Patent Publication Number: US-2004041620-A1

Title: LED driver with increased efficiency

Description:
RELATED APPLICATIONS  
     [0001] This application claims the benefit of a U.S. Provisional Patent Application Serial No. 60/407,127 entitled “LED Driver with Increased Efficiency” filed Sep. 3, 2002. The disclosure of that provisional application is incorporated in this document by reference. 
    
    
     
       TECHNICAL FIELD  
       [0002] 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  
       [0003] 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.  
       [0004] 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.  
       [0005] 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 D . FIG. 2 shows a similar topology where the current source has been replaced by a current setting resistor. Multiple LEDs driven in parallel is also possible using this approach, but the brightness variation problem is potentially exacerbated by both resistor and forward voltage mismatch.  
       [0006] To maximize efficiency and battery life, both the voltage regulator and the current controller must be optimized to minimize dissipate power dissipation. The efficiency of the current controller is equal to the ratio of its input and output voltages, and is optimized by lowering that ratio. Optimizing voltage regulator efficiency is more involved. As shown in FIG. 3, a typical LED driver places a regulated charge pump in series with a battery and current source (or other current controller). For this configuration, efficiency η is equal to V D  (the forward diode voltage) divided by V BAT  (the input power supply) times CP. In the case where a doubler regulated charge pump is used, CP is equal to 2. For typical applications where V D  and V BAT  are 3.5 volts, the resulting efficiency is 50%. Alternately, when a fractional charge pump is used, CP is equal to 1.5 and the resulting efficiency (for V D  and V BAT  equal to 3.5 volts) is 67%. For this reason, the use of a fractional charge pump is strongly indicated where efficiency is paramount. In either case, it is clear that the use of a regulated charge pump results in a significant reduction in efficiency.  
       [0007] As shown in FIG. 4A, a second method for driving LEDs places an inductor based DC/DC boost converter in series with a battery and current source (or other current controller). The driven LED&#39;s are configured in series, and the regulated voltage is equal to the number of LED&#39;s multiplied by the LED forward voltage V D  plus the voltage drop across the current controller. FIG. 4B shows a similar topology where the current source has been replaced by a current controlling resistor.  
       [0008] In practice, LED drivers of this type must be configured to generate relatively high regulated voltages, often in the range of twenty volts. For monolithic implementations, this means that the driver has to be implemented using a special high voltage wafer fabrication process. The high voltage process is typically expensive and unique and often prevents inductor based DC/DC boost converters from being implemented along with other functions in power management ASICs. Furthermore, the higher cost, increased noise and larger PC board areas makes boost converter based implementations undesirable, especially in portable products.  
       [0009] As the preceding paragraphs describe, available LED drivers have known disadvantages and there is a need for drivers that provide greater efficiency. This need is particularly relevant to portable electronic systems where increased efficiency is directly related to increased battery life.  
       SUMMARY OF THE INVENTION  
       [0010] The present invention provides several topologies for driving white LEDs (and related devices) with high efficiency. One of the topologies combines a charge pump, a DC/DC converter and a current source. The charge pump is unregulated and, as a result, has a high efficiency. The efficiency of the DC/DC converter is also high and the combination yields an overall efficiency of potentially more than 92%. A second topology combines a voltage regulator and a current source. The voltage regulator is connected to monitor the forward voltage of a driven LED and uses the forward voltage as a reference to produce an adaptive regulated voltage. This allows the voltage regulator to react to changes in the LED forward voltage by setting the regulated voltage to the lowest appropriate level. This second topology may also be configured to disable the voltage regulator when battery voltage exceeds a predetermined level.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a block diagram of a prior art LED driver using a current source in series with a voltage regulator.  
     [0012]FIG. 2 is a block diagram of a prior art LED driver using a current setting resistor in series with a voltage regulator.  
     [0013]FIG. 3 is a block diagram of a prior art LED driver using a current source in series with a regulated charge pump.  
     [0014]FIG. 4A is a block diagram of a prior art LED driver using a current source in series with step up (Boost) converter.  
     [0015]FIG. 4B is a block diagram of a prior art LED driver using a current limiting resistor in series with step up (Boost) converter.  
     [0016]FIG. 5 is a block diagram of a LED driver using a current source in series with an unregulated charge pump followed by a step-down (Buck) DC-DC converter.  
     [0017]FIG. 6 is a block diagram of a LED driver that automatically adapts its regulated voltage output to reflect the forward current flowing through a driven LED.  
     [0018]FIG. 7 is a block diagram of the LED driver of FIG. 6 including circuitry to compensate for voltage overhead of an internal current source.  
     [0019]FIG. 8 is a block diagram of the LED driver of FIG. 7 including circuitry to disable an internal voltage regulator when an input battery voltage exceeds a predetermined level.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0020] The present invention provides several topologies for driving white LEDs (and related devices) with high efficiency. The first of the topologies is shown in FIG. 5 and generally designated  500 . As shown, topology  500  combines a charge pump  502 , a Buck DC/DC converter  504  and a current source  506 . These three components are placed in series between a power source, LED and ground. Charge pump  502  boosts the voltage available from the power source at the expense of introducing a degree of voltage fluctuation at the output of charge pump  502 . Buck DC/DC converter  504  reduces the fluctuations to create a regulated voltage to supply current source  506 . Current source  506  creates the forward current required to drive the LED. In general, the use of a Buck converter results in lower peak currents than a Boost converter for equivalent output currents. Topology  500  capitalizes on this by using the combination of charge pump  502  followed by followed by Buck DC/DC converter  504 . The overall result is a topology that generates less noise than would be produced by a high voltage step up (boost) converter.  
     [0021] Unlike the charge pump of FIG. 3, charge pump  502  is unregulated, and, as a result, has an efficiency that can be as high as: η=95%. The efficiency of DC/DC converter  504  can be even higher at: η=97%. Combined with the efficiency of current source  506  (η=1 m) yields an overall efficiency of  
       η   =       0.95   *   0.97     m                   
 
     [0022] (or potentially more than 92%) for topology  500 .  
     [0023] Importantly, it is generally practical to combine both charge pump  502 , and DC/DC converter  504  in the same package or even in the same silicon substrate. This makes topology  500  an appropriate choice for monolithic implementations in high efficiency portable electronic devices. Monolithic implementation is especially attractive in cases where DC/DC converter operates at a relatively high switching frequency allowing charge pump  502  and DC/DC converter  504  to be implemented using a single (and relatively small) inductor.  
     [0024] A second topology for driving white LEDs (and related devices) is shown in FIG. 6 and generally designated  600 . Topology  600  is based on the observation that forward voltage of an LED increases as a function of forward current. As a result, the voltage used to drive an LED may be decreased (and power saved) whenever the LED is operating at less than its maximum current.  
     [0025] As shown in FIG. 6, topology  600  includes a voltage regulator  602  and a current source  604 . As described for other topologies, these components are connected in series between a battery, LED and ground. The voltage regulator  602  is connected to monitor the forward voltage of the LED (V D ) and uses the forward voltage as a reference to produce an adaptive regulated voltage (V REG ). This means that voltage regulator  602  reacts to changes in V D  by setting V REG  at the lowest appropriate level.  
     [0026] Within topology  600 , current source  604  is used to drive the LED current. While doing this, current source  604  has an associated voltage overhead. The voltage overhead must be accounted for by voltage regulator  602 . FIG. 7 shows a topology  700  that accomplishes this objective. As shown in FIG. 7, voltage regulator  702  includes a linear regulator  706  and a charge pump  708 . Linear regulator  706  further includes a comparator driving a MOSFET. Other suitable components and topologies may also be used to implement voltage regulator  702 .  
     [0027] Voltage regulator  702  also includes two resistors labeled R 1 and R   2 . These two resistors form a voltage divider that multiples the regulated output of voltage regulator  702  (V REG ) by a predetermined percentage. The voltage divider output (in this case, eighty percent of V REG ) is used as the feedback voltage for voltage regulator  702 . Multiplication of V REG  to form the feedback voltage works because the voltage overhead of current source  704  (like the forward voltage of the driven LED) increases as a function of the forward current. As a result, the LED forward voltage (V D ) can be calculated as a percentage of the regulated output of voltage regulator  702  (V REG ). For example, for the case shown in FIG. 7 (i.e., where the regulator feedback voltage is eighty percent of V reg ), a forward diode voltage (V D ) of 3.8 volts corresponds to a regulated voltage (V reg ) of 4.75 volts.  
     [0028] The batteries used to power portable electronic systems typically operate over a voltage range, starting from an initial high voltage and decreasing over time. For Lithium Ion battery cells, this range typically starts at 4.2 Volts and decrease to approximately 2.8 Volts. The forward voltage required to drive an LED (typically 3.5 Volts) falls almost in the middle of that range. This implies that there is a voltage range where the output of a Lithium Ion battery is sufficient to drive an LED without any form of voltage regulation. For example, if a typical forward LED voltage is 3.5 volts, and the voltage overhead required by the LED&#39;s current source is 250 mV, then any battery voltage greater than 3.75 volts can drive the LED without voltage regulation. The same no-regulation-range, with different boundaries, may also exist for other battery chemistries.  
     [0029]FIG. 8 shows a driver topology  800  that is optimized to distinguish between high battery voltages (where regulation is not required) and low battery voltages (where regulation is required). As shown in FIG. 8, topology  800  adds a load switch  810  and a comparator  812  to the components already described for topology  700 . Load switch  810  is positioned in parallel with linear regulator  806  and fractional charge pump  808 . The output of comparator  812  alternately enables either load switch  810  or the combination of linear regulator  806  and fractional charge pump  808 . The inputs to comparator  812  are the LED forward voltage (V D ) and the difference between the battery voltage V BAT  and an offset voltage V os , where V os  is the overhead required by current source  804 .  
     [0030] During operation, comparator  812  enables load switch  810  and disables the combination of linear regulator  806  and fractional charge pump  808  whenever battery voltage (V BAT ) minus offset voltage (V os ) exceeds the LED forward voltage (V D ). This means that when battery voltage (V BAT ) is high (typically when V BAT  exceeds 3.75 volts) topology  800  operates without voltage regulation (load switch mode). As battery voltage (V BAT ) decreases, comparator  812  enables the combination of linear regulator  806  and fractional charge pump  808  and disables load switch  810 . This means that when battery voltage (V BAT ) is low (typically when V BAT  is less than 3.75 volts) topology  800  operates with voltage regulation (voltage regulation mode). Importantly, using the LED voltage to decide which mode (load switch mode or voltage regulation mode) allows the same circuit to drive LED&#39;s with arbitrary forward voltages.  
     [0031] The efficiency of topology  800  is described by analyzing operation in two modes: load switch mode and voltage regulation mode. As previously described, the efficiency of topology  800  during operation in voltage regulation mode is defined as:  
             η   =       P   out       P     i                 n                     =         V   REG     *     I   OUT           V   BAT     *     I   OUT     *   cp                   =       V   REG       1.5   *     V   BAT                             
 
     [0032] The efficiency of topology  800  during operation in load switch mode is a combination of the efficiencies of current source  804  and load switch  810 . The efficiency of current source  804  is defined as:  
             η   =       P   out       P     i                 n                     =         V   D     *     I   LED           V   REG     *     I   LED                     =       V   D       V   REG                           
 
     [0033] and the efficiency of load switch  810  is defined as:  
             η   =       P   out       P     i                 n                     =         V   REG     *     I   LED           V   BAT     *     I   LED                     =       V   REG       V   BAT                           
 
     [0034] This yields a total efficiency for load switch mode of:  
       η   =         V   D       V   BAT       .                   
 
     [0035] The following table shows how the overall efficiency of topology  800  changes as a Lithium Ion battery is discharged: 
 
 
     [0036] 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.