Abstract:
A circuit for driving light emitting elements, such as LEDs, includes a first transistor having a source coupled to ground through a first resistive element, and a second transistor having a gate electrically coupled to a gate of the first transistor, a source electrically coupled to ground, and a drain for electrical connection to a first group of light emitting elements. The circuit also includes circuitry to provide a predetermined voltage at the source of the first transistor, circuitry to compensate for a difference in respective gate-source voltages of the first and second transistors, and circuitry to compensate for a difference in respective drain-source voltages of the first and second transistors. In some implementations, the circuit can achieve relatively low power consumption.

Description:
BACKGROUND 
       [0001]    This disclosure relates to circuits for driving light emitting elements such as light emitting diodes (LEDs). 
         [0002]    LEDs are current-driven devices whose brightness is proportional to their forward current. Forward current can be controlled in various ways. For example, one technique is to use the LED current-voltage (I-V) curve to determine what voltage needs to be applied to the LED to generate a desired forward current. Another technique of regulating LED current is to drive the LED with a constant-current source. The constant-current source can help eliminate changes in current due to variations in forward voltage, which results in constant LED brightness. In this technique, rather than regulating the output voltage, the input power supply regulates the voltage across a current-sense resistor. The power supply reference voltage and the value of the current-sense resistor determine the LED current. 
         [0003]    One issue that arises in some LED driver circuits is high power consumption. 
       SUMMARY 
       [0004]    The subject matter described in this disclosure relates to LED driver circuits, which in some implementations, can help reduce power consumption. 
         [0005]    For example, in one aspect, a circuit for driving light emitting elements includes a first transistor having a source coupled to ground through a first resistive element, and a second transistor having a gate electrically coupled to a gate of the first transistor, a source electrically coupled to ground, and a drain for electrical connection to a first group of light emitting elements. The circuit also includes circuitry to provide a predetermined voltage at the source of the first transistor, circuitry to compensate for a difference in respective gate-source voltages of the first and second transistors, and circuitry to compensate for a difference in respective drain-source voltages of the first and second transistors. 
         [0006]    In a second aspect, a circuit for driving a string of light emitting diodes includes a first transistor having a gate, a source coupled to ground through a first resistive element, and a drain. Circuitry is included to provide a voltage having a predetermined value to the source of the first transistor. A second transistor has a gate, a source electrically coupled to ground, and a drain for electrical connection to the string of light emitting diodes. A second resistive element has a first end coupled electrically to a gate of the first transistor and a second end coupled electrically to the gate of the second transistor. A first current source is coupled electrically between the second end of the second resistive element and ground. A third resistive element has one end coupled electrically to the drain of the first transistor and a second end coupled electrically to the drain of the second transistor. 
         [0007]    Various apparatus that can include the driving circuits, as well as methods of operation, are described below. 
         [0008]    Some implementations include one or more of the following advantages. For example, as noted above, in some implementations, the circuits can achieve relatively low power consumption. The second transistor generates a relatively controlled and stable drive current that, in some implementations, varies little, if at all, with changes in the voltage of the LED string. 
         [0009]    Other aspects, features and advantages will be readily apparent from the following detailed description, the accompanying drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a simplified diagram illustrating driving of multiple LED strings. 
           [0011]      FIG. 2  illustrates details of an example circuit for driving a single LED string. 
           [0012]      FIG. 3  is a flow chart of a method of operation of the driving circuit. 
           [0013]      FIG. 4  illustrates details of an example circuit for driving multiple LED strings. 
           [0014]      FIG. 5  illustrates another example of a circuit for driving multiple LED strings. 
           [0015]      FIG. 6  illustrates an example of a LED driver circuit with protection circuitry. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    As illustrated in  FIG. 1 , outputs from a LED driver circuit  10  are coupled, respectively, to LED strings  11 . In the example of  FIG. 1 , there are six LED strings  11  connected in parallel, each of which includes ten LEDs  11 A connected in series. In some implementations, however, the driver circuit  10  may drive a different number of LED strings (e.g., eight or sixteen) and, in some cases, may drive only a single LED string. Furthermore, in some implementations, the number of LEDs in each string  11  may differ from ten. 
         [0017]    The number of LED strings, as well as the number of LEDs in each string, may depend on the particular type of device and application. For example, the LED driver technology described here can be used, for example, in backlighting and solid-state lighting applications. Examples of such applications include LCD TVs, PC monitors, specialty panels (e.g., in industrial, military, medical, or avionics applications) and general illumination for commercial, residential, industrial and government applications. The LED driver technology described here can be used in other applications as well, including backlighting for various handheld devices. The driver circuit  10  can be implemented as an integrated circuit fabricated, for example, on a silicon or other semiconductor substrate. 
         [0018]    As illustrated in  FIG. 1 , the driver circuit  10  includes connections to a power supply voltage (VCC) and to ground. The LED strings  11  are coupled to a LED power supply voltage (V POWER-LED ). 
         [0019]    As illustrated in the example of  FIG. 2 , the driver circuit  10  includes several current sources  12 ,  14 ,  16 , an operational amplifier  18 , resistors R 1 , R 2 , R 3 , R 4 , and transistors M 1 , M 2 . The reference current source  12  generates a current that flows through resistor R 1 . This current flow generates a reference voltage V 1  at the non-inverting input (in+) of the operational amplifier  18 . Substantially the same voltage (V 1 ) appears at the inverting input (in−) of the operational amplifier  18 , and this voltage appears across the resistor R 2 , which is coupled between the source of the transistor M 1  and ground. Thus, the operational amplifier  18  regulates the voltage appearing at the source of transistor M 1  by maintaining the voltage at the inverting input (in−) at the same level as the voltage appearing at the non-inverting input (in+). 
         [0020]    As further shown in  FIG. 2 , the output of the operational amplifier  18  is coupled to the gate of transistor M 1  and (through resistor R 3 ) to the gate of transistor M 2 . The transistors M 1 , M 2  can be implemented, for example, as MOS transistors. In the illustrated example, the size (i.e., area) of transistor M 2 , which provides the current for an LED string coupled to the drain of transistor M 2 , is X times larger than the size of transistor M 1 . The value of X can vary over a wide range depending on the particular circuit design. In some implementations, the ratio of the sizes of the transistors M 1 :M 2  is on the order of about 1:1000. The relative sizes of the transistors M 1 , M 2  can be used to generate a larger current for the LED string. For example, if the gate-source voltages (Vgs) of the transistors M 1 , M 2  were substantially the same, then transistor M 2  would provide a controllable, substantially stable current that is about X times as large as the current through transistor M 1 . However, in actual implementations, the gate-source voltages on the transistors differ from one another due to the fact that the source of the transistor M 2  is connected directly to ground, whereas the source of the transistor M 1  is connected to ground through resistor R 2 . Without additional circuit components such as those described below (e.g., resistors R 3 , R 4  and current source  16 ), the current generated by transistor M 2  will typically depend on the voltage of the LED string because of the difference in the gate-source voltages. Thus, in the absence of the additional circuit components (e.g., resistors R 3 , R 4  and current source  16 ), the current generated by transistor M 2  for the LED string will vary and, thus, is not well-controlled or stable. 
         [0021]    To help ensure that the current generated by second transistor M 2  remains at the desired level, additional circuit components (e.g., resistors R 3 , R 4  and current source  16 ) are provided to compensate for differences in the gate-source voltages of the transistors M 1 , M 2  and to compensate for differences in their drain-source voltages. 
         [0022]    To compensate for the difference in the gate-source voltages of the transistors M 1 , M 2 , resistor R 3  is coupled between the gates of the transistors M 1 , M 2 . In addition, a current source  16  is coupled between the gate of transistor M 2  and ground. The values of the resistor R 3  and the current source  16  should be selected such that the voltage V 1  across resistor R 2  is substantially equal to the value of the resistor R 3  multiplied by the current I 3  generated by the current source  16  (i.e., V 1 =I 3 ×R 3 ). The voltage generated by the current I 3  (from source  16 ) flowing through resistor R 3  compensates for the difference in gate-source voltages of the transistors M 1 , M 2 . Furthermore, to compensate for the difference in drain-source voltages (Vds) of the transistors M 1 , M 2 , resistor R 4  is coupled between the respective drains of the transistors. 
         [0023]    As indicated by  FIG. 3 , in operation, the circuit  10  provides a predetermined voltage at the source of the first transistor M 1  ( 102 ), compensates for a difference in respective gate-source voltages of the first and second transistors M 1 , M 2  ( 104 ), and compensates for a difference in respective drain-source voltages of the first and second transistors M 1 , M 2  ( 106 ). 
         [0024]    As an illustrative example, it is assumed that the values of resistors R 2 , R 3  and R 4  are the same. In that case, half the current from the current source  14  flows through transistor M 1  and resistor R 2 , and the same amount of current flows through resistor R 4 . Thus, in this example, a current I 2 /2 flows through transistor M 1  (and resistor R 2 ). Likewise, when the voltage of the LED string is lower than the power supply voltage (VCC), a current I 2 /2 also flows through resistor R 4  to compensate for the difference in drain-source voltages between the transistors M 1  and M 2 . 
         [0025]    Continuing with the foregoing example, the voltage V 1  at the source of the transistor M 1  is equal to the product of the resistance R 2  and the current flowing through that resistor (i.e., V 1 =I 2 /2×R 2 ). The voltage V 1  also is equal to the product of the current from current source  12  and the resistance R 1  (i.e., V 1 =I 1 ×R 1 ). Values of the current sources  12 ,  14  and the resistors R 1 , R 2  can be selected using the foregoing information. 
         [0026]    As explained above, the values of the resistor R 3  and the third current source  16  are selected such that V 1 =I 3 ×R 3 . Using the foregoing example in which R 3 =R 2 , the value of the current source would be set equal to I 2 /2 so as to compensate for the difference in gate-source voltages of the transistors M 1  and M 2 . 
         [0027]    In some implementations, the values of the resistors and current sources may differ from the foregoing example. 
         [0028]    By using the driver circuit  10  of  FIG. 2 , the current generated by transistor M 2  can be substantially independent of the voltage of the LED string. The circuit  10  can, therefore, provide a more controllable drive current. 
         [0029]    The extent of power savings that can be achieved in some implementations can be appreciated by considering a driver circuit without transistor M 2 , current sources  14 ,  16  and resistors R 3 , R 4 , but with the drain of transistor M 1  coupled to the LED string. If V 1  were 250 mV and the current required of transistor M 1  were 60 mA, the power consumption would be on the order of 0.015 Watts. If there are eight LED strings in the device, power consumption would be on the order of 0.12 Watts. The requirement of a voltage and current on resistor R 2  results in significant waste or loss of power. In contrast, the driver circuit  10  of  FIG. 2  can achieve a significant reduction in power consumption, for example, on the order of 99% in some implementations. 
         [0030]    Furthermore, the drive circuit  10  of  FIG. 2  can result in a significant reduction in the amount of die area. For a driver circuit without transistor M 2 , current sources  14 ,  16  and resistors R 3 , R 4 , but with the drain of transistor M 1  coupled to the LED string, the ratio of R 1 :R 2  may need to be on the order of 1,000 for some implementations, which can require a large die area for resistor R 2 . In contrast, the driver circuit of  FIG. 2  does not require such a high ratio of resistor values and, therefore, can significantly reduce the amount of die area required (e.g., by as much as about 20% for some implementations). 
         [0031]      FIGS. 4 and 5  illustrate examples of circuits for driving multiple LED strings. If the PWM control of the respective LED strings is to be substantially the same (e.g., same phase and frequency), the circuit  20  of  FIG. 4  can be used. Circuit  20  is similar to circuit  10  of  FIG. 2  except that an additional transistor M 3  is provided to generate the current for the second LED string. As illustrated in the example of  FIG. 4 , the gate of transistor M 3  is coupled to the gate of transistor M 2 , which is coupled to the gate of transistor M 1  through resistor R 3  as described above. The drain of transistor M 2  is coupled to the first LED string, whereas the drain of transistor M 3  is coupled to the second LED string. The source of transistor M 3 , like the source of transistor M 2  is coupled directly to ground. In this example, the size of transistor M 3  can be substantially the same as the size of transistor M 2 . 
         [0032]    On the other hand, if the PWM control of the respective LED strings is to differ from one another, then the circuit  30  of  FIG. 5  can be used. Different LED strings may require different currents, for example, if the strings contain different types of LEDs (e.g., the first string contains LEDs that emit light of a first color, and the second string contains LEDs that emit light of a second color, different from the first color). The circuit  30  of  FIG. 5  includes multiple copies (in this case two) of the circuit  10  of  FIG. 2 . Each circuit  10  is coupled to one of the LED strings. 
         [0033]    Although  FIGS. 4 and 5  illustrate only two LED strings, some implementations may include a greater number of LED strings. In that case, additional circuitry can be added as needed. For example, in  FIG. 4 , additional transistors similar to M 2  and M 3  can be provided to generate the current needed to drive the additional LED strings. Likewise, in  FIG. 5 , additional copies of the circuit  10  can be provided to generate the current needed to drive the additional LED strings. 
         [0034]      FIG. 6  illustrates a drive circuit  40  that is similar to the circuit of  FIG. 2 , but which also includes protection diode  42  or other circuit components to protect the current source  14  in the event that the voltage of the LED string becomes greater than the power supply voltage VCC. Instead of the diode  42 , other circuit components can be used, such as a clamp. Furthermore, the protection circuitry can be separate from the current source  14  or can be part of the current source  14 . 
         [0035]    Each resistive element R 1 , R 2 , R 3 , R 4  can be implemented, respectively, for example, as a single resistive component or as a combination of resistive components connected in series and/or in parallel. 
         [0036]    Other implementations are within the scope of the claims.