Abstract:
A method for controlling a current flowing through one or more light emitting diodes (LEDs) comprising receiving a Pulse Width Modulation (PWM) control signal, which includes rising and falling edges; receiving a first voltage signal; generating a second voltage signal based on the PWM control signal and the first voltage signal, wherein the second voltage increases gradually in response to one of the rising and falling edges of the PWM signal and decreases gradually in response to the other of the rising and falling edges of the PWM signal; and providing a current to the one or more LEDs, wherein the current varies gradually according to the second voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority to Chinese Patent Application No. 201210219812.6, filed with the Chinese Patent Office on Jun. 28, 2012, and entitled “Circuit and Method for Controlling Light Emitting Diode,” which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The subject matter of the present application relates to methods and circuits for controlling Light Emitting Diodes (LEDs), in particular, to methods and circuits for controlling the light intensity of LEDs by integrated circuits having a soft dimming capability. 
       BACKGROUND INFORMATION 
       [0003]    Many electronic products including cellular phones, Personal Digital Assistant (PDA) devices, electronic books (e-books), digital cameras, MP3 players, Global Positioning Systems (GPS), and digital photo frames use Liquid Crystal Displays (LCDs). Many of these products use Light Emitting Diodes (LEDs) to provide backlight to the LCDs. An LED control circuit is often used to drive the LEDs to turn the lights on or off or to obtain a desired light intensity. The LED control circuit can include a DC-to-DC converter, which converts a direct current (DC) signal from one voltage level to another. A DC-to-DC converter can also regulate an output current flowing through the LEDs, and thus, adjust the light intensity of the LEDs. 
         [0004]    The DC-to-DC converter of the LED control circuit can be a boost or buck converter, and include a Pulse Width Modulation (PWM) circuit. In such a circuit, a PWM signal is an input to the DC-to-DC converter. The PWM signal, which is an electrical pulse signal, can have, for example, a high voltage level and a low voltage level. A frequency of the electrical pulse signal can be fixed but the width of the electrical pulse signal can be varied. By varying the pulse width, an average value of the current flowing through the LEDs can be varied accordingly, thus adjusting the light intensity of the LEDs. But changing the pulse width can cause a severe output voltage fluctuation. 
         [0005]    Therefore, there is a need to avoid severe and sudden fluctuations of the output voltage and current of the LED control circuit. That is, it is desirable to increase or decrease the output current in a controllable manner. 
       SUMMARY 
       [0006]    The present disclosure provides a method for controlling a current flowing through one or more light emitting diodes (LEDs). According to one embodiment, the method includes receiving a Pulse Width Modulation (PWM) signal, which includes rising and falling edges; receiving a first voltage signal; generating a second voltage signal based on the PWM signal and the first voltage signal, wherein the second voltage increases gradually in response to one of the rising and falling edges of the PWM signal and decreases gradually in response to the other of the rising and falling edges of the PWM signal; and providing a current to the one or more LEDs, wherein the current varies gradually according to the second voltage. 
         [0007]    According to a further embodiment, a method for controlling a current flowing through one or more light emitting diodes (LEDs) includes receiving a Pulse Width Modulation (PWM) signal, which includes rising and falling edges; receiving a voltage signal; generating a first current based on the voltage signal; generating a second current based on the first current and the PWM signal, wherein the second current increases gradually in response to one of the rising and falling edges of the PWM signal and decreases gradually in response to the other of the rising and falling edges of the PWM signal; and providing a third current to the one or more LEDs, wherein the third current varies gradually according to the second current. 
         [0008]    The present disclosure further provides a system for controlling a current flowing through one or more light emitting diodes (LEDs). According to one embodiment, the system includes a voltage regulator configured to receive a first voltage signal; a current regulator coupled to the voltage regulator and configured to generate a second voltage signal based on a PWM signal and the first voltage signal, wherein the second voltage increases gradually in response to one of the rising and falling edges of the PWM signal and decreases gradually in response to the other of the rising and falling edges of the PWM signal; and a current controller configured to provide a current to the one or more LEDs, wherein the current varies gradually according to the second voltage. 
         [0009]    According to a further embodiment, a system for controlling a current flowing through one or more light emitting diodes (LEDs) includes a voltage regulator configured to receive a voltage signal; a current regulator configured to generate a first current based on the voltage signal and a second current based on the first current and a PWM control signal, wherein the second current increases gradually in response to one of the rising and falling edges of the PWM signal and decreases gradually in response to the other of the rising and falling edges of the PWM signal; and a current controller configured to provide a third current to the one or more LEDs, wherein the third current varies gradually according to the second current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of an exemplary LED control system. 
           [0011]      FIG. 2  is a schematic diagram illustrating an exemplary embodiment of an LED current controller shown in  FIG. 1 . 
           [0012]      FIG. 3  is a block diagram of an exemplary LED control system with an LED current regulator. 
           [0013]      FIG. 4A  is a schematic diagram of an exemplary LED current regulator shown in  FIG. 3 . 
           [0014]      FIG. 4B  is a schematic diagram of an exemplary multiplexer shown in  FIG. 4A . 
           [0015]      FIG. 4C  is a schematic diagram of an exemplary LED current regulator shown in  FIG. 3 . 
           [0016]      FIG. 5A  is an exemplary timing diagram illustrating timing relations between a PWM input signal, an LED current, and a dimming control signal, corresponding to the dimming control circuit shown, for example, in  FIG. 1 . 
           [0017]      FIG. 5B  is an exemplary timing diagram illustrating timing relations between a PWM input signal, an LED current, and a dimming control signal, corresponding to the LED control system shown, for example, in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    Reference will now be made in detail to the exemplary embodiments consistent with the embodiments disclosed herein, the examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or similar parts. 
         [0019]      FIG. 1  is a block diagram of an exemplary LED control system  100 . LED control circuit  100  can have one or more input signals including, for example, a voltage input signal Vin  102 A, an enable signal  102 B, and a PWM input  102 C. LED control circuit  100  can also include a bandgap and Vcc regulator  110 , a boost controller  120 , an LED current controller  140 , a transistor Q1  180 , an inductor  182 , a DC voltage input Pvin  183 , a diode  184 , two resistors R1  185  and R2  187 , an output capacitor  188 , and one or more LEDs  190 A˜ 190 D. In this exemplary embodiment, a boost controller is used as an example. A person having ordinary skill in the art should appreciate that a buck controller or a buck-boost controller can also be used. 
         [0020]    As shown in  FIG. 1 , bandgap and Vcc regulator  110  receives external input signals including Vin  102 A as an input DC voltage and enable signal  102 B. When enable signal  102 B is ON, for example, bandgap and Vcc regulator  110  can generate a reference voltage Vref  112  and an internal power supply Vcc (not shown in  FIG. 1 ). The internal power supply Vcc provides a more stable power supply compared to outside power supplies. The reference voltage Vref  112 , generated from bandgap and Vcc regulator  110 , is later provided to boost controller  120  and LED current controller  140 . 
         [0021]    In addition to the reference voltage Vref  112 , boost controller  120  can receive a PWM input signal  102 C and a feedback signal  192 . Boost controller  120  can generate a control signal  122  as its output signal to control the transistor Q1  180  through the connection to the control terminal of transistor Q1  180 . 
         [0022]    As shown in  FIG. 1 , LED current controller  140  receives PWM input  102 C and Vref  112  as its input signals. LED current controller  140  is electrically coupled to LEDs  190 A˜ 190 D through connection  192 . As described in detail below, LED current controller  140  can supply or suppress current flowing through LEDs  190 A˜ 190 D. In some embodiments, the current flowing through connection  192 , i.e., the current flowing through LEDs  190 A˜ 190 D, can also be provided directly or indirectly as a feedback signal to boost controller  120 . 
         [0023]    Transistor Q1  180  can be a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) device, which includes a gate terminal electrically coupled to signal  122 , which is generated from boost controller  120 . Thus, transistor Q1  180  can be turned on or off depending on the voltage level of the control signal  122 . Transistor Q1  180  further includes a source terminal electrically coupled to a ground potential and a drain terminal electrically coupled to a first terminal of inductor  182  and a first terminal (e.g. anode) of diode  184 . Inductor  182  includes a second terminal, which receives an input DC voltage PVin  183 . When transistor Q1  180  is turned off, Pvin  183  provides a voltage  181  to LEDs  190 A˜ 190 D through diode  184 . When transistor Q1  180  is turned on, however, the voltage  181  can be pulled toward a lower value or a ground potential. 
         [0024]    Further, in  FIG. 1 , Diode  184  includes a second terminal electrically coupled to a first terminal of capacitor  188 . A second terminal of capacitor  188  is electrically connected to the ground potential. Diode  184  and capacitor  188  can stabilize the output voltage Vout  189 . For example, when transistor Q1  180  is turned off, Pvin  183  provides a current flowing through diode  184  to charge capacitor  188 . When transistor Q1  180  is turned on and thus the voltage  181  is pulled toward the ground potential, diode  184  cuts off the current path from capacitor  188  to transistor Q1  180 . Capacitor  188  releases its electrical charge through LEDs  190 A˜ 190 D, thus temporarily maintaining the current flowing through LEDs  190 A˜ 190 D. In some embodiments, the capacitance value can be large in order to prevent or reduce the sudden change of the output voltage Vout  189 , i.e., reduce the output voltage fluctuation. 
         [0025]    The second terminal (e.g. a cathode terminal) of diode  184  is also electrically coupled to a voltage divider, which can include resistors R1  185  and R2  187 . Resistors R1  185  and R2  187  can generate a voltage division and produce an overvoltage protection signal OVP  186 . OVP  186  is provided to boost controller  120  as a feedback signal so that boost controller  120  can provide overvoltage protection for LEDs  190 A˜ 190 D by adjusting the voltage of control signal  122  accordingly. 
         [0026]    As shown in  FIG. 1 , the second terminal of diode  184  is also electrically coupled to a first terminal of LED  190 A in LEDs  190 A˜ 190 D. It is readily appreciated that the number of LEDs is not limited to four and can be any number desired. In some embodiments, LEDs  190 A˜ 190 D can be sequentially connected as shown in  FIG. 1 . LEDs  190 A˜ 190 D are electrically coupled to LED current controller  140 , which can form a part of the current path from Pvin  183  and can sense the current flowing through LEDs  190 A˜ 190 D. 
         [0027]    In operation, when PWM input  102 C is high, i.e., the input pulse voltage level is high, both boost controller  120  and LED current controller  140  can be turned on. Transistor Q1  180  can be turned off so that the current flowing through LEDs  190 A˜ 190 D is at a high level. Conversely, if PWM input  102 C is low, i.e., the input pulse voltage level is low, both boost controller  120  and LED current controller  140  can be turned off. 
         [0028]    When transistor Q1  180  is turned on and the voltage of signal  181  is pulled toward the ground potential, capacitor  188  can temporarily maintain the voltage Vout  189  or reduce its rate of decay. As discussed above, however, a dimming control circuit implemented by using a PWM input, which has two voltage levels, exhibits output voltage fluctuations and a ripple effect. 
         [0029]      FIG. 2  is a schematic diagram illustrating an exemplary embodiment of an LED current controller  140  shown in  FIG. 1 . LED current controller  140  can receive a reference voltage Vref  112 , for example, from bandgap and Vcc regulator  110  shown in  FIG. 1 . LED current controller  140  can also receive a dimming control signal DIMB  172 . DIMB  172  can be the same as PWM input  102 C in  FIG. 1  or a signal derived therefrom. LED current controller  140  can include an operational amplifier  144 , a transistor Q1  148 , a resistor Riset  150 , a transistor M1  154 , a transistor M2  160 , a resistor Rx1  164 , an operational amplifier  168 , a transistor Q0  173 , a transistor Q2  174 , and a resistor Rx2  178 . LED current controller  140  can be electrically coupled to LEDs  190 A˜ 190 D through connection  192 . 
         [0030]    As discussed above, in some embodiments, the reference voltage Vref  112  can be generated from bandgap and Vcc regulator  110  and thus can be any desired value. In  FIG. 2 , operational amplifier  144  receives Vref  112  and Vset  142  as its input signals and generates an output signal  146  to control a gate terminal of transistor Q1  148 . Operational amplifier  144  can enforce Vset  142  to be equal to or substantially equal to Vref  112 , depending on the characteristics of operation amplifier  144 , such as its gain, input frequency range, etc. Operational amplifier  144 , transistor Q1  148 , and resistor Riset  150  form a feedback loop, which dynamically adjusts Vset  142  to closely track Vref  112 . Thus, the current flowing through transistor Q1  148  can be equal to the voltage value of Vref  112  divided by the resistance value of resistor Riset  150 . This current can be the same as, or substantially the same as, the current flowing through transistor M1  154 . Transistor M1  154  and transistor M2  160  form a current mirror. Current flowing through transistor M2  160  can closely follow that of transistor M1  154 , depending on the current gain ratio (M2/M1) of the current mirror, which is related to relative dimensions of the gate of transistor M1  154  and transistor M2  160 . For example, if transistor M1  154  and transistor M2  160  are identical, current flowing through the two transistors are the same or substantially the same. 
         [0031]    In some embodiments, resistor Rx1  164  can convert current flowing through transistor M2  160  to a voltage signal  162 , which is one of the input signals to operational amplifier  168 . Operational amplifier  168  has a second voltage input  166 . Operational amplifier  168  can enforce the voltage  166  to be equal to or substantially equal to the voltage  162 , depending on the characteristics of operation amplifier  168 , such as its gain, input frequency range, etc. The output signal  170  of operation amplifier  168  is electrically coupled to a gate terminal of transistor Q2  174 . A source terminal of transistor Q2  174  is connected to resistor Rx2  178 . When transistor Q2  174  is turned on, the current flowing through transistor Q2  174  and resistor Rx2  178  can be determined to be equal to, or substantially equal to, the voltage  162  divided by the resistance value of resistor Rx2  178 . Consequently, since LEDs  190 A˜ 190 D are electrically coupled to the drain terminal of transistor Q2  174 , the current flowing through the LEDs can be the same as, or substantially the same as, the current flowing through transistor Q2  174  and resistor Rx2  178 . 
         [0032]    Further, in  FIG. 2 , as discussed above, Vref  112  can determine the current flowing through resistor Riset  150 . And this current is mirrored or multiplied by the transistor M1 and transistor M2 pair. Therefore, the current flowing through resistor Rx1  164  can be determined by the voltage value of Vref  112  and the current gain ratio (M2/M1) of the current mirror. Later, the current flowing through the resistor Rx2  178  can also be determined through the current-voltage conversion by resistor Rx1 and through the function of operational amplifier  168 . Thus, the current flowing through LEDs  190 A˜ 190 D can be expressed as: iLED=(Vref/Riset)×(M2/M1)×(Rx1/Rx2)=K×(Vref/Riset), where K=(M2/M1)×(Rx1/Rx2). If transistor M1  154  and transistor M2  160  are identical, K=(Rx1/Rx2). 
         [0033]    As shown in  FIG. 2 , DIMB  172  can be the same as or derived from PWM input  102 C shown in  FIG. 1 . As discussed above, PWM input  102 C can be a PWM signal, and thus DIMB  172  can also be a PWM signal. When DIMB  172  is at a low voltage level, transistor Q0  173  can be turned off and LED current controller  140  operates to supply foregoing calculated iLED current to LEDs  190 A˜ 190 D. Conversely, when DIMB  172  is at a high voltage level, transistor Q0  173  can be turned on and voltage  170  is pulled toward the ground potential. Consequently, transistor Q2  174  can be turned off, and the current supply to LEDs  190 A˜ 190 D can be reduced or eliminated. Thus, by adjusting the control signal DIMB  172 —for example, adjusting its pulse width—the LED control circuit can adjust the current passing through LEDs  190 A˜ 190 D and thus adjust the light intensity. As discussed earlier, this method can result in a significant output voltage fluctuation, and consequently, the current flowing through LEDs  190 A˜ 190 D (i.e., the LED load current) can experience a large ripple effect. 
         [0034]      FIG. 3  is a block diagram of an exemplary LED control system  300  having an LED current regulator  440 . In  FIG. 3 , it will be readily appreciated by one of ordinary skill in the art that the illustrated blocks and circuit elements can be altered in their numbers or their relative positions. LED control system  300  can also include additional blocks or circuit elements. 
         [0035]    LED control system  300  can have one or more input signals including, for example, a voltage signal Vin  402 A, an enable signal  402 B, and a PWM input  402 C. LED control system  300  can also include a bandgap and Vcc regulator  410 , an LED current regulator  440 , one or more LED current controllers  460 A˜ 460 N, a voltage converter  470 , and LEDs  490 . For example, the system can have any number of LED arrays, and hence corresponding number of LED current controllers. In  FIG. 3 , the LED control system  300  is illustrated in block diagram. A person having ordinary skill in the art should appreciate that the blocks are divided for illustration purpose; and the functional blocks may be integrated on the actual circuit. 
         [0036]    As shown in  FIG. 3 , bandgap and Vcc regulator  410  can receive external signals including voltage signal Vin  402 A and enable signal  402 B. When enable signal  402 B is high, for example, bandgap and Vcc regulator  410  can generate a reference voltage Vref  412  and an internal power supply Vcc (not shown in  FIG. 3 ), which is a more stable power supply voltage compared with an outside power supply voltage. Vref  412  can be an input signal to both LED current regulator  440  and voltage converter  470 . 
         [0037]    LED current regulator  440  can receive reference voltage Vref  412  and PWM input  402 C. Similar to PWM input  102 C in  FIG. 1 , PWM input  402 C can be a PWM signal. LED current regulator  440  can generate output signals including a voltage signal  482  and a control signal DIM  483 . Signals  482  and  483  can be input signals to one or more LED current controllers  460 A˜ 460 N, which form part of the current path through the LEDs  490  and voltage converter  470 . LEDs  490  can include one or more light emitting diodes the same as, or similar to, those shown in  FIG. 1 , i.e., LEDs  190 A˜ 190 D. The details of LED current regulator  440  will be discussed below in association with  FIGS. 4A˜4C . Signal  482  can be controlled to increase or decrease in a desirable manner so that the output currents from LED current controllers  460 A˜ 460 N (i.e., the currents flowing through LEDs  490 ) are also controlled in a desirable manner. Consequently, the output voltage fluctuation and ripple effect can be reduced or avoided. For example, signal  482  can increase or decrease in eight relatively small steps until it reaches its final value. As a result, the current flowing through LEDs  490  can also change gradually, i.e., in eight relatively small steps. Therefore, significant fluctuations of the output voltage or ripple effect can be reduced or avoided. Signal DIM  483  can be a control signal similar to PWM input signal  402 C, or derived therefrom. Signal DIM  483  can control the on or off of LED current controllers  460 A˜ 460 N. 
         [0038]    Further, in  FIG. 3 , LED current controllers  460 A˜ 460 N can be any type, for example, the same or similar type as LED current controller  140  or part of LED current controller  140  shown in  FIG. 1 . As an example, referring to LED current controller  140  in  FIG. 2 , LED current controllers  460 A˜ 460 N in  FIG. 3  may include only circuit elements corresponding to operational amplifier  168 , transistor Q0  173 , transistor Q2  174 , and resistor Rx2  178 , but may not include the remaining circuit elements in  FIG. 2 . The remaining circuit elements shown in  FIG. 2  may be included in LED current regulator  440  in  FIG. 3 , as will be discussed in details below in association with  FIG. 4A . The circuit elements included in LED current controllers  460 A˜ 460 N can be connected in the same or similar way and thus have the same or similar functions as their counterparts in  FIG. 2 . Therefore, their descriptions are not repeated here. The output signals from LED current controllers  460 A˜ 460 N can be provided to voltage converter  470  directly or indirectly through connections  492 A˜ 492 N as feedback signals. 
         [0039]    As shown in  FIG. 3 , voltage converter  470  can be any type, for example, the same or a similar type as boost controller  120  as shown in  FIG. 1 . Voltage converter  470  can be a boost converter, which generates an output signal having a voltage level higher than that of the input signal. Voltage converter  470  can also be a buck converter, which generates an output signal having a voltage level lower than that of the input signal. Moreover, voltage converter  470  can be a buck-boost converter, which generates an output signal having a voltage level either higher or lower than that of the input signal. 
         [0040]      FIG. 4A  is a schematic diagram of an exemplary LED current regulator  440 A corresponding to LED current regulator  440  shown in  FIG. 3 . In  FIG. 4A , it will be readily appreciated by one of ordinary skill in the art that the illustrated blocks and circuit elements can be altered in their numbers (e.g., the number of resistors are not limited to eight as shown in  FIG. 4A ) or their relative positions; and LED current regulator  440 A can further include additional blocks or circuit elements. 
         [0041]    In  FIG. 4A , LED current regulator  440 A can receive one or more input signals including, for example, PWM input  402 C and Vref  412  from bandgap and Vcc regulator  410  as shown in  FIG. 3 . LED current regulator  440 A can include an operational amplifier  442 , a transistor  444 , a power supply  446 , a voltage divider including two or more resistors such as  448 A˜ 448 H, and a selection circuit, which includes a multiplexer  452  and a counter  454 . LED current regulator  440 A can generate one or more output signals including a voltage signal  482  and a control signal DIM  483 . 
         [0042]    As shown in  FIG. 4A , in some embodiments, operational amplifier  442  can receive Vref  412  and enforce the voltage  447  to be equal to or substantially equal to voltage Vref  412  in a similar way as discussed in association with operational amplifier  144  in  FIG. 2 , depending on the characteristics of operation amplifier  442 , such as its gain, input frequency range, etc. Operational amplifier  442  has an output voltage  443 , which is connected to a gate terminal of transistor  444 . The current flowing through transistor  444  can be determined by the voltage value of Vref  412  divided by the resistance value of a sum of resistors  448 A˜ 448 H. In some embodiments, resistors  448 A˜ 448 H can have resistance values on the ten kilo-ohm (10 KΩ), twenty kilo-ohm (20 KΩ) or thirty kilo-ohm (30 KΩ) scales. The current flowing through resistors  448 A˜ 448 H can be the same as, or substantially the same as, the current flowing through transistor  444 . 
         [0043]    Further in  FIG. 4A , the voltages  450 A˜ 450 H are intermediate voltages between the resistors and can be fractions of the voltage  447 , i.e., fractions of the voltage Vref  412 . For example, if the eight resistors  448 A˜ 448 H, each having the same resistance value, are used as shown in  FIG. 4A  in the resistor network, the voltage  450 G is ⅛ of Vref  412 , the voltage  450 F is 2/8 of Vref  412 , and so forth. It can be readily appreciated by one skilled in the art that the number of resistors in the network is not restricted to eight, but can be any number greater than one. In some embodiments, for example, the number of resistors can be an integer between two and sixteen. Moreover, the resistance values of resistors  448 A˜ 448 H are not required to be equal to each other and can be different in any manner desired. It can also be readily appreciated by one skilled in the art that the input signals to multiplexer  452  can be arranged in anyway desired. 
         [0044]    In some embodiments, multiplexer  452  can receive signals  450 A˜ 450 H, voltage  447  and PWM input  402 C as its input signals and generate output signals including voltage Vref−dim  458 . The voltage Vref−dim  458  can be selected or derived from the voltages  450 A˜ 450 H, voltage  447 , and an electrical ground. For example, the voltage Vref−dim  458  can be selected to be equal to or substantially equal to any of the voltages  450 A˜ 450 H, depending on the control signals (e.g., Ctl1˜Ctl4) from counter  454 . Vref−dim  458  can also be further refined to be equal to any voltage between ground and voltage  447  (i.e., the voltage Vref  412  or the voltage that is substantially similar to that of Vref  412 ). As an example, multiplexer  452  can interpret voltages  450 A and  450 B, either linearly or nonlinearly, and generate the voltage Vref−dim  458  to be any voltage between ⅞ of Vref  412  and 6/8 of Vref  412 . An exemplary embodiment of multiplexer  452  will be discussed in association with  FIG. 4B . 
         [0045]    Multiplexer  452  can be controlled by any logic including, for example, by counter  454 . Counter  454  is electrically coupled to multiplexer  452  and can be any type of counters such as up/down counter, asynchronous (ripple) counter, synchronous counter, etc. Counter  454  can be binary coded, Gray coded, or coded with any other type of coding. 
         [0046]    In some embodiments, counter  454  can receive a control signal to initiate counting, stop counting, or reset the counter. One example of the control signal can be PWM input  402 C or a signal derived therefrom. For example, when PWM input  402 C falls, counter  454  can initiate counting. During counter  454 &#39;s first counting period, multiplexer  452  can select signal  450 A (i.e., ⅞ of Vref  412 ) and generate the voltage Vref−dim  458  to be equal to or substantially equal to the voltage  450 A. During counter  454 &#39;s second counting period, multiplexer  452  can select voltage  450 B (i.e., 6/8 of Vref  412 ), and during the third counting period, select voltage  450 C (i.e., ⅝ of Vref  412 ), and so forth. When the voltage Vref−dim  458  reaches voltage  450 H, counter  454  can stop counting. The selection of signals  450 A˜ 450 H can be controlled by signals such as Ctl1˜Ctl4 from counter  454 . The details of an exemplary counter  454  will be discussed in association with  FIG. 4B . 
         [0047]    In some embodiments, Vref−dim  458  can be an input reference voltage to the subsequent circuits similar to those shown in  FIG. 2 . That is, Vref−dim  458  can replace Vref  112  as shown in  FIG. 2 . In other words, operational amplifier  464 , transistor Q1  468 , resistor Riset  469 , transistor M1  474 , transistor M2  481 , and resistor Rx1  484 , can correspond to their respective counterparts in  FIG. 2 , i.e., operational amplifier  144 , transistor Q1  148 , resistor Riset  150 , transistor M1  154 , transistor M2  160 , and resistor Rx1  164 . Therefore, the functions of these circuit elements in LED current regulator  440 A are not repeated. 
         [0048]    LED current regulator  440 A can generate an output voltage signal  482 , which can be the input to one or more LED current controllers  460 A˜ 460 N. Each of LED current controllers  460 A˜ 460 N can include an operational amplifier  569 , a transistor Q0  575 , a transistor Q2  572 , and a resistor Rx2  573 , corresponding to operational amplifier  168 , transistor Q0  173 , transistor Q2  174 , and resistor Rx2  178  as shown in  FIG. 2 , respectively. That is, LED current controllers  460 A˜ 460 N can be similar to the corresponding part of LED current controller  140  as shown in  FIG. 2  and thus the functions of these circuit elements are not repeated. 
         [0049]    As discussed above, Vref−dim  458  can replace Vref  112  as the input reference voltage shown in  FIG. 2 . Thus, similar to the discussion earlier that referred to  FIG. 2 , current flowing through the LEDs  490  can now be controlled by Vref−dim  458 , instead of Vref  112 . That is, iLED=(Vref−dim/Riset)×(M2/M1)×(Rx1/Rx2)=K×(Vref−dim/Riset), where K=(M2/M1)×(Rx1/Rx2). If transistor M1  474  and transistor M2  481  are identical, K=(Rx1/Rx2). As seen in the above equation, the current passing through the LEDs (iLED) is proportional to Vref−dim. Therefore, during the process of turning off the current flowing through LEDs  490  (i.e., iLED), counter  454  and multiplexer  452  can select the voltage levels from high to low, and the current can decrease in smaller steps corresponding to ⅞, 6/8, ⅝ . . . ⅛ of Vref, until the current decreases to zero. It is readily appreciated by those skilled in the art that the LED current iLED can decrease at any step desired, linear or nonlinear, and is not restricted to the eight steps corresponding to the eight voltage levels divided by the resistance value of a sum of resistors  448 A˜ 448 H. 
         [0050]    In some embodiments, when PWM input  402 C rises, counter  454  can also initiate counting. During counter  454 &#39;s first counting period, multiplexer  452  can select voltage  450 H (i.e., the ground potential) and generate the voltage Vref−dim  458  to be equal or substantially equal to the voltage  450 H. During counter  454 &#39;s second counting period, multiplexer  452  can select voltage  450 G (i.e., ⅛ of Vref  412 ), and during the third counting period, select voltage  450 F (i.e., 2/8 of Vref  412 ), and so forth. Counter  454  can stop counting when voltage  450 A (i.e., ⅞ of Vref  412 ) is selected and the voltage Vref−dim  458  equals or substantially equals to the voltage  450 A. Or counter  454  can stop counting when voltage  447  is selected and the voltage Vref−dim  458  equals or substantially equals to voltage  447  (i.e., the voltage Vref  412 ). Because the current flowing through LEDs  490  (i.e., iLED) corresponds to the voltage Vref−dim  458 , as counter  454  and multiplexer  452  select the voltage levels from low to high, the LED current iLED can increase in smaller steps, until it reaches its final value. It is readily appreciated that iLED can be controlled to increase or decrease in any manner desired, linearly or nonlinearly, and is not restricted to the eight steps corresponding to the eight resistors  448 A˜ 448 H as shown in  FIG. 4A . 
         [0051]    Further, in  FIG. 4A , LED current controllers  460 A˜ 460 N can also receive a dimming control signal DIM  483  from LED current regulator  440 A. DIM  483  can have, for example, a waveform  716  as shown in  FIG. 5B . The dimming control signal DIM  483 , which can be derived from PWM input  402 C, can generate a signal such as DIMB  574  for controlling transistor Q0  575 . DIM  483  can rise when PWM input  402 C rises, i.e., when the current of LEDs  490  rises above its initial level. DIM  483  may fall to a low voltage level when current flowing through LEDs  490  (i.e., iLED) falls back to its initial level (also referring to waveform  716  in  FIG. 5B ). This allows LED current controller  460 A˜ 460 N to continue supplying current to LEDs  490  corresponding to the voltage level of signal  482 . As will be explained below, DIMB  574 , derived from DIM  483 , can turn on and turn off LEDs  490  shown in  FIG. 3 . 
         [0052]    As shown in  FIG. 4A , in LED current controllers  460 A˜ 460 N, DIMB  574  is coupled to a gate terminal of transistor Q0  575 . A drain terminal of transistor Q0  575  is electrically coupled to a gate terminal of transistor Q2  572 , and a source terminal of transistor Q0  575  is coupled to the ground potential. A drain terminal of transistor Q2  572  is coupled to LEDs  490  through connections such as  492 A, and a source terminal of transistor Q2  572  is coupled to the ground through resistor Rx2  573 . When DIMB  574  is high, transistor Q0  575  is turned on, the voltage of the drain terminal of transistor Q0  575  is pulled toward ground, and transistor Q2  572  is turned off. Consequently, the current flowing through LEDs  490  (shown in  FIG. 3 ) can be reduced or eliminated. When DIMB  574  is low, transistor Q0  575  is turned off, the voltage of the drain terminal of transistor Q0  575  is high, and transistor Q2  572  is on. The current flowing through the LEDs  490  can flow through resistor Rx2 to the ground potential. 
         [0053]      FIG. 4B  is a schematic diagram of an exemplary multiplexer  452  shown in  FIG. 4A . Multiplexer  452  can include one or more inverters  502 A˜ 502 D, one or more switches  504 A˜ 504 H controlled by an input signal Ctl1  505 A, one or more switches  506 A˜ 506 D controlled by an input signal Ctl2  505 B, one or more switches  508 A  508 B controlled by an input signal Ctl3  505 C, and one or more switches  510 A˜ 510 B controlled by an input signal Ctl4  505 D. Multiplexer  452  can also include additional logics or circuits such as switches  512  and  514 . Further, in  FIG. 4B , it will be readily appreciated by one of ordinary skill in the art that the illustrated blocks and circuit elements can be altered in their numbers or their relative positions. For Example, the multiplexer  452  is not limited to have four control signals CM  505 A˜Ctl4  505 D and eight voltage levels corresponding to the signals  450 A˜ 450 H. 
         [0054]    As shown in  FIG. 4B , in some embodiments, multiplexer  452  can take, for example, signals  450 A˜ 450 H as its input signals and generate an output signal Vref_dim  458  based on the control signals Ctl1  505 A˜Ctl4  505 D. For example, when the dimming control is initiated, PWM input  402 C can rise to a high voltage level (i.e., PWM input  402 C=1). The corresponding control signal DIM  483  (shown in  FIG. 4A ) can also rise immediately, and turn on LED current controllers  460 A˜ 460 N by turning off transistor Q0  575  as discussed above in  FIG. 4A . Counter  454  shown in  FIG. 4A  can thus start counting from “0000,” i.e., signals Ctl4  505 D˜CM  505 A=“0000,” respectively. Switches  504 H,  506 D,  508 B, and  510 A are closed and thus the voltage of the output signal Vref_dim  458  equals or substantially equals to voltage  450 H (i.e., ⅛ of Vref  412 ). When counter  454  advances one counting period and “Ctl4Ctl3Ctl2Ctl1” equals “0001,” respectively, switches  504 G,  506 D,  508 B, and  510 A are closed and thus the voltage Vref_dim  458  equals or substantially equals voltage  450 G (i.e., 2/8 of Vref  412 ). When “Ctl4Ctl3Ctl2Ctl1” equals “0010,” the voltage Vref_dim  458  equals or substantially equals voltage  450 F, and so forth. When “Ctl4Ctl3Ctl2Ctl1” equals “0111,” the voltage Vref_dim  458  equals or substantially equals voltage  450 A (i.e., ⅞ of Vref  412 ). Counter  454  may also count one more period such that when “Ctl4Ctl3Ctl2Ctl1” equals to “1000,” the voltage Vref_dim  458  equals voltage  447 , i.e., 8/8 of Vref  412 . Counter  454  can then stop counting. The logic relations of the input and the output signals for multiplexer  452  in  FIG. 4B , as discussed above, are summarized in Table 1 below. It is readily appreciated by one of ordinary skill in the art that the logic relations shown in Table 1 are for illustration purpose only and any other logic can be designed to achieve the same or similar voltage selection purpose. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Logic relations between input and output signals 
               
               
                 shown in FIG. 4B, when PWM input 402C = 1. 
               
             
          
           
               
                 PWM 
                   
                   
                   
                   
                   
                   
               
               
                 Input 
                 Ctl1 
                 Ctl2 
                 Ctl3 
                 Ctl4 
                 Vref_dim 
                 DIM 
               
               
                   
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 Vref * 1/8 
                 1 
               
               
                 1 
                 1 
                 0 
                 0 
                 0 
                 Vref * 2/8 
                 1 
               
               
                 1 
                 0 
                 1 
                 0 
                 0 
                 Vref * 3/8 
                 1 
               
               
                 1 
                 1 
                 1 
                 0 
                 0 
                 Vref * 4/8 
                 1 
               
               
                 1 
                 0 
                 0 
                 1 
                 0 
                 Vref * 5/8 
                 1 
               
               
                 1 
                 1 
                 0 
                 1 
                 0 
                 Vref * 6/8 
                 1 
               
               
                 1 
                 0 
                 1 
                 1 
                 0 
                 Vref * 7/8 
                 1 
               
               
                 1 
                 1 
                 1 
                 1 
                 0 
                 Vref * 8/8 
                 1 
               
               
                 1 
                 0 
                 0 
                 0 
                 1 
                 Vref 
                 1 
               
               
                   
               
             
          
         
       
     
         [0055]    In  FIG. 4A , as another example, PWM input  402 C falls to a low voltage level (i.e., PWM input  402 C=0). The corresponding control signal DIM  483 , however, may not fall to a low voltage level until the current flowing through LEDs  490  (i.e., iLED) falls back to its initial level (also referring to waveform  716  in  FIG. 5B ). This allows LED current controllers  460 A˜ 460 N to continue supplying current to LEDs  490  corresponding to the voltage level of signal  482 . 
         [0056]    In  FIG. 4B , when PWM input  402 C falls, counter  454  shown in  FIG. 4A  can start counting from, for example, “0111.” When “Ctl4Ctl3Ctl2Ctl1” (i.e., signals  505 D  505 A) equals “0111,” respectively, switches  504 A,  506 A,  508 A, and  510 A are closed and thus the voltage of the output signal Vref_dim  458  equals or substantially equals voltage  450 A (i.e., ⅞ of Vref  412 ). When counter  454  advances one counting period and “Ctl4Ctl3Ctl2Ctl1” equals “0110,” respectively, switches  504 B,  506 A,  508 A, and  510 A are closed and thus the voltage Vref_dim  458  equals or substantially equals voltage  450 B (i.e., 6/8 of Vref  412 ). When “Ctl4Ctl3Ctl2Ctl1” equals “0101,” the voltage Vref_dim  458  equals to substantially equals voltage  450 C, and so forth. When “Ctl4Ctl3Ctl2Ctl1” equals “0000,” the voltage Vref_dim  458  equals or substantially equals voltage  450 H (i.e., 0/8 of Vref  412 ). Counter  454  may also count one more period and when “Ctl4Ctl3Ctl2Ctl1” equals “1000,” the voltage Vref_dim  458  equals or substantially equals that of a ground signal GND  511 . The ground signal GND  511  can be generated internally or externally to multiplexer  452 . Counter  454  can then stop counting. The logic relations of input and output signals for multiplexer  452  shown in  FIG. 4B , as discussed above, are summarized in Table 2 below. It is readily appreciated by one of ordinary skill in the art that the logic relations shown in Table 2 are for illustration purpose only and any other logic can be designed to achieve the same or similar voltage selection purpose. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Logic relations between input and output signals 
               
               
                 shown in FIG. 4B, when PWM input 402C = 0. 
               
             
          
           
               
                 PWM 
                   
                   
                   
                   
                   
                   
               
               
                 Input 
                 Ctl1 
                 Ctl2 
                 Ctl3 
                 Ctl4 
                 Vref_dim 
                 DIM 
               
               
                   
               
               
                 0 
                 1 
                 1 
                 1 
                 0 
                 Vref * 7/8 
                 1 
               
               
                 0 
                 0 
                 1 
                 1 
                 0 
                 Vref * 6/8 
                 1 
               
               
                 0 
                 1 
                 0 
                 1 
                 0 
                 Vref * 5/8 
                 1 
               
               
                 0 
                 0 
                 0 
                 1 
                 0 
                 Vref * 4/8 
                 1 
               
               
                 0 
                 1 
                 1 
                 0 
                 0 
                 Vref * 3/8 
                 1 
               
               
                 0 
                 0 
                 1 
                 0 
                 0 
                 Vref * 2/8 
                 1 
               
               
                 0 
                 1 
                 0 
                 0 
                 0 
                 Vref * 1/8 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 Vref * 0/8 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 1 
                 GND 
                 0 
               
               
                   
               
             
          
         
       
     
         [0057]    In some embodiments, counter  454  shown in  FIG. 4A  can have a counting frequency in the range of 100 kHz˜1 MHz for the Ctl1 output signal. That is, the output signal Ctl1  505 A, which is the fastest switching output signal among the four signals Ctl1˜Ctl4, can switch at the frequency of 100 kHz˜1 MHz. Signals Ctl2, Ctl3 and Ctl4, for example, can then be switching at a frequency that is a fraction of the Ctl1 frequency. For example, Ctl2  505 B can have a switching frequency of 50 KHz˜500 Khz, Ctl3  505 C can have a switching frequency of 25 KHz˜250 Khz, and so forth. It is readily appreciated by one skilled in the art that the switching frequencies of the control signals Ctl1, Ctl2, Ctl3 and Ctl4 can also have other desired relations. 
         [0058]      FIG. 4C  is a schematic diagram of another exemplary LED current regulator  440 B corresponding to the LED current regulator  440  as shown in  FIG. 3 . In  FIG. 4C , it will be readily appreciated by one of ordinary skill in the art that the illustrated blocks and circuit elements can be altered in their numbers (e.g., number of slave stages of the current mirror transistors are not limited to eight as shown in  FIG. 4C ) or their relative positions. LED current regulator  440 B can further include additional blocks or circuit elements. 
         [0059]    In  FIG. 4C , LED current regulator  440 B can include a power supply  541 , an operational amplifier  562 , a resistor Riset  551 , a transistor Q1  564 , a transistor M1  566 , two or more transistors  542 A˜ 542 H, two or more switches  546 A˜ 546 H, a selection circuit, which includes a counter  550 , and a resistor Rx1  571 . LED current regulator  440 B can receive one or more input signals including, for example, PWM input  402 C and Vref  412 , and generate one or more output signals including a dimming control signal DIM  483  and a voltage signal  549 . 
         [0060]    Similar to that in  FIG. 4A , operational amplifier  562  in  FIG. 4C  can enforce the voltage Vset  547  to be equal to or substantially equal to that of Vref  412 , depending on the characteristics of operation amplifier  562 , such as its gain, input frequency range, etc. Thus, the current flowing through transistor Q1  564  and transistor M1  566  can be equal to or substantially equal to the voltage value of Vref  412  divided by the resistance value of resistor Riset  551 . This current can be mirrored to the slave transistors M20  542 A˜M27  542 H. The control terminals of the master transistor M1  566  and the slave transistors M20  542 A˜M27  542 H are connected to each other. Thus, the current flowing through the slave transistors M20  542 A˜M27  542 H can closely follow the current in the master transistor M1  566 , depending on the current gain ratio (M20˜M27/M1) of the current mirror, which is related to relative gate dimensions of the master transistor M1  566  and the slave transistors M20  542 A˜M27  542 H. As an example, if switch  546 A is closed and the slave transistor M20  542 A is identical to the master transistor M1  566 , the current flowing through M20  542 A can be equal or substantially equal to that flowing through transistor M1  566 . Similarly, if any other switches S1  546 B˜S7  546 H are closed, current can flow through these slave transistors M21  542 B˜M27  542 H in relation to the current of transistor M1  566 . 
         [0061]    In some embodiments, switches  546 A˜ 546 H can be controlled by any logic including, for example, by counter  550 . Counter  550  can be any type of counter such as up/down counter, asynchronous (ripple) counter, synchronous counter, etc. Counter  550  can be binary coded, Gray coded, or coded with any other type of coding. 
         [0062]    Further, in  FIG. 4C , counter  550  can receive control signals to initiate counting, stop counting or reset the counter. One example of the control signal can be PWM input  402 C. For example, when PWM input  402 C rises to a high voltage level, counter  550  can initiate counting. The corresponding control signal DIM  483  also rises immediately, turning on LED current controllers  460 A˜ 460 N by turning off transistor Q0  575  as discussed above. 
         [0063]    In operation, all switches S0  546 A˜S7  546 H may be disconnected so that no current can flow. During counter  550 &#39;s first counting period, switch S0  546 A can be closed so that the total current flowing through transistors M20  542 A˜M27  542 H can be increased by the current flowing through transistor M20  542 A. For example, if all of the slave transistors M20  542 A˜M27  542 H are identical, then the total current is increased by ⅛. During counter  550 &#39;s second counting period, switch S1  546 B can be closed so that the total current can be further increased by the current flowing through transistor M21  542 B (e.g., increased by another ⅛), and so forth. When all switches S0  546 A˜S7  546 H are closed, the total current can be increased to a desired level (e.g., a current level that is required for LEDs  490  to have the highest light intensity) and counter  550  can then stop counting. The total current flowing through resistor Rx1  571  is the same as, or substantially the same as, the total current flowing through switches S0  546 A˜S7  546 H. The relations between input and output signals of counter  550 , when PWM input  402 C=1, are shown in Table 3 below. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Relations between input and output signals of counter 550, 
               
               
                 when PWM input 402C = 1. 
               
             
          
           
               
                 PWM 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 input 
                 S7 
                 S6 
                 S5 
                 S4 
                 S3 
                 S2 
                 S1 
                 S0 
                 DIM 
               
               
                   
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 1 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 1 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 1 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
             
          
         
       
     
         [0064]    As another example, when PWM input  402 C falls from a high voltage level to a low voltage level, counter  550  can also initiate counting. The corresponding control signal DIM  483 , however, may not fall until the total current flowing through LEDs  490  falls to its initial level. That is, signal DIM  483  falls only after all switches  546 A˜ 546 H are disconnected, and LED current controllers  460 A˜ 460 N are turned off so that no current is supplied to LEDs  490 . This allows LED current controllers  460 A˜ 460 N to continue supplying current to LEDs  490  corresponding to the voltage level of signal  549 , which reflects the total current flowing through the slave transistors M20  542 A˜M27  542 H. 
         [0065]    Further, in  FIG. 4C , initially, all switches S0  546 A˜S7  546 H may be closed so that current is flowing through all these switches. During counter  550 &#39;s first counting period, switch S7  546 H can be disconnected so that the total current flowing through transistors M20  542 A˜M27  542 H can be reduced by the current flowing through transistor M27  542 H. For example, if all of the slave transistors M20  542 A˜M27  542 H are identical, then the total current is reduced by ⅛. During counter  550 &#39;s second counting period, switch S6  546 G can be disconnected so that the total current flowing through transistors M20  542 A˜M27  542 H can be further reduced by the current flowing through transistor M26  542 G (e.g., reduced by another ⅛), and so forth. When all switches S0  546 A˜S7  546 H are disconnected, the total current can be reduced to zero or close to zero and counter  550  can then stop counting. Because resistor Rx1  571  is electrically coupled to switches S0  546 A˜S7  546 H, the total current flowing through Rx1  571  is the same as, or substantially the same as, the total current flowing through switches S0  546 A˜S7  546 H. The relations between input and output signals of counter  550 , when PWM input  402 C=0, are shown in Table 4 below. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Relations between input and output signals of counter 550, 
               
               
                 when PWM input 402C = 0. 
               
             
          
           
               
                 PWM 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Input 
                 S7 
                 S6 
                 S5 
                 S4 
                 S3 
                 S2 
                 S1 
                 S0 
                 DIM 
               
               
                   
               
               
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
         [0066]    As shown in  FIG. 4C , LED current regulator  440 B generates an output signal  549 . Signal  549  is a voltage signal that is converted by resistor Rx1  571  from the total current flowing through the slave transistors M20  542 A˜M27  542 H of the current mirrors. Signal  549  can be an input to one or more LED current controllers  460 A  460 N. Each of LED current controllers  460 A˜ 460 N can include an operational amplifier  569 , a transistor Q0  575 , a transistor Q2  572 , and a resistor Rx2  573 , corresponding to operational amplifier  168 , transistor Q0  173 , transistor Q2  174 , and resistor Rx2  178  as shown in  FIG. 2 , respectively. That is, LED current controllers  460 A˜ 460 N can be same as or similar to the corresponding portion of LED current controller  140  as shown in  FIG. 2  and thus the descriptions of these elements in LED current controllers  460 A˜ 460 N are not repeated. 
         [0067]    As shown in  FIG. 4C , LED current controllers  460 A˜ 460 N can also receive a dimming control signal DIM  483  generated from LED current regulator  440 B, similar to that in  FIG. 4A . DIM  483  can have a waveform  716  as shown in  FIG. 5B . The dimming control signal DIM  483 , which can be derived from PWM input  402 C, can generate a signal DIMB  574  for controlling transistor Q0  575 . DIM  483  can start to rise when PWM input  402 C rises, i.e., when the current of LEDs  490  rises above its initial level. DIM  483  may not fall until the current of LEDs  490  gradually falls to its initial level. After DIM  483  falls, LED current controllers  460 A˜ 460 N are turned off. 
         [0068]    Further, in  FIG. 4C , the dimming control signal DIMB  574  is coupled to a gate terminal of the transistor Q0  575 . A drain terminal of transistor Q0  575  is electrically coupled to a gate terminal of transistor Q2  572 , and a source terminal of transistor Q0  575  is coupled to the ground potential. A drain terminal of transistor Q2  572  is coupled to LEDs  490  through signals such as  492 A˜ 492 N, and a source terminal of transistor Q2  572  is coupled to the ground through resistor Rx2  573 . When the signal DIMB  574  is high, transistor Q0  575  is turned on, the voltage of the drain terminal of transistor Q0  575  is pulled toward ground, and transistor Q2  572  is turned off. Consequently, the current flowing through LEDs  490  can be reduced or eliminated. When the signal DIMB  574  is low, transistor Q0  575  is turned off, the voltage of the drain terminal of transistor Q0  575  is high, and transistor Q2  572  is on. The current flowing through LEDs  490  can be increased or maintained. 
         [0069]    Further, in  FIG. 4C  and similar to the discussion above by referring to  FIG. 2 , current flowing through LEDs  490  can be expressed as iLED=K×(Vref/Riset), where K=(M20−M27/M1)×(Rx1/Rx2). The current value will depend on which of the switches  546 A˜ 546 H is closed. 
         [0070]      FIG. 5A  is an exemplary timing diagram illustrating timing relations between waveform  702  (corresponding to PWM input  102 C shown in  FIG. 1 ), waveform  704  (corresponding to the current flowing through LEDs  190 A˜ 190 D shown in  FIG. 1 ), and waveform  706  (corresponding to the dimming control signal DIMB  172  shown in  FIG. 2 ). In  FIG. 5A , waveform  706  closely follows waveform  702 . That is, waveform  706  rises when waveform  702  rises and falls when waveform  702  falls. As shown in  FIG. 2 , DIMB  172  controls turn-on and turn-off of the LED current controller  140 . Therefore, the current flowing through LEDs  190 A˜ 190 D (i.e., iLED) can have a sudden change and exhibits only two current levels, i.e., a high current level and a low current level. The average LED current iLED I LED-AVG  can be calculated as I LED-AVG =IL*D*T/T=IL*D, where IL is the maximum value of LED current, T is the period of PWM signal  102 C, D is the duty cycle of the PWM signal  102 C and D*T is the period of time that PWM signal  102 C is high. An exemplary range of the frequency of PWM signal  102 C can be from 200 Hz to 20 KHz. 
         [0071]      FIG. 5B  is an exemplary timing diagram illustrating timing relations between waveform  712  (corresponding to PWM input  402 C shown in  FIG. 3 ), waveform  714  (corresponding to the current flowing through LEDs  490  shown in  FIG. 3 ), and waveform  716  (corresponding to the dimming control signal DIM  483  shown in  FIG. 3 ). In  FIG. 5B , waveform  716  rises when waveform  712  rises. Waveform of the dimming control signal DIM  483  (i.e., waveform  716 ), however, does not fall immediately after waveform of PWM input  402 C (i.e., waveform  712 ) falls. Instead, it falls when the LED current iLED (i.e., waveform  714 ) falls to its initial level. And the LED current iLED, can rise or fall gradually as shown in  FIG. 5B . In the example shown in  FIG. 5B , LED current iLED rises or falls in eight smaller steps, corresponding to the eight voltage levels of signals  450 A˜ 450 H shown in  FIG. 4A  or the eight current levels of the slave current mirror stages M20  542 A˜M27  542 H shown in  FIG. 4C . 
         [0072]    Further, in  FIG. 5B , the average value of LED current iLED can still be kept the same as that in the circuit shown in  FIG. 1 , but the undesired current fluctuation and ripple effect can be suppressed or eliminated. As an illustration, when the LED current iLED rises of falls in eight steps, the average LED current I LED-AVG  can be calculated as I LED-AVG =IL*D*T*(⅛+ 2/8+⅜+ 4/8+⅝+ 6/8+⅞+ 8/8)+IL*D*T*(⅞+ 6/8+⅝+ 4/8+⅜+ 2/8+⅛+ 0/8)+IL*(D*T−8*D*T)=IL*D, where IL is the maximum value of the LED current, T is the period of the PWM input, D is the duty cycle of the PWM input, and D*T is the period of time that the PWM input is high. 
         [0073]    In the preceding specification, the subject matter has been described with reference to specific exemplary embodiments. It will, however, be evident that various modifications and changes may be made without departing from the broader spirit and scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded as illustrative rather than restrictive. Other embodiments may be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein.