Abstract:
An LED driver circuit is disclosed that can drive a plurality of LED strings that are arranged in parallel, each LED string having a plurality of component LEDs that are series-connected. The LED strings can be the same type of LEDs in each string, or have different types of LEDs from one string to another. The LED driver includes a voltage control loop that dynamically regulates the output voltage across the parallel arrangement of LED strings. The output voltage is dynamically adjusted to accommodate the LED string with the largest operational voltage drop. This enables LED displays to constructed using different types of LEDs strings, but still supply the LED strings in a power efficient manner. Further, each LED string also includes its own individual current regulation loop so that the current, and therefore brightness, of each LED string can be individually adjusted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/790,221, filed on Apr. 24, 2007, now allowed, titled “Single Inductor Serial-Parallel LED Driver”, which claims the benefit of U.S. Provisional Application No. 60/841,543, filed on Sep. 1, 2006, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is directed to a driver circuit for a Light Emitting Diode (LED) display system that can be used, for example, in cellular phones, PDAs, and other similar visual display units. More particularly, this invention is directed to providing driver circuitry to drive a multi-string configuration of different types of LEDs used in display systems. 
         [0004]    2. Background Art 
         [0005]    Modern day display systems increasingly rely upon LEDs to render a high quality visual display. The applications can include displays for cell phones, personal digital assistants (PDAs), wireless email devices, etc. It is important that the color, tone (hue), contrast, brightness, and other visual parameters remain consistent per the tolerance limits of the desired application and the specifications. To this aim, the driver circuitry used to drive any such LED display system plays a crucial role in terms of controlling the variations in the parameters and providing stability in the operation of the LED grid used in the display. 
         [0006]    Depending on the particular application, the display system could have a submodule where a series of flash LEDs are used in parallel with regular back-lighting LEDs. The voltage and current requirements for these two types of LEDs are different and so is their operation. Also, different LEDs will have different forward voltage drops due to variations in manufacturing tolerances. This results in different output voltage requirements across the LED terminals, and therefore a different optical output for each LED if a common voltage driver is applied. Since multiple LEDs are in use in display systems, such a variation in the optical output of the LEDs leads to a degradation in the overall quality of the display images and may lead to failure in operation due to reduction in the life of an LED. 
         [0007]      FIG. 1  shows the schematic for a conventional serial LED driver  100  that drives a single string of LEDs  110  containing ‘n’ number of LEDs, where ‘n’ is an integer. Each of the n LEDs  110   a  to  110   n  of the single LED string  110  is connected in a series configuration. The cathode of the last LED  110   n  of the LED string  110  is connected to one of the two terminals of a resistive component RB  112  at a node  124 . The second terminal of the resistive component RB  112  is connected to a ground  128 . Since the member LEDs  110   a - 110   n  of the LED string  110  are connected in a series configuration, there is a voltage drop as one moves along the electrical path connecting nodes  123  and  124 . As the number of LEDs increases in the LED string  110 , the voltage drop across the nodes  123  and  124  increases. Accordingly, the output voltage VOUT  108  that is required to drive the single LED string  110  will increase as the number of LEDs increases. For example, assuming a voltage drop of 3-4 volts per LED, an input voltage VIN  102  of approximately 30 Volts would be needed to drive an LED string  110  with 8 LEDs. If the input voltage VIN  102  is powered from a battery, it can be as low as 2.7 Volts for a Li-ion battery and even lower for a single cell or a two cell alkaline battery, which is the common energy source for most displays used in mobile devices. Boosting from such low input voltages to 30 Volts is not very efficient and not feasible since the duty cycle required for such a process approaches 100%. 
         [0008]    The feedback loop  122  is a voltage feedback loop that includes the control loop  117  and the FET  116  to regulate the voltage Vout  108  that supplies the LED string  110 . The feedback loop  122  measures the voltage across the resistor  112  at node  124  and controls the FET  116  to drive the voltage at node  124  to equal the reference voltage  102 . In other words, the control loop  117  compares voltage at node  124  to the reference voltage  102 , and controls the on-off duty-cycle of the FET  116  to increase or decrease the output voltage  108  so as to drive the voltage at the node  124  to be equal to the reference voltage  102 . The feedback loop  122  operates so as to time average the output voltage  108  by controlling the on-off duty-cycle of the FET  116 . Finally, the Schottky diode  106  prevents any reverse current flow from the charge stored on the capacitor  114 . 
         [0009]    The conventional serial LED driver  100  is undesirable if all the member LEDs  110   1 - 110   n  of the single LED string  110  are not of the same type. For example, if some of the LEDs are used for back-lighting and others are used for flash (flash LEDs), due to the series configuration, the same current will flow through all of them. However, flash LEDs need a higher current than that needed by LEDs for back-lighting purposes. 
         [0010]      FIG. 2  shows another conventional LED driver  200  that is used to drive multiple parallel LED strings  210   a - n  that are terminated in corresponding resistors  216   a - 216   n . These strings can be part of a main display, a sub display, a flash LED or a key pad LED, among many other things. Each of the LED strings  210  are parallel connected with each, but the LEDs in a particular string are series connected with each other. Therefore, the same current flows through each LED in a particular string. The feedback loop  122  provides a voltage feedback path to control the Vout  208 , similar to that described in  FIG. 1 . More specifically, the control loop  117  measures the voltage at the midpoint  218  of the resistor divider  220 , and controls the FET  116  to drive Vout  208  so that the midpoint  218  is equal to Vref  102 . 
         [0011]    The conventional serial LED driver  200  has poor performance if all the LED strings  210  do not have the same voltage/current characteristics. Since there is no individual current regulation for the each LED string, then the LED brightness from one string to another will vary if the LEDs are not matched. As the forward voltage of the LEDs in each of the parallel LED strings  210  changes, the current flowing in them also changes. Accordingly, there is a variation in the brightness or the optical output of the display system. The variation in forward voltage of the LEDs can be attributed, amongst many other factors, to temperature variations or manufacturing mismatches. Further, different types of LEDs require different voltage drops, for example, flash LEDs have different voltage drop requirements when compared to other LEDs. 
         [0012]    In addition, the LED current matching in the parallel LED strings  210  is not guaranteed and depends on the forward voltages of the individual LEDs. Such a current mismatch again leads to a degradation in the output of the display. In other words, the LED driver  200  does not have any method to regulate the current in the individual LED strings, and thus falls short of attaining maximum optical output efficiency of the display system. 
         [0013]    Additionally, the LED driver  200  is not power efficient. The output voltage VOUT  208  needs to be set to drive the LEDs with the largest forward voltage drop. If the output voltage VOUT  208  is less than the largest forward voltage drop, the whole string containing that particular LED will not light up. For example, if the maximum expected LED forward voltage drop is 4 Volts, then to drive 4 LEDs in series, the output voltage VOUT  208  needs to be set higher than 4×4 Volts=16 Volts. However, if one of the LED strings only requires 3 volts/per LED for a total of 12 Volts, then the extra 4 Volts is dissipated across one of the resistors  216   a - n , which is an efficiency loss of 25% (4 Volts/16 Volts*100). 
         [0014]    In view of the foregoing, there is a need for a low cost LED driver for display systems which overcomes the problems associated with the fluctuations in current and voltage for each of the strings of the LEDs and the concomitant fluctuations and inconsistencies in the optical output of the display systems. 
       SUMMARY OF THE INVENTION 
       [0015]    According to one embodiment of the present invention, an LED driver circuit is described that can drive a plurality of LED strings that are arranged in parallel, each LED string having a plurality of component LEDs that are series-connected. The LED strings can be the same type of LEDs in each string, or have different types of LEDs from one string to another. The present invention overcomes the limitations discussed above by providing a means to control the voltage output across the parallel LED strings, and a means to control the current in each of the parallel LED strings. 
         [0016]    The LED driver includes a voltage control loop that dynamically regulates the output voltage across the parallel arrangement of LED strings. The output voltage is dynamically adjusted to accommodate the LED string with the largest operational voltage drop, which allows for LED strings with different voltage drops to be powered in an efficient manner. Further, each LED string also includes an individual current regulation loop so that the current, and therefore optical brightness, of each LED string can be individually adjusted. 
         [0017]    As discussed, the mechanism of voltage regulation ensures that the output voltage is boosted to a high enough value for all of the LED strings to operate properly. Further, this ensures that all the parallel LED strings have enough headroom available for proper current regulation. Since each LED string has its own current regulation loop, it allows for tight and precise current matching across all LED strings, if desired. In addition, the current in each LED string can be independently programmed and each LED string can be independently enabled or disabled. Furthermore, due to an additional degree of design freedom obtained by providing each LED string with its own current regulation mechanism, each LED string can have a different LED type from the others. Even further, the individual LEDs of a particular string could be different from each other as well. For example, some of the LEDs could be Flash LEDs while others could be regular LEDs. Additionally, for the same reason, the tolerance of the overall system to LED mismatch due to manufacturing processes also increases. 
         [0018]    Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description. The advantages of the invention will be realized and attained by the structure and particularly pointed out in the written description and claims hereof as well as the accompanying drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed, but are not meant to limit the claimed invention in any way. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements and in which: 
           [0020]      FIG. 1  illustrates a serial LED driver with a single string of LEDs connected in series. 
           [0021]      FIG. 2  illustrates an LED driver to drive a set of parallel LED strings, where the individual strings have LEDs connected in series. 
           [0022]      FIG. 3  illustrates an LED circuit having a set of parallel LED strings that are driven according to one embodiment of the present invention. 
           [0023]      FIG. 4  illustrates main components of a current regulation loop used to control current variations in each of the parallel LED strings. 
           [0024]      FIG. 5  further illustrates an exemplary implementation of the LED circuit according one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    An LED driver circuit is disclosed for an LED display system that includes the ability to control the current in each LED string and the voltage output. The present invention also provides improved power efficiency and scalability. In the following description, specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail, as they are well known to those skilled in the art. 
         [0026]      FIG. 3  illustrates a LED display system  300  having an LED driver  301  driving a plurality LED strings  310   1  to  310   n  according to embodiments of the invention, where each LED string  310  includes a plurality of LEDs  309 . As shown, the LED strings  310   1  to  310   n  are connected in parallel with each other, but the LEDs  309  in each LED string are series connected. The total number of LEDs  309  in each individual string  310  may vary depending on the particular type of display system  300  that is intended. Further, the type of LEDs  309  may vary from string to string. For example, a first string  310   1  may contain one or more flash LEDs, and a second string  310   2  may contain backlighting LEDs. 
         [0027]    The LED driver  301  includes both a voltage regulation loop  324  and current regulation loops  314   1  to  314   n , according to one embodiment of the present invention. As such, the voltage Vout  308  is regulated to the minimum voltage necessary to supply all of the LED strings  310 . In other words, the Vout  308  is determined to meet the voltage requirement of the LED string  310  that requires the most voltage drop to be operational. Further, the current in each LED string  310  can be individually regulated by the corresponding current regulators  314   1  to  314   n . The LED driver  301  further includes an output voltage capacitor  315 , one terminal of which is connected to a ground  330  and the other to a common node connecting the output of a voltage regulation loop  324  and anodes of the LED strings  310   1  to  310   n . 
         [0028]    The input to the voltage regulation loop  324  includes an input voltage source VIN  326  via an inductor  325 , a reference voltage input VREF  322 , and a connection to a feedback signal  328 . The feedback signal  328  originates from an output line  320  of a minimum voltage selector  312 . The output voltages  316   1  to  316   n  from the cathodes of the last LEDs of the serial LED strings  310   1 - 310   n  are fed into the input of the minimum voltage selector  312 . The number of input lines to the minimum voltages selector  312  corresponds to the LED strings  310   1 - 310   n . As will be discussed further below, the minimum voltage selector  312  selects the lowest of the input voltages  316   1 - 316   n  from the ends of the LED strings  310 , and then the voltage control loop  324  drives Vout  308  so that these the lowest input voltage is set approximately equal to Vref  322 . This ensures that Vout  308  is sufficient to drive all of the LED strings  310  regardless of any differing voltage requirements among the LED strings  310 . The current regulation loop  314   1  is connected to the cathode of the last LED in the LED string  310   1  to set and maintain the current in the LED string  310   1 . Similarly, the other serial LED strings  310   2 - 310   n  have their individual current regulation loops  314   2 - 314   n . The internal circuitry of the current regulation loops  314   1 - 314   n  will be described in more detail with respect to  FIG. 4 . 
         [0029]    The operation of the voltage control loop  324  and the minimum voltage selector  312  will now be described in more detail by means of an example. Consider two exemplary serial LED strings  310   1  and  310   n . Assume that the total voltage drop across serial LED string  310   1  is 6 Volts and that across serial LED string  310   n  is 8 Volts, due to differing LED characteristics. The minimum voltage selector  312  receives the two voltage values  316   1  and  316   n  corresponding to the two voltage drops. If Vout=8 volts, then voltage  316   1 =2 v and voltage  316   n =0 v. However, the current regulation loops  314  require some minimum voltage drop to be operational. Therefore, at Vout=8 v, LED string  310   n  may not be fully turned-on at Vout=8 v, if there is 0 volts at  316   n . Therefore, the minimum voltage selector  312  selects the lowest voltage values from nodes  316   1  to  316   n  and outputs the minimum voltage to the feedback input  328 . The control loop  332  then compares the minimum voltage to Vref  322 , and drives the FET  330  so that the minimum voltage  316  is equal to Vref  322 . Specifically, the control loop  332  increases or decreases the on-off duty-cycle of FET  330  so that Vout  308  adjusted as necessary in order for the minimum voltage  316  to be equal to Vref  322 . In doing so, a minimum voltage at each of the nodes  316  is guaranteed so that the current regulation loops  314  are all operational. Further, each of the LEDs string  310   1 - 310   n  is also guaranteed to have enough voltage drop to remain operational. In this specific example herein, Vref  322  may be set to say 0.4 v, which requires Vout=8.4 volts, so as accommodate the 8 v drop across the LED string  310   n . 
         [0030]    In summary, minimum voltage selector  312  and the voltage regulation loop  324  operate so that the Vout  308  to accommodate the LED string  310  with the highest voltage drop, in order to achieve dynamic voltage regulation. But Vout  308  is not set unnecessarily high, so as to minimize power requirements. Using this technique, all the parallel serial LED strings  310   1 - 310   n  will have the sufficient voltage for the individual LEDs, which are a part of a particular string. 
         [0031]      FIG. 4  illustrates one embodiment of the current regulation loops  314 . Referring to  FIG. 4 , the current regulation loop  314  includes an operational amplifier  416  (hereinafter, referred to as “OPAMP  416 ”) having a positive input terminal  402 , a negative input terminal  404  and an output terminal  418  connected to the gate of a FET  408 . Other types of transistors could be used besides FETs, including BJTs. The positive terminal  402  receives a reference voltage VREF 1   404 , which is determined based on the desired current that is to flow through the serial LED string  310 . The negative terminal  404  is connected to the source of the FET  408  at node  410 , which is connected to one terminal of a resistor  412 . The second terminal of the resistor  412  is connected to a ground  414 . Resistor  412  is preferably a highly accurate, stable resistor so that a voltage measurement at node  410  will be used to accurately determine the current through the LED string  310 . The drain of the FET  408  is connected to the cathode of the last LED  309  of the corresponding serial LED string  310  at a node  316 , as shown. 
         [0032]    Still referring to  FIG. 4 , during operation, OPAMP  416  detects the voltage drop across resistor  412  by measuring the voltage at node  410  and comparing it to VREF 1   404 . The OPAMP  416  generates an output voltage  418  that controls the gate voltage of the FET  408 , and therefore the conductivity of FET  408  based on the difference between the voltage at node  410  and the reference voltage  404 . More specifically, the OPAMP  416  measures the voltage across the resistor  412  and drives the FET  408  so that the voltage across the resistor  412  substantially matches the reference voltage  404 . As such, the conductivity of FET  416 , and therefore the current flow through the corresponding LED string  310 , can be adjusted higher or lower (i.e. regulated) by adjusting the reference voltage  404 . The reference voltage VREF 1   404  can be different for each of the serial LED strings  310   1 - 310   n  so as to individually tailor the current flow through each LED string  310 . In sum, the current regulation loops  314  individually regulate the current in each LED string  310 , according to adjustments made to the corresponding voltage reference  404 . Since the current flow controls the brightness of an LED, then adjusting the reference voltage in a particular current regulation loop also controls the brightness of the LED string  310 . 
         [0033]      FIG. 5  further shows the LED driver  301  with the current regulation loops  314  illustrated in  FIG. 4 . As discussed above, the voltage regulation loop  324  provides dynamic voltage regulation by setting the output voltage Vout  308  so as to satisfy the LED string  310  with the highest voltage drop requirements. Further, the current regulation loops  314  also provide individual current regulation for each of the LED strings  310 , based on the corresponding reference voltages Vref  404 . 
         [0034]    Let the currents flowing through each of the serial LED string  500   1 ,  500   2  . . . be denoted be i 1 , i 2 , . . . . The current i 1  has the value: 
         [0000]    
       
         
           
             
               
                 
                   
                     i 
                     1 
                   
                   = 
                   
                     
                       VREF 
                        
                       
                           
                       
                        
                       1 
                     
                     
                       RB 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where VREF 1 =reference voltage VREF 1   404  at the positive terminal  402  of the current regulation loop  400  and RB 1 =resistor  412  shown in  FIG. 4 . Since RB 1  is a precision resistor, it is almost of a constant value. Therefore, as can be seen from equation (1), reference voltage VREF 1   404  can be used to vary the value of the current i 1  in the first serial LED string  310   1 . The same holds true for the other serial LED strings  310   2  to  310   n , as was discussed in reference to  FIG. 4  above. 
         [0035]    If reference voltages in the current regulation loop are equal (i.e. Vref 1 =Vref n ), then the voltage drop between Vout  308  and node  410   1  is equal to the voltage drop between Vout  308  and node  410   n . However, the voltage differences between the node Vout  308  and the cathode of the last LEDs  309  (node  316 ) of each of the serial LED strings  310  can vary depending upon the brightness requirements for each serial LED string  310 . The extra or differing voltage drop between LED strings  310  is accounted for by the FETs  408  in the current loops. In other words, if the one LED string  310  requires a higher voltage drop than another LED string, the extra voltage in the LED string with the lower voltage drop is dropped across the corresponding FET  408 , assuming the current loop reference voltages  404  are equal. Since different regions of the display may need different optical outputs, the flexibility in varying the output voltage VOUT  308 , if needed, adds to the design features of the LED driver  301 . Therefore, a stable output voltage VOUT  308  across the terminals of the output capacitor  315  is maintained while attaining different brightness levels for different LED strings. Meanwhile, the ability to adjust the current draw of each LED string through the current loop adds addition brightness adjusting, and power efficiency savings. 
         [0036]    As is mentioned elsewhere,  FIG. 5  is an exemplary embodiment of the present invention. Depending upon whether a constant (or a static) display is required or a varying (or a dynamic) display is required, different features of the claimed invention can be implemented, thereby resulting in more embodiments. Such embodiments will be apparent to those skilled in the art and can be learnt by the practice of the invention. 
       CONCLUSION 
       [0037]    Example embodiments of the methods, circuits, and components of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible and are covered by the invention. Such embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.