Patent Publication Number: US-2005128168-A1

Title: Topology for increasing LED driver efficiency

Description:
RELATED APPLICATIONS  
      This application claims the benefit of a U.S. Provisional Patent Application Ser. No. 60/527,570 entitled “Topology for Increasing LED Driver Efficiency” filed Dec. 8, 2003. The disclosure of that provisional application is incorporated in this document by reference. 
    
    
     TECHNICAL FIELD  
      The present invention relates to drivers used to power light emitting diodes (LEDs) and other devices. More particularly, the present invention relates to efficient drivers for white LED applications in portable electronic systems.  
     BACKGROUND OF THE INVENTION  
      Extending battery life is one of the most important tasks faced by designers of portable electronic systems. This is particularly true for consumer electronics, such as cellular phones, digital cameras, portable computers and other handheld equipment. Designers of these products are faced with a continual need to reduce package size (and battery size) while increasing battery life to match or exceed competitive products.  
      White LEDs are commonly used to illuminate color displays in portable electronic systems. The forward voltage of these LEDs is usually higher than the voltage available from common battery chemistries and configurations. As a result, some form of driver is typically used to regulate voltage and current whenever white LEDs are powered by batteries. The relatively large amount of current handled by drivers of this type makes their efficiency (typically denoted η) a critical consideration for designers of portable electronic systems.  
      As shown in  FIG. 1 , a typical LED driver includes a voltage regulator and a current controller. The voltage regulator is generally a step-up type DC/DC converter circuit, employing either an inductor-based switching converter or a capacitive charge pump. For many applications, the current controller is a current source powered by the output of the voltage regulator and is placed in series with the LED and electrical ground. With this combination, multiple LEDs can be driven in parallel. Powering multiple parallel connected LEDs from a single-output current source, however, suffers from variation in LED brightness resulting from random mismatch in LED forward voltage V f .  
      U.S. patent application Ser. No. 10/369,982 describes an efficient driver for light emitting diodes. For that driver, LED&#39;s are driven using a combination of a current source and a boost converter (a charge pump in most implementations). The boost converter is enabled when the forward voltage (Vf) of the driven LED requires it, otherwise the charge pump remains disabled. This architecture works well and is highly efficient for single LEDs. Unfortunately, LEDs exhibit great variability in forward voltage. Vf can differ by as much as one volt even for LEDs from the same production lot. This creates an undesired inefficiency when the previously described LED driver is used to drive multiple LEDs. This results because the boost converter in that architecture becomes enabled whenever the forward voltage of any single LED requires it, even if all other LEDs do not.  
      For this reason, there is a need for an efficient LED driver for use with multiple LEDs. This need is particularly relevant to portable electronic systems where increased efficiency is directly related to increased battery life.  
     SUMMARY OF THE INVENTION  
      The present invention provides a high efficiency topology for driving multiple LEDs (and related devices) with high efficiency. A representative implementation of the driver topology pairs each LED to be driven with a current source. Each current source is connected through a pair of switches to a battery and to a charge pump. This allows each LED to operate in two modes: battery mode and charge pump mode. Comparators monitor the forward voltage of each LED. Battery mode is used when an LED&#39;s forward voltage combined with the overhead of the LED&#39;s associated current source is less than the battery voltage. Charge pump mode is used when an LED&#39;s forward voltage combined with the overhead of the LED&#39;s associated current source exceeds the battery voltage. The choice between battery mode and charge pump mode is made on a LED-by-LED basis. Thus, each LED only enters charge pump mode when required by its own forward voltage (and the voltage overhead of its associated current source). The charge pump is only activated when one or more LEDs are operating in charge pump mode and is disabled otherwise. By selecting between charge pump and battery mode individually for each LED, efficiency is increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a prior art LED driver using a current source in series with a voltage regulator.  
       FIG. 2  is a block diagram of an LED driver topology as provided by an embodiment of the present invention.  
       FIG. 3  is a diagram that shows the operation of a series of LEDs driven using a topology of the present invention as a function of battery voltage.  
       FIG. 4  is a diagram that shows how the efficiency of the topology of  FIG. 2  changes as a function of battery voltage.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention provides a high efficiency topology for driving multiple LEDs (and related devices) with high efficiency. As shown in  FIG. 2 , a representative implementation of the topology  200  includes a series of LEDs, represented by LED  202   a  and  202   b . In general, although two LEDs  202  are shown, it should be appreciated topology  200  is intended to be support any useful number. Each LED is connected to a current source  204  which are driven, in turn by either a battery (not shown) or a charge pump  208 . The choice between battery and charge pump  208  is controlled by two switches per current source. These are designated  210  (closed when current sources  204  are driven by the battery) and  212  (closed when current sources  204  are driven by charge pump  208 ).  
      Switches  210  are controlled by the outputs of respective comparators  214 . In this way, the output of each comparator  214  determines if its associated LED is driven by the battery or by charge pump  208 . The inputs to comparators  214  are the LED forward voltage (V f ) (of the associated LED) and the difference between the battery voltage V BAT  and an offset voltage V os , where V os  is the overhead required by each current source  204 . The battery is selected for a given LED  202  as long as V f  (for that LED  202 ) stays below V BAT  minus V os . Otherwise, charge pump  208  is selected. In other words, the battery is used as long as the battery voltage is sufficient to run the combination of an LED  202  and its associated current source  204 . Otherwise, that LED  202  is switched to be supplied by charge pump  208 . The choice between battery and charge pump  208  is made on a case-by-case basis. Thus, there will typically be cases where some LEDs  202  are battery drive while others are power by charge pump  208 .  
      The operation of charge pump  208  is controlled by an or gate  216 . Or gate  216  is driven by the output of comparators  214 . As a result, charge pump  208  is enabled whenever required to run one of LEDs  202 . Otherwise, charge pump  208  is inactive. Since in the case of a charge pump, the input current is 1.5× or 2× (depending on charge pump topology) times the diode current, any LEDs  202  not powered by charge pump  208  give a significant current savings.  
      An example of this is shown in  FIG. 3 . For this particular example, it is assumed that topology  200  is configured to drive four LEDs  202 . One of these has a forward voltage of 3.7V. The remaining three have forward voltages of 3.2V. Battery voltage is assumed to start at 4.2V and V os  is assumed to be approximately 0.26V (this example ignores the fact that V os  will typically vary between current sources  204 ). At the initial battery voltage of 4.2V all LEDs  202  operate using the battery. As the battery voltage falls below ˜3.96V (i.e., V f  plus V os  for the first LED  202 ), the first LED  202  is switched to be powered by charge pump  208 . The remaining three LEDs  202  are powered by the battery until battery voltage falls below ˜3.46V (i.e., V f  plus V os  for the remaining LEDs  202 ). At that point, the remaining LEDs  202  are switched to be powered by charge pump  208 .  
       FIG. 4  shows how the efficiency of topology  200  changes as a function of battery voltage. Two traces are shown. The first (labeled “A”) shows efficiency for the LEDs  202  just described (i.e., V f   1 =3.7V, V f   2 =V f   3 =V f   4 =3.2V). The second trace (labeled “B”) shows efficiency compared to battery voltage for a second series of LEDs. The LEDs in that series all have a forward voltage of 3.5V. By inspection, it is clear the architecture of the current invention is able to run the first series of LEDs (with their mixed forward voltages) with greater efficiency (e.g., efficiency is improved 18% at V BAT =3.6 v).  
      In general, it should be noted that the topology described above is adaptable for a number of different implementations. This specifically includes an implementation where charge pump  208  is replaced by the combination of a charge pump and a DC/DC converter. A second implementation uses a voltage regulator in place of charge pump  208 . The details of these configurations along with several variations are described in U.S. patent application Ser. No. 10/369,982. It should also be noted that while driving white LEDs is a prime application of the described topology, applications exist for colored LEDs as well. This occurs, for example when white LEDs are used to illuminate a display in combination with colored LEDs that illuminate a translucent keyboard (a description that matches) many current cellular telephone designs. In such cases, the white and colored LEDs typically have different forward voltages and benefit from the driver topology described above. Another example occurs when driving RGB (red, green, blue) LED arrays. Once again, the different LED colors tend to have different forward voltages and may benefit from the described driver topology.  
      Although particular embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made without departing from the present invention in its broader aspects, and therefore, the appended claims are to encompass within their scope all such changes and modifications that fall within the true scope of the present invention.