Patent Publication Number: US-7903058-B1

Title: Forward LED voltage monitoring for optimizing energy efficient operation of an LED driver circuit

Description:
FIELD OF THE INVENTION 
     The present invention relates to driver circuits and more specifically to an apparatus and method for monitoring forward LED voltages to enable optimal energy efficiency in LED driver circuits. 
     BACKGROUND 
     Portable battery power devices are increasingly common in modern life, e.g., mobile telephones, MP3 players, personal digital assistants (PDAs), notebook computers, DVD players, CD players, radios, televisions, and the like. However, the relatively short span of time before a fully charged battery becomes discharged and needs to be either recharged or replaced is a common problem in the operation of most battery powered devices To extend the “lifetime” of a battery&#39;s effective use, energy efficient circuitry is often included in battery powered devices. 
     Many battery powered devices include displays that provide information regarding the operation of the devices. Often, these displays are backlit to enable their use in low light environments. However, since backlit displays can consume a relatively large percentage of the available energy in the battery, relatively efficient LED driver solutions are preferred. 
     In the past, current regulated switch capacitor LED drivers have been employed to achieve a relatively optimized energy efficiency by switching between different gains. These different gains are switched to follow the voltage drop across the battery as it discharges. For example, when the battery is fully charged a gain of 1 is often selected because the battery&#39;s voltage is high enough to efficiently drive the LEDs. However, as the battery discharges and its voltage drops, a gain greater than 1 is then selected (1.5X for example) to boost the output of the LED driver above the battery&#39;s voltage. Previously, it has been difficult to determine the optimal point for this gain transition. Thus, it is with respect to these considerations and others that the present invention has been made. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. 
       For a better understanding of the present invention, reference will be made to the following Detailed Description of the Invention, which is to be read in association with the accompanying drawings, wherein: 
         FIG. 1  illustrates the change in efficiency in regard to the input Voltage (V IN ) and different gains provided by the LED driver circuit; 
         FIG. 2  schematically illustrates an exemplary high side LED driver; 
         FIG. 3  schematically illustrates a high side LED driver; 
         FIG. 4  schematically illustrates a MOS maximum selector circuit implementation; 
         FIG. 5  schematically illustrates a bipolar maximum selector implementation; 
         FIG. 6A  schematically illustrates a headroom replica implementation; 
         FIG. 6B  schematically illustrates simplified model of the switch array of the charge pump in unity gain; 
         FIG. 6C  schematically illustrates a device level gain select implementation for a high side driver circuit; 
         FIG. 7  schematically illustrates a low side driver system; 
         FIG. 8  schematically illustrates a device level gain select implementation for a low side driver circuit; and 
         FIG. 9  shows a flow chart for a process in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is described fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the present invention may be embodied as methods or devices. Accordingly, the present invention may take the form of an entirely hardware embodiment or an embodiment combining software and hardware aspects. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Briefly stated, the present invention is directed to an apparatus and method for monitoring the forward voltage for a plurality of LEDs in a battery powered device so that the gain in the LED driver circuit can be switched at a point that optimizes the energy provided by the battery to illuminate the LEDs. The invention provides for sensing each LED&#39;s voltage, V LED , and determining the maximum forward voltage, V LEDmax , between the plurality of LEDs. The invention uses the knowledge of V LEDmax  in conjunction with V IN , converter output resistance and LED current, and current source/sink minimum headroom to switch from an initial gain to some final gain (higher than initial gain) just before the current sinks/sources would drop out. Similarly, the invention provides for switching from the higher gain back to the initial gain in the event that the battery voltage rises back to its initial voltage after being momentarily pulled down by a heavy load or other factors. 
     In part, because of differences in the gain selection circuitry when implemented for a high side LED driver versus a low side LED driver, the invention provides for two complementary embodiments that enable gain selection on the high side and low side. The conditions for gain switching that account for the above mentioned factors are developed in the implementation section below. 
     As discussed above, after discharging for some period of time, the battery&#39;s voltage drops. For example, the cell voltage of a lithium ion battery (typical battery for mobile phones) usually ranges from 4.2 Volts at full charge down to 2.5 Volts at deep discharge. Since this lower voltage is often less than the desired power supply voltage for a battery powered device, a boost converter is employed to extend the period of time that the battery can be a useful energy source for the device. 
       FIG. 1  illustrates the change in efficiency in regard to the input Voltage (V IN ) and different gains provided by the LED driver circuit. As shown, a gain of 1.5X is provided for a relatively low V IN  and a gain of unity (one) for higher V IN . 
     Although the invention can be implemented with both high side and low side LED drivers,  FIG. 2  illustrates an exemplary high side LED driver. 
     Additionally, as shown in  FIG. 3 , in an ideal case, the Gain G is unity and so V OUT =V IN . In a non-ideal case, V OUT =V IN −R OUT * I OUT , with R OUT  defined as the charge pump open loop output resistance and I OUT  the output current as shown in  FIG. 3 . Also, I OUT =N.I LED  with N equivalent to the number of LEDs (N=4 in  FIG. 2 ). 
     The efficiency of the LED driver depends on the gain that the LED driver is operating in: 
                     n   ≡       P   OUT       P   IN         =           NV   LED     *     I   LED           V   IN     *     I   IN         ≅         NV   LED     *     I   LED           V   IN     *     (       G   *     I   OUT       +     I   Q       )                   Equation   ⁢           ⁢   1               
The quiescent current I Q  is negligible for moderate to high output current applications. This equation highlights that the efficiency is optimal when the LED driver operates in the lowest possible gain that can still provide a V OUT  voltage to enable the operation of the battery powered electronic device.
 
     For the case where the LED driver is operating with a gain of one, the following second equation applies: 
                     I   OUT     =         I   IN     ⁢           ⁢   and   ⁢           ⁢   η     ≅       V   LED       V   IN                 Equation   ⁢           ⁢   2               
For the case where the LED driver is operating with a 1.5 times gain (1.5X boost mode), the following third equation applies:
 
     
       
         
           
             
               
                 
                   
                     I 
                     IN 
                   
                   = 
                   
                     
                       1.5 
                       * 
                       
                         I 
                         OUT 
                       
                       ⁢ 
                       
                           
                       
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                         V 
                         LED 
                       
                       
                         1.5 
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                           V 
                           IN 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
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                   3 
                 
               
             
           
         
       
     
     The LED current sources that are connected between V OUT  and the diodes need enough headroom, V HR , across them to provide the desired current in the LED. The headroom is the voltage across each current source V HR =V OUT −V LED . The current sources require a sufficient amount of headroom voltage to be present across them in order to regulate properly. The minimum headroom voltage V HRmin  is proportional to the current flowing through the current source as described by the equation: V HRmin =R HR *I LED , then R HR  represents the ON resistance of the current source. 
     For LED drivers, the optimal efficiency is achieved by switching gains based on the value of V IN  and the forward LED voltage V LED  as Equations 2 and 3 highlight it. This optimal efficiency can be achieved by enabling its DC-DC converter to stay in a gain of one (unity) over the largest input voltage range possible, while at the same time preventing the headroom of the current sources from dropping below V HRmin . The input voltage at which the converter switches gains depends on the forward voltage of the LEDs that are being driven. 
     The invention provides for initially sensing each LED voltage, V LED , and determining the maximum forward voltage, V LEDmax , between several LEDs. The invention employs the knowledge of V LEDmax  in conjunction with what is known about V IN , converter output resistance and LED current, and current source/sink minimum headroom to switch from an initial gain to some higher gain just before the current sinks/sources drop out or from a higher gain to a lower gain in the event of the battery voltage going back to its initial value after being momentarily pulled down by a heavy load. 
     The conditions for gain switching that account for the above mentioned factors are developed in the implementation section below. Because of the uniqueness of the gain selection circuitry when implemented for a high side LED drive solution versus a low side LED drive solution, there are two subsections in the implementation. The first subsection describes the circuitry used for gain selection in a high side drive system and the second describes circuitry used for gain selection in a low side drive system. 
     Implementation of Gain Selectors for High Side and Low side Drivers 
     (1) HIGH SIDE SELECTOR AND FORWARD VOLTAGE MONITOR CIRCUITRY 
     The forward voltage of different LEDs driven by the same amount of current may vary considerably. As shown in  FIG. 4 , a circuit based on source followers in parallel can be used to determine the maximum forward LED voltage of several LED (V LEDmax ). 
     The LED with the largest forward voltage forces a larger gate voltage onto the NMOS which is sampling the LED voltage. The NMOS with the maximum gate drive tends to take most of the current i 1  and the voltage at its gate appears at its source minus its Vgs as illustrated in the fourth equation:
 
 V   A   =V   LEDmax −Vgs  Equation 4
 
     The maximum LED forward voltage is hence measured. Although this example is based on 4 LEDS, the invention can be implemented for any greater or lesser number of LEDs. 
     As shown in  FIG. 4 , if VF 1 =VF 2 =VF 3 =VF 4 , each NMOS will have the same amount of current: i 1 /4. The Vgs of all of the NMOS will be less than in the case when one is dominating the others as illustrated by the fifth equation as follows: 
     
       
         
           
             
               
                 
                   Vgs 
                   = 
                   
                     Vt 
                     + 
                     
                       
                         
                           Id 
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                             .2 
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                   Equation 
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                   ⁢ 
                   5 
                 
               
             
           
         
       
     
     Additionally, NPN transistors can be used instead of MOS transistors to reduce the impact of current density differences. In the case of a bipolar transistor, equation 4 is modified as follows: V A =V LEDmax −Vbe Equation 4b 
     One embodiment, of a final high side maximum forward LED voltage selector circuit is shown in  FIG. 5 . 
     Gain Transitions 
     To improve efficiency, a gain of unity (one) is selected instead of 3/2 if V IN  is high enough. For example, if Vin is greater than the required headroom across the current sources, the drop across the charge pump and the maximum LED voltage, then Vin can be passed to Vout. In one embodiment, to avoid gain chattering, a hysteresis voltage V hys  is added when switching gain from 3/2 to 1. 
     The equations for further implementing the invention are as follows:
 
 G= 1 if Vin&gt; V   hr   +V   LEDmax +Rout 1X * Iout+ V   hys   Equation 6
 
 G= 3/2 if Vin&lt; V   hr   +V   LEDmax +Rout 1X * Iout  Equation 7
 
Equation 6 can be reworked as:
 
 G= 1 if  V   LED max   &lt;V   IN −V HR −Rout 1X   * I   OUT   −V   hys   Equation 8
 
In this case, the gain selection is based on a comparison between a headroom replica circuit that models the right part of the equation 8 and V LEDmax . The headroom replica circuit is yet another exemplary embodiment of the maximum selector circuit. Also, since the determination of VLEDmax introduces Vbe (or Vgs)—see equation 4b above—the headroom replica circuit also uses a source follower:
 
 G= 1 if  V   LEDmax −Vbe&gt; V   IN   −V   HR −Rout 1X   * I   OUT   −V   hys −Vbe  Equation 9
 
An exemplary schematic that implements equation 9 is shown in  FIG. 6 . The left part of equation 9 is connected to the negative input of the comparator, the right part of the equation to the positive input of the comparator.
 
     As shown in  FIG. 6A , the current it can be either fixed and representing the worst case or proportional to the current in the LED to achieve better efficiency at lower LED currents. Efforts are made to have the replica impedances track the real Rout ix , R Vhr  and R Vhys  over temperature and supply. However, since no replica can be perfect, additional error correction can be built into each small replica block. 
     Switching too early into a gain of 1 due to a too small headroom replica could be a real issue for the LED: the charge pump would not be able to deliver the right current lout and the LED light would be dimmed. Also, changes in headroom voltage from one current source to the next, which is possible with LED forward voltage mismatch can result in different output currents and LED brightness mismatch. However, if too much error compensation is added, it can negatively impact efficiency by causing switching between gains to occur too late. 
     To model the charge pump output impedance in gain of 1, Rout 1X , a small PMOS M 7  is chosen in  FIG. 6C  and the ratio of M 7  to the switch array PMOS M 6  and M 8  of  FIG. 6B  is substantially the same as the ratio chosen for Ir to the total output current N*I LED . 
     For this case, the ratio selected is 1:1000 as shown below: 
     
       
         
           
             ir 
             ≈ 
             
               
                 
                   N 
                   * 
                   
                     i 
                     LED 
                   
                 
                 1000 
               
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                   M 
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                   M 
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                   6 
                 
               
               1000 
             
           
         
       
     
       FIG. 6B  illustrates one embodiment of an exemplary headroom replica circuit. 
     Referring to  FIG. 6C , the device level gain select implementation in High Side LED driver system is presented. The first block called Rout 1X  replica fed by it generates a voltage across a fully driven PMOS M 7  that replicates the voltage drop in one possible implementation of a gain of 1 which is a PMOS pass transistor from V IN  to V OUT . The headroom replica is in this example made of a PFET M 9  and a resistor R that replicates the current sources regulating current in the LEDs. That sub block generates a voltage that will track the dropout voltage of a current source driver made of a PMOS mirror with source degeneration. Devices Q 5  and Q 6  are used as balancing level shifters: they match the level shift generated by the minimum selector made of Q 1 -Q 4 . The hysteresis generated by Rhys and i 1  is a fixed voltage that is added or taken out by PMOS switch M 11 . A voltage mode comparison is hence used for the high side gain selector. 
     (2) LOW SIDE SELECTOR AND FORWARD VOLTAGE MONITOR CIRCUITRY 
       FIG. 7  shows an exemplary LED driver utilizing low side current sinks. The LED forward voltage sensing and gain selecting circuitry described in this section are designed for application in the example system shown in  FIG. 7  that has four LEDs. However, this circuitry can be modified to operate correctly in an “N” LED driver topology, and the charge pump configuration need not be identical to that shown in  FIG. 7  for the gain selection circuit to be useful. 
       FIG. 7  provides the context in which the implemented gain select circuitry operates. Each of the LED anodes are connected to the output of the charge pump, Vo, that is regulated to a value of V REG  if the charge pump has sufficient headroom. The inputs to the gain select circuit are the input voltage Vi, the LED supply voltage Vo, and the voltage across each of the current sinks V HR . The gain select circuit uses each of these inputs to select the gain transition voltage for optimum efficiency. 
       FIG. 8  shows a device level implementation of the gain select circuit used in a low side LED driver system. This circuit is described by partitioning functionality into four separate subcircuits that are shown and labeled in  FIG. 8 . The gain selection criteria for the low side driver is as follows: 
     The charge pump should switch from gain of 1 to 3/2 as V in  falls below:
 
 V   in     1x→3/2X     =V   LEDmax +NI LED   R   O   +V   HRmin   (Equation 10)
 
     The charge pump should switch from gain of 3/2 to 1 as V in  rises above:
 
 V   in     3/2X→1X     =V   LEDmax +NI LED   R   0   +V   HRmin   +M   hyst   (Equation 11)
 
Where V hyst  is a hysteresis voltage needed so that gain chattering does not occur.
 
     While a voltage mode type comparison can be used for the high side gain selector because a level shifted V LEDmax  (relative to ground) can be directly measured using the maximum voltage selector circuit shown in  FIGS. 4 and 5 , a current mode approach is employed for the low side gain selector to extract the V LEDmax  information from the set of LEDs. 
     Referring to  FIG. 8 , the voltage present across each of the current sinks (shown in  FIG. 7 ) is routed into the minimum voltage selector circuit composed of Q 1 -Q 4  and a current source. The signal at the gate of M 6  depends on the voltages V LEDmax  and V O . By buffering and level shifting the output of the minimum selector circuit to the bottom side of a resistor connected to V O , a current I 1  is generated that will vary only with V LEDmax , Vbe, and Vsg. Therefore V O -V LEDmax +Vbe is measured using the minimum selector circuit and the voltage V O -(V O -V LEDmax +Vbe+Vsg)=V LEDmax −Vbe-Vsg is placed across a resistor with the value R. 
     The right most subcircuit in  FIG. 8  is designed to generate a minimum headroom voltage reference that replicates the current sinks regulating current in the LEDs. In this example, R degen , M 12 , and CS 2  are used to generate a voltage that will track the drop out voltage of a current sink driver made of an NMOS mirror with source degeneration. Note that CS 2  sources a current that is proportional to I LED , so that the reference headroom tracks the needed headroom of the actual current sink. Devices Q 5  and Q 6  are used as balancing level shifters (they match the level shift generated by the minimum selector made of Q 1 -Q 4 ), and the resistor R hyst  in conjunction with current source Ib generate a hysteresis voltage that is added in or taken out by NMOS switch M 11 . Device M 7  level shifts the headroom reference and hysteresis voltage to the bottom side of a resistor connected to V in , placing a voltage equal to V in -(V Href +V hyst +Vsg+Vbe) across a resistor with a value R. 
     The left most circuit composed of M 1 -M 5 , R, and CS 1  is used to generate and route a current that is proportional to the voltage across a fully driven PMOS device that replicates the voltage drop in one possible implementation of a gain of one, which is a PMOS pass transistor from V in  to V O . Again, the current sink CS 1  pulls a current that is proportional to the current being supplied by the charge pump so that the gain select circuit optimizes efficiency over LED current as well as input voltage. Care is taken to size M 2  and M 3  so that A is equal to the current density ratio between the devices. 
     Finally, the current mode comparator resolves the comparison between currents I 1  and I 2 . 
     If I 2 &gt;I 1  then G=1 (high state=gain of 1 selected) or: 
                         V   in     -     (       V   HRref     +     V   hyst     +     V   SG     +     V   BE       )     -         V     DSM   ⁢           ⁢   1       R     ⁢   R       R     &gt;         V   O     -     (       V   O     -     V   LEDmax     +     V   SG     +     V   BE       )       R             (     Equation   ⁢           ⁢   12     )               
or after appropriate regrouping and cancellation of terms:
 
 G= 1 when:  V   in   &gt;V   LEDmax   +V   HRref   +V   hyst   +V   DSM1   (Equation 13)
 
And,
 
 G= 0 when:  V   in   &lt;V   LEDmax   +V   HRref   +V   DSM1   (Equation 14)
 
     The exemplary circuit as shown in  FIG. 8  implements the desired gain selection criteria for the low side LED driver. 
       FIG. 9  illustrates a block diagram of a method for determining a switching point of the gain of circuit to drive a plurality of LEDs. Moving from a start block, the process performs actions at four blocks in parallel, i.e., block  902 , block  904 , block  906 , and block  908 . At block  902 , the output impedance of a charge pump is determined. Also, at block  904 , the min headroom for each current source and each current sink is determined. Further, at block  906 , a modeled determination of the load current to drive each of the LEDs is performed. Additionally, at block  908 , the forward maximum voltage for each of the LEDs is determined. 
     Next, the process advances to block  910  where these determined values and the input voltage are employed to switch from an initial gain to a final gain to drive the plurality of LEDS. The determined switching point enables the plurality of LEDs to be driven in an optimally energy efficient manner. Further, the process steps to a return block and returns to performing other actions. Additionally, although the actions of blocks  902 ,  904 ,  906 , and  908  are shown performed in parallel, it is understood that in other embodiments, some or all of these actions could be performed serially without departing from the spirit and scope of the invention. 
     The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.