Abstract:
An LED driver includes an embedded non-volatile memory (NVM) capable of being programmed and storing control data for setting a variety of features of the LED driver, such as the maximum current for driving the LEDs, analog parameters such as the resistance of the internal resistor for setting the reference current for the LEDs, and the operation modes of the charge pump of the LED driver. This enables implementation of multiple LED driver product options without the need for different metallization steps during the fabrication process for the LED driver.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an LED (Light-Emitting Diode) driver, and more specifically to a programmable LED driver with an embedded non-volatile memory storing control data for custom programming of a variety of features of the LED driver. 
         [0003]    2. Description of the Related Arts 
         [0004]    White LEDs are being used increasingly in display devices. For example, some modern liquid crystal display (LCD) devices use white LEDs as the backlight for the LCD display. These LEDs are typically driven by an LED driver. White LED drivers are typically constant current devices where a constant sink current is fed through the white LEDs to provide a constant luminescence. The anode of the white LEDs is driven by a charge pump circuit. 
         [0005]      FIG. 1  illustrates a conventional LED driver  100  driving LEDs  112 ,  114 . For example, the LEDs  112 ,  114  can be white LEDs. The LED driver  100  includes 2 main circuit blocks, a charge pump  102  and a current regulator  110 . The charge pump  102  typically converts a battery voltage (V IN ) into an output voltage (V OUT ) coupled to the anodes of the LEDs  112 ,  114 . The output voltage (V OUT ) drives the LEDs  112 ,  114 . 
         [0006]    Current through the LEDs  112 ,  114  sets their intensity and associated luminescence. Thus, in order to obtain accurate intensity, which is very important for displays, the current through the LEDs  112 ,  114  must be set accurately. Typically, the current regulator  110  is responsible for driving the LEDs with constant current. The current regulator  110  includes, among other components, a bandgap voltage generator  104 , an error amplifier comprised of the amplifier  106  and the transistor  119 , a current mirror  108  comprised of transistors  116 ,  118 , and LED drive transistors  122 ,  124 ,  126 . 
         [0007]    The bandgap voltage generator  104  generates a bandgap voltage Vref, and the error amplifier ( 106 ,  119 ) ensures that the voltage at node  121  across the resistor R EXT    120  is set at Vref. Typically, the resistor R EXT    120  is external to the LED driver circuit  100 . The reference current I REF  through the external resistor R EXT    120  is set by the bandgap voltage Vref and the external resistor R EXT    120 . That is, the reference current I REF  is set by Vref/R EXT . The reference current I REF  is repeated through the transistor  122  by the current mirror  108 , and eventually drives the LEDs  112 ,  114  by the transistors  122 ,  124  and the transistors  122 ,  126 , respectively. The size (W/L ratio, or width/length ratio) of the transistors  124 ,  126  relative to the size of the transistor  122  determines how large the current I D1 , I D2  through the LEDs  112 ,  114  is relative to the reference current I REF  through the transistor  122 . Thus, the current I D1 , I D2  through the LEDs  112 ,  114  is also determined by the bandgap voltage Vref and the external resistor R EXT    120 . The resistance R EXT  of the external resistor  120  needs to be set accurately in order to control the luminescence of the LEDs  112 ,  114  precisely. In conventional LED drivers  100 , there is no convenient way to change the current through the LEDs  112 ,  114  without changing the resistance value of the resistor  120 . 
         [0008]    Typical LED drivers  100  may use an external resistor  120  to set the current in the LEDs  112 ,  114 . Such external resistor  120  adds a pin to the LED driver IC (integrated circuit), extra board space for the overall LED driver circuitry, and results in an increase in the Bill-of-Materials (BOM) cost for the overall LED driver circuitry. Note that different applications might require different maximum currents from the LED driver  100 . This is because different LEDs  112 ,  114  from different manufacturers may give different intensity for different current values. With a conventional LED driver  100 , the only way to control the reference current I REF  is to change the resistance value of the external resistor  120  so that the current through the LEDs  112 ,  114  change accordingly. The resistor  120  is typically external to the LED driver  100  in order to have its resistance value changed, which results in waste of a pin, board space, and cost, as explained above. 
         [0009]    The charge pump  102  typically operates in multiple operation modes. Initially at power up of the LED driver  100 , the input voltage V IN  is attached to the output voltage V OUT  via the charge pump  102  so that V IN  equals V OUT . This mode is often called the 1× mode. The charge pump  102  typically changes operation modes as time goes by and the battery voltage V IN  drops over time, because the LEDs  112 ,  114  typically have a voltage drop. The typical voltage drop V LED  in a white LED may be, for example, 3.4 V. 
         [0010]    As the input voltage V IN  decreases over the lifetime of the battery (not shown), the output voltage V OUT  decreases in the same proportion since V IN  equals V OUT  when the charge pump is in 1× mode. Thus, the voltage at nodes  115 ,  117  (the LED driver pins) is given by V OUT −V LED . When the voltage at nodes  115 ,  117  becomes too low, typically 200 mV, the current regulator  110  goes out of saturation and can no longer provide an accurate current through the LEDs  112 ,  114 . This causes the charge pump  102  to switch to a higher operation mode, typically a 1.5× mode that generates the output voltage V OUT  to be 1.5×V IN . As a result, the LED driver pin voltage at nodes  115 ,  117  rises high enough to push the current regulator  110  back into saturation. This process is repeated, and when the battery voltage V IN  further decreases to cause the current regulator  110  to go out of saturation even under 1.5× mode, the charge pump switches to 2× mode that generates the output voltage V OUT  to be 2×V IN . 
         [0011]    Although the charge pump  102  may automatically switch to different operation modes as explained above, some LED applications may need to set the operation mode of the charge pump  102  to a single operation mode or have only selected ones of multiple operation modes, even when the charge pump  102  itself has circuitry to operate in multiple operation modes. In order to set the operation mode of the charge pump  102  in a conventional LED driver  100 , fixed circuitry has to be used in the charge pump  102  to permanently set the operation mode, which essentially requires manufacturing different LED driver integrated circuits using different metallization processes during the fabrication process of the LED driver IC. 
         [0012]    Therefore, there is a need for a more convenient technique to change the maximum current through the LEDs. There is also a need for a technique to bring the resistor for generating the reference current internal to the LED driver and be able to trim the resistor. Finally, there is a need for a more convenient technique to set the operation mode of the charge pump of the LED driver. 
       SUMMARY OF THE INVENTION 
       [0013]    Embodiments of the present invention include an LED driver with an embedded non-volatile memory (NVM) capable of being programmed and storing control data for setting a variety of features of the LED driver, such as but not limited to the maximum current for driving the LEDs, analog parameters such as the resistance of the internal resistor for setting the reference current for the LEDs, and operation modes of the charge pump of the LED driver. This enables the implementation of multiple LED driver product options without the need for different metallization steps during the fabrication process for the LED driver. 
         [0014]    In one embodiment, a programmable LED driver for driving one or more LEDs comprises a charge pump configured to operate in one or more operation modes for receiving an input voltage and generating an output voltage to be applied to said one or more LEDs, a current regulator for generating a reference current, and a non-volatile memory module storing first control data, where current through the one or more LEDs is determined based on the reference current and the first control data. 
         [0015]    In another embodiment, the current regulator includes a trimmable resistor internal to the programmable LED driver, and the reference current is generated based upon a reference voltage and the resistance of the trimmable resistor. The non-volatile memory further stores second control data, and the resistance of the trimmable resistor is adjusted based upon the second control data. 
         [0016]    In still another embodiment, the charge pump is configured to operate in one or more of a plurality of operation modes, where each operation mode is configured to generate a different output voltage based on the input voltage. The non-volatile memory further stores third control data, and the one or more of the plurality of operation modes are activated or inactivated based upon the third control data. 
         [0017]    The present invention has the advantage that a variety of features of the LED driver, such as the LED current, internal resistance for setting the reference current for the LEDs, and the operation modes of the charge pump, and potentially a variety of other analog parameters of the LED driver may be conveniently set simply by programming the LED driver with the appropriate control data value in the non-volatile memory. Thus, an LED driver with different functionalities and features can be implemented as a single IC from the same die in the semiconductor fabrication process without having to go through different metallization processes for the different functionalities during the fabrication of the IC for the LED driver. 
         [0018]    The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The teachings of the embodiments of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
           [0020]      FIG. 1  illustrates a conventional LED driver for driving LEDs. 
           [0021]      FIG. 2  illustrates an LED driver for driving LEDs, according to one embodiment of the present invention. 
           [0022]      FIG. 3  illustrates using the control data stored in the non-volatile memory (NVM) to trim the internal resistance of the LED driver, according to one embodiment of the present invention. 
           [0023]      FIG. 4  illustrates the charge pump of  FIG. 2  that is configurable using the control data stored in the NVM, according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0024]    The Figures (FIG.) and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention. 
         [0025]    Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
         [0026]      FIG. 2  illustrates an LED driver  200  for driving LEDs  112 ,  114 , according to one embodiment of the present invention. For example, the LEDs  112 ,  114  can be white LEDs. The LED driver  200  includes 2 main circuit blocks, a configurable charge pump  201  and a current regulator  210 . 
         [0027]    Current through the LEDs  112 ,  114  sets their intensity and associated luminescence. The current regulator  210  is responsible for driving the LEDs  112 ,  114  with constant current. The current regulator  210  includes, among other components, a bandgap voltage generator  104 , an error amplifier comprised of the amplifier  106  and the transistor  119 , a current mirror  108  comprised of transistors  116 ,  118 , a non-volatile memory (NVM)  250 , and LED drive transistors  122 ,  202 ,  204 ,  206 ,  208 . Although the NVM  250  is shown in  FIG. 2  as part of the current regulator  210 , the NVM  250  may be part of, or separate from, the current regulator  210 . 
         [0028]    The NVM  250  stores control data for controlling the operation of various features of the LED driver  200 . For example, the NVM  250  stores control data A 1 , A 0 , B 1 , B 0  for controlling the current through the LEDs  112 ,  114 , control data C 1 , C 0  for trimming the internal resistance R INT    220 , and control data D 2 , D 1 , D 0  for setting the operation mode of the charge pump  201 , as will be explained in more detail below. The control data A 1 , A 0 , B 1 , B 0 , C 1 , C 0 , D 2 , D 1 , D 0  stored in the NVM  250  may be 1-bit digital data, although they may be in other form of data. Such control data may be written into the NVM  250  via the write (WR) line  252  through, for example, an external computer (not shown). The data written into the NVM  250  are not deleted even when the NVM  250  is powered off. The NVM  250  can be a flash memory, an SRAM (Synchronous Random Access Memory), or any other type of non-volatile memory. 
         [0029]    The bandgap voltage generator  104  generates a bandgap voltage Vref, and the error amplifier ( 106 ,  119 ) ensures that the voltage at node  260  across the resistor R INT    220  is set at Vref. Note that the resistor  220  is internal to the LED driver  200 , contrary to the external resistor  120  for use with the conventional LED driver  100  of  FIG. 1 . The reference current I REF  through the internal resistor R INT    220  is set by the bandgap voltage Vref and the internal resistance R INT    220 . That is, the reference current I REF  is set by Vref/R INT . The reference current I REF  is repeated through the transistor  122  as current I REF ′ by the current mirror  108 , and eventually drives the LEDs  112 ,  114  by the transistors  202 ,  204  and transistors  206 ,  208 , respectively. 
         [0030]    The current I REF ′ through the transistor  116  may be identical to or different from the reference current I REF  through the transistor  118 , depending upon the relative size or width/length (W/L) ratio of the transistor  116  compared to that of the transistor  118 . In addition, the current I REF ′ through the transistor  116  is repeated through the transistors  202 ,  204 ,  206 ,  208 , according to their relative size or W/L ratio compared to that of the transistor  122 . 
         [0031]    Note that the transistor  202  has a size or a width/length (W/L) ratio that is twice the W/L ratio of the transistor  204 , and the transistor  206  has a size or W/L ratio that is twice the W/L ratio of the transistor  208 . Thus, the transistor  202  draws twice as much the current drawn by the transistor  204 , both of which are added to drive the LED  112 . Likewise, the transistor  206  draws twice as much the current drawn by the transistor  208 , both of which are added to drive the LED  114 . 
         [0032]    The control data A 1 , A 0  stored in the NVM  250  determine the maximum current through the LED  112 , and the control data B 1 , B 0  stored in the NVM  250  determine the maximum current through the LED  114 . Specifically, the control data A 1 , A 0  control the on/off state of the switches  210 ,  212 , respectively. For example, the switches  210 ,  212  may be on (closed) when the control data A 1 , A 0  are “1”, respectively, and off (open) when the control data A 1 , A 0  are “0”, respectively. The control data B 1 , B 0  control the on/off state of the switches  214 ,  216 , respectively. For example, the switches  214 ,  216  may be on (closed) when the control data B 1 , B 0  are “1”, respectively, and off (open) when the control data B 1 , B 0  are “0”, respectively. 
         [0033]    For illustration, assume that the sizes or W/L ratios of all the transistors  118 ,  116 ,  122 ,  204 , and  208  are identical, and the W/L ratio of the transistors  202 ,  206  is twice the W/L ratio of the transistors  204 ,  208  and that I REF  is 1 mA. When A 1 , A 0  are “1” and “1” respectively, the maximum current through the LED  112  is 3 mA because both switches  210 ,  212  are on. When A 1 , A 0  are “1” and “0” respectively, the maximum current through the LED  112  is 2 mA because the switch  210  is on and the switch  212  is off. When A 1 , A 0  are “0” and “1” respectively, the maximum current through the LED  112  is 1 mA because the switch  210  is off and the switch  212  is on. When A 1 , A 0  are “0” and “0” respectively, the maximum current through the LED  112  is 0 mA because both switches  210 ,  212  are off. Similarly, when B 1 , B 0  are “1” and “1” respectively, the maximum current through the LED  114  is 3 mA because both switches  214 ,  216  are on. When B 1 , B 0  are “1” and “0” respectively, the maximum current through the LED  114  is 2 mA because the switch  214  is on and the switch  216  is off. When B 1 , B 0  are “0” and “1” respectively, the maximum current through the LED  114  is 1 mA because the switch  214  is off and the switch  216  is on. When B 1 , B 0  are “0” and “0” respectively, the maximum current through the LED  114  is 0 mA because both switches  214 ,  216  are off. 
         [0034]    The resistance R INT  of the internal resistance module  220  needs to be set accurately in order to control the reference current I REF  and the luminescence of the LEDs  112 ,  114  precisely. The use of an internal resistor  220  results in saving a pin of the LED driver IC and cost and board area associated with the additional pin. Since the resistor  220  is brought internal to the LED driver  200  according to the present invention, it should be capable of being trimmed internally and accurately as necessary. Although conventionally it was possible to use a polysilicon fuse to trim the internal resistor  220 , that has the disadvantage of increasing overall area and adding to manufacturing costs. Moreover, polysilicon or metal fuses have long term reliability problems due to fuse re-growth concerns. 
         [0035]      FIG. 3  illustrates using the control data stored in the NVM  250  to trim the internal resistance module  220 , according to one embodiment of the present invention. Referring to both  FIGS. 2 and 3 , the trimmable internal resistance module  220  of  FIG. 2  includes a plurality of resistors connected in series with each other, in this example R 1 , R 2 , R 3 . The resistance module  220  also includes switches  302 ,  304  that are connected in parallel to resistors R 2 , R 3 , respectively. 
         [0036]    The switches  302 ,  304  are turned on (closed) or off (open) in response to the control data C 0 , C 1  of the NVM  250 . For example, when the control data C 0 , C 1  are “1”, the switches  302  and  304  are turned on (closed), thereby shorting the connected resistors R 2 , R 3 , respectively. When the control data C 0 , C 1  are “0”, the switches  302  and  304  are turned off (open), and thus the resistors R 2  and R 3  become connected to R 1  in series. In other words, the switches  302 ,  304  effectively remove or connect the corresponding resistors R 2 , R 3 , respectively to the resistor R 1 . 
         [0037]    When C 0  is “1” and C 1  is “1”, the total resistance R INT =R 1 +R 2 +R 3  and I REF =Vref/(R 1 +R 2 +R 3 ). When C 0  is “1” and C 1  is “0”, the total resistance R INT =R 1 +R 2  and I REF =Vref/(R 1 +R 2 ). When C 0  is “0” and C 1  is “1”, the total resistance R INT =R 1 +R 3  and I REF =Vref/(R 1 +R 3 ). When C 0  is “0” and C 1  is “0”, the total resistance R INT =R 1  and I REF =Vref/R 1 . In this manner, the LED driver  120  of the present invention may trim the resistance R INT  of the internal resistance module  220  and also set the reference current I REF  through the internal resistor  220  and eventually the current through the LEDs  112 ,  114  accurately without using fuses. The resistance R INT  of the internal resistance module  220  and also set the reference current I REF  through the internal resistor  220  are programmable simply by programming appropriate control data C 1 , C 2  of the NVM  250  that is internal to the LED driver  200  IC. 
         [0038]      FIG. 4  illustrates the charge pump  201  of  FIG. 2  that is configurable using the control data stored in the NVM  250 , according to one embodiment of the present invention. The configurable charge pump  201  converts a battery voltage (V IN ) into an output voltage (V OUT ) in one of the plurality of operation modes, a 1× mode, 1.5× mode, and 2× mode. The charge pump  201  includes a 1× mode voltage generation module  402 , a 1.5× mode voltage generation module  404 , and a 2× mode generation module  406 . The 1× mode voltage generation module  402  receives the battery input voltage V IN  and generates an output voltage V OUT  where V OUT =V IN . The 1× mode voltage generation module  402  requires a running clock signal (Clock) coupled to its CLK input in order to operate and generate the output voltage V OUT . The 1.5× mode voltage generation module  404  receives the battery input voltage V IN  and generates an output voltage V OUT  where V OUT =1.5×V IN . The 1.5× mode voltage generation module  404  also requires a running clock signal (Clock) coupled to its CLK input in order to operate and generate the output voltage V OUT . The 2× mode voltage generation module  406  receives the battery input voltage V IN  and generates an output voltage V OUT  where V OUT =2×V IN . The 2× mode voltage generation module  406  also requires a running clock signal (Clock) coupled to its CLK input in order to operate and generate the output voltage V OUT . The output voltage (V OUT ) of the charge pump  201  drives the LEDs  112 ,  114 . The internal circuitry itself of the 1× mode voltage generation module  402 , 1.5× mode voltage generation module  404 , and 2× mode voltage generation module  406  are conventional and known in the art, and is not the subject of the invention disclosed herein. 
         [0039]    A typical charge pump has 3 modes of operation as explained above, 1×, 1.5× and 2×. However, some LED applications may only need 1 mode of operation (1×) in the charge pump, in which case the charge pump  201  behaves as a low voltage dropout regulator. In other LED applications, all three operation modes may be needed in the charge pump  201  because the battery input voltage V IN  can drop low enough and the voltage drop V LED  across the LEDs  112 ,  114  can be high enough. Thus, it would be very useful to activate or inactivate one or more of the 1× mode voltage generation module  402 , 1.5× mode voltage generation module  404 , 2× mode voltage generation module  406  in a convenient way. 
         [0040]    The control data D 0 , D 1 , D 2  of the NVM  250  determines which one(s) of the 1× mode voltage generation module  402 , 1.5× mode voltage generation module  404 , 2× mode voltage generation module  406  becomes active. As shown in  FIG. 4 , the control data D 0 , D 1 , D 2  are input to the AND gates  408 ,  410 ,  412 , respectively, to be AND&#39;ed with the clock signal  270 . Thus, when D 0 =1, the signal  414  to the CLK input of the 1× mode voltage generation module  402  is the same as the clock signal  270  and thus the 1× mode voltage generation module  402  is active. But when D 0 =0, the signal  414  to the CLK input of the 1× mode voltage generation module  402  is inactive and thus the 1× mode voltage generation module  402  is inactive. When D 1 =1, the signal  416  to the CLK input of the 1.5× mode voltage generation module  404  is the same as the clock signal  270  and thus the 1.5× mode voltage generation module  404  is active. But when D 1 =0, the signal  416  to the CLK input of the 1.5× mode voltage generation module  404  is inactive and thus the 1.5× mode voltage generation module  404  is inactive. When D 2 =1, the signal  418  to the CLK input of the 2× mode voltage generation module  406  is the same as the clock signal  270  and thus the 2× mode voltage generation module  406  is active. But when D 2 =0, the signal  418  to the CLK input of the 2× mode voltage generation module  406  is inactive and thus the 2× mode voltage generation module  406  is inactive. 
         [0041]    Therefore, activating or inactivating one or more of the operation modes of the charge pump  201  can be accomplished simply by programming the control data D 0 , D 1 , D 2  of the NVM  250 . If D 0 =1 but D 1 =0 and D 2 =0, the charge pump  201  is a single mode (1×) charge pump. However, if D 0 =D 1 =D 2 =1, the charge pump  201  becomes a tri-mode charge pump. Thus, there is no need to make 2 separate LED drivers with different mode charge pumps. 
         [0042]    The present invention has the advantage that a variety of features, such as the LED current, internal resistance for setting the reference current for the LEDs, and the operation modes of the charge pump, may be conveniently set simply by programming the LED driver with the appropriate control data value in the NVM. Thus, an LED driver with different functionalities and features can be implemented as a single IC from the same die in the semiconductor fabrication process. 
         [0043]    Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs for a programmable LED driver. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.