LED array  110 . Current regulating device  130  senses the current across resistor  135  and regulates the current through LED array  110  in order to provide a constant current through display  110 . Those skilled in the art will recognize that the arrangement of stacks  105 ,  106  and  107  along with the current limiting device  130  provide a constant voltage and current source across LED array  110 , thus providing a constant illuminosity output of display  110 . In this way, those skilled in the art will recognize that each LED will receive the same current flow as every other LED in the array  110 , ensuring a constant luminosity across the LED array  110  for any given current flow. 
     FIG. 2 illustrates a traditional DC-DC converter  120  utilized in a boosting circuit to provide the necessary voltage needed for back lighting LEDs. Shown in FIG. 2 is a DC/DC converter control chip  203 . DC/DC converter control chip  203  is a generic, standard DC/DC converter control chip such as Advance Micro Device&#39;s ADP 1110. Various inputs are needed for DC/DC converter control chip  203  to operate in its normal mode. Pin  1   235  is supplied with V IN    250  which may correspond to the bus voltage within the display. Additionally, DC/DC converter control chip  203  receives a feedback signal (FB)  245  at pin  3 . The feedback voltage samples the voltage across a typical voltage divider resistor circuitry. The voltage divider circuitry consists of resistors  210  and  220 . The DC-DC converter  120  compares the feedback voltage with a reference voltage determined by DC/DC converter control chip  203 , internal to the DC-DC converter  120 . The reference voltage source commonly used in DC-DC voltage converters varies from 0.22 volts to 1.245 volts or more. 
     Switch (SW)  230  at pin  2  regulates the current through inductor  202 . By regulating the current through inductor  202 , those skilled in the art will recognize that, in combination with rectifier  205  and capacitor  225 , inductor  202  will operate to provide a boosted, V BUS    260  which is greater than V IN . Using a resistor divider feedback circuitry, a constant V BUS    260  is maintained. Thus, a separate and distinct current regulating circuit as shown in FIG. 1, at  130 , is needed to provide a constant current through the LEDs. 
     FIG. 3 provides an example of typical current regulating circuitry  130  utilized in a back light LED circuit in prior art circuits. In FIG. 3, a certain V BUS    260  is provided from DC-DC converter  120  as a constant voltage source to the current regulating circuit  130 . Four LEDs  101  are placed in series to create LED stack  105  as described above in FIG.  1 . Transistor  320  and low ohmic resistor R 1    135  are placed in series with LED stack  105 . Operational amplifier  330  senses the voltage across resistor  135  and compares the voltage across resistor  135  with a reference voltage  340  maintained by xenor diode  335  and resistors  336  and  338  which are aligned in a typical resistor divider network. When current flow through LED stack  105  varies from a predetermined range, those skilled in the art will recognize that Operational amplifier  330  will sense the current divergence by comparing the two voltages. If the current flow through the LED stack  105  is less than the predetermined value, the circuitry will bias transistor  320  on or off accordingly to maintain the proper current flow through LED stack  105 . It is important to note that in the prior art example shown in FIG. 3, a constant voltage source  260  is provided as a V BUS  as described above. Thus, the current regulating circuitry  130  directly affects the current flow through LED stack  105  without affecting the voltage  260 . 
     Because LEDs tend to be non-linear resistive devices, a stack of LEDs is unable to be used in a voltage divider network in order to regulate the current through the stack. Stated another way, LED stack  105  cannot replace resistor  135 , simply because of LED&#39;s non-linear characteristics do not allow for a predictive voltage to occur across a LED or LED stack. Thus, the voltage across a LED or LED stack cannot be used as the feedback voltage for a constant current output. Hence, in prior art solutions, a DC-DC converter  120  alone is unable to regulate the current through the LED back light display  110  to maintain a constant current through the LEDs. Thus, prior art solutions incorporated a separate current sensing circuit  130  electrically coupled to the DC-DC converter to regulate the current through the LED stack to within the proper limits to maintained the desired luminosity as described above. Using two separate circuits to provide the necessary voltage and current flow through the LED array  110  is inefficient because of the additional circuitry needed. 
     Thus, a need in the art exists for a simplified DC-DC converter/current regulating device that, all in one, provides the proper voltage range, while maintaining the required current needed to provide a constant luminescence in the back light LED art. 
     SUMMARY OF THE INVENTION 
     Accordingly, provided is a DC-DC voltage converting means that takes the input voltage and either boosts or bucks the voltage to an output voltage, the output voltage being placed across a LED array and low ohmic resistor, such that the voltage across the low ohmic resistor may be utilized as a feedback signal to control the output current, thus providing a constant current through the LED array. If the LED array demands it, an op amp is used to properly amplify the voltage sampled across the low ohmic resistor such that the output voltage from the op amp is within the specifications of the DC-DC converting circuitry. 
     Additionally, provided is a circuit comprising a DC-DC converting means for providing a boosted or bucked output voltage, the output voltage being provided to an electrical device connected in series with a current sensing means which provides a voltage feedback to the DC-DC converter, the voltage feedback being used to regulate the output voltage such that the current through the electrical device is constant within the data processing system. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a LED back light display circuitry used in the prior art, 
     FIG. 2 illustrates a step-up or boosting DC-DC converter circuitry utilized in the prior art; 
     FIG. 3 illustrates a constant current regulating circuitry utilized in the prior art; 
     FIG. 4 illustrates a LED back light display circuitry utilizing an embodiment of the present invention; 
     FIG. 5 illustrates a circuit diagram of an embodiment of the present invention; 
     FIG. 6 illustrates a circuit diagram of another embodiment of the present invention; and 
     FIG. 7 illustrates a data processing system implementing an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. 
     For the most part, details concerning timing consideration and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art. 
     Referring now to FIG. 4, there is illustrated a schematic drawing of one embodiment of the present invention  401 . In FIG. 4, as described above, diodes  101  are arranged in series to create stack  105 . Stacks  105 ,  106  and  107  are placed in parallel to create display  110 . Additionally, display  110  receives a voltage from the voltage boosting circuit  120  which receives an input voltage  125 . The present invention  401  provides a current sensing circuitry  430  which senses the current through low ohmic resistor  435  by sampling the voltage across resistor  435 , and provides this sampled voltage as a feedback signal to the voltage boosting circuit  120 . The operation of current sensing circuitry  430  will be described in more detail below. 
     Referring now to FIG. 5, there is illustrated a circuit diagram of one embodiment of the present invention. In FIG. 5, the step-up voltage current regulating device  501  is comprised of the generic step-up DC/DC converter control chip  203  shown in FIG. 2 in the prior art.  501  is a more detailed view of  401  in FIG.  4 . The boosting or step-up operation of DC/DC converter control chip  203 , inductor  202 , rectifier diode  205 , and capacitor  225  have been previously discussed in relationship to FIG.  2  and will not be repeated here for brevity. In FIG. 5, LED stack  105  consists of four LEDs placed in series with each other as described above. 
     The LED stack  105  is placed in series with resistor  435 . Resistor  435  is a low ohmic resistor. The resistance of resistor  435  must be low enough as to maximize the voltage drop across LED stack  105 . However, resistor  435  must be large enough to allow a voltage to be sampled across resistor  435 . A resistor value of 0.27 ohms is used in the present example. The voltage across resistor  435  is used to provide a feedback voltage signal at input  245  to DC/DC converter control chip  203 . However, because resistor  435  is a low ohmic resistor, the voltage across resistor  435  may not be within DC/DC converter control chip  203 &#39;s voltage specification range. Thus, op amp  530  provides the necessary amplification of the voltage across resistor  435  such that DC/DC converter control chip  203  will be able to utilize it as a feedback signal  245 . Resistors  533  and  536  are gain setting resistors for op amp.  530 . In the present invention, op amp  530  senses the voltage at resistor  435  and amplifies it. DC/DC converter control chip  203  senses the feedback signal  245  from op amp  530  at pin  3 . Those familiar in the art will understand that the feedback signal is utilized by DC/DC converter control chip  203  to control the current flow through the inductor at pin  2 , point  230  as discussed above in FIG.  2 . 
     If current flow through LED stack  105  varies, circuitry  501  will vary the voltage across LED stack  105  in order to minimize the current variance. For example, if current flow through LED stack  105  should drop because of temperature variances or system losses, then the voltage across resistor  435  will drop. Op amp  530  will sense the voltage across resistor  435  and amplify the voltage to a usable level and provide the amplified voltage as feedback signal  245  to DC/DC converter control chip  203 . DC/DC converter control chip  203  will sense the drop in feedback voltage  245  by comparing it to a reference voltage determined by the characteristics of the DC/DC converter control chip  203  as described above, and operate to more rapidly switch the current on and off at point  240 . By switching the current across inductor  202  more rapidly, those in the art will understand that a higher voltage is produced at across cap  225 . This higher voltage provides the higher voltage across and correspondingly a higher current flow through LED stack  105 , thus maintaining the proper current flow through LED stack  105 . 
     In this way, unlike in the prior art where the output voltage or bus voltage was regulated by the DC-DC converting circuitry within a certain range, in the present invention the bus voltage across the LED stack  105  is allowed to float within the DC-DC converting circuitry, while maintaining the current flow across resistor  435 . In this way, the voltage across the LED stack  105  is allowed to vary in accordance to the current needs of LED stack  105 . By ensuring that the current flow through LED stack  105  remains within a constant range, circuitry  501 , maintains a constant luminosity of stack  105  all within one circuit  501 . 
     Referring now to FIG. 6, there exists another embodiment of the present invention. FIG. 6 illustrates the circuitry as described in FIG. 5 with the exception that op amp  530  and its corresponding resistors  533  and  536  are removed from circuitry  501 . Thus, circuitry  601  operates without the need of op amp  530 . The deletion of this portion of the circuitry can occur when power consumption limitations allow for it. In other words, when input power available is such that resistor  435  can be increased to the point that the voltage across resistor  435  equals the reference voltage internal to the DC-DC converter, then op amp  530  is no longer needed. Thus, resistor  620  is a low ohmic resistor and function in a similar manner as resistor  435 . However, because of power availability levels, the resistive value of  620  can be high enough to allow for the deletion of op amp  530  from the circuitry. 
     A representative hardware environment for practicing the present invention is depicted in FIG. 7, which illustrates a typical hardware configuration of data processing system  713  in accordance with the subject invention having central processing unit (CPU)  710 , such as a conventional microprocessor, and a number of other units interconnected via system bus  712 . Data processing system  713  includes random access memory (RAM)  714 , read only memory (ROM)  716 , and input/output (I/O) adapter  718  for connecting peripheral devices such as disk units  720  and to bus  712 , user interface adapter  722  for connecting keyboard  724 , mouse  726 , and/or other user interface devices such as a touch screen device (not shown) to bus  712 , communication adapter  734  for connecting data processing system  713  to a data processing network, and display adapter  736  for connecting bus  712  to display device  738 . The present invention, within data processing system  713 , would be located within display device  738 . CPU  710  may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU  710  may also reside on a single integrated circuit. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.