Abstract:
Disclosed are LED controllers for dimming. An LED controller includes a current driver, a pulse-width modulator, a feedback circuit, and a decoupling circuit. The current driver, selectively in response to a dimming signal, causes a driving current flowing through one LED string. The dimming signal is capable of defining a dimming ON period and a dimming OFF period. The pulse-width modulator generates a PWM signal to control a power switch, in order to buildup a driving voltage at a power node of the LED string. The PWM signal is generated in response to a compensation signal. The feedback circuit, based upon a feedback voltage from the light emitting device, drives a compensation capacitor to generate the compensation signal. The decoupling circuit defines a decoupling period at the start of the dimming ON period and causes the feedback circuit not driving the capacitor during the decoupling period.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Taiwan Application Series Number 102132127 filed on Sep. 6, 2013, which is incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates generally to control circuit and method for driving light emitting diodes (LEDs), and more particularly to the control of dimming LEDs. 
         [0003]    LEDs have obtained popularity in the field of lighting and backlight modules, due to their excellent lighting efficiency, compact product size, and long lifespan. For instance, the backlight modules used in computer monitors or televisions have largely turned to employ LEDs as their light sources, instead of cold cathode fluorescent lamps that were commonly used years ago. 
         [0004]      FIG. 1  demonstrates a LED driver  10  in the art, which drives four LED strings S 1 ˜S 4  in parallel. Even though each LED strings in  FIG. 1  has LEDs connected in series, one LED string in other example might have only one LED. A booster  12  boosts up an input power voltage V IN  and generates a driving power voltage V OUT  to a common power node of the LED strings S 1 ˜S 4 . Current control circuits CD 1 ˜CD 4  govern the driving current I 1   LED ˜I 4   LED  through LED strings S 1 ˜S 4  respectively. LED controller  14  controls the operation of the current control circuit CD 1 ˜CD 4  and the booster  12 . 
         [0005]      FIG. 2  shows a conventional LED controller  14  in the art. A minimum voltage selector  20  provides a minimum feedback voltage V FBMIN  based on the minimum of the feedback voltages V 1   FB ˜V 4   FB , which are the voltages at feedback nodes FB 1 ˜FB 4  of LED strings S 1 ˜S 4 . Transconductor  22  drives a compensation node COM based on the difference between the minimum feedback voltage V FBMIN  and a reference voltage V REF , so as to charge or discharge a compensation capacitor  23  and to build a compensation voltage V COM . A pulse-width modulator  24 , in response to the compensation voltage V COM , generates a PWM signal S DRV , which turns ON and OFF the power switch  28  periodically. Simply speaking, minimum voltage selector  20 , transconductor  22 , pulse-width modulator  24 , booster  12 , and LED strings S 1 ˜S 4  as a whole forms a loop with a negative loop gain, capable of stabilizing the minimum feedback voltage V FBMIN  at the reference voltage V REF . 
         [0006]    Constant current driver CC 1 ˜CC 4  correspond to current control circuits CD 1 ˜CD 4  respectively. Only the constant current driver CC 1  is detailed herein because other constant current drivers are analogous in view of the teaching of the constant current driver CC 1 . The constant current driver CC 1  has an operational amplifier  30 , which is configured to make a current sense voltage V 1   CS  about the same with the setting voltage V CSSET , which, depending on the dimming signal S DIM , is either 0V or a predetermined voltage V CSON . As the current sense voltage V 1   CS , in a way, represents the driving current I 1   LED , the constant current driver CC 1  can stabilize the driving current I 1   LED . 
         [0007]    The dimming signal S DIM  is capable of adjusting the brightness of the LED strings S 1 ˜S 4 , or dimming the LED strings S 1 ˜S 4 . The dimming signal S DIM  is a PWM signal, for example. When the dimming signal S DIM  is “1” in logic, the minimum feedback voltage V FBMIN  could be stabilized to be about the reference voltage V REF  and each of driving currents I 1   LED ˜I 4   LED  is about a constant corresponding to the predetermined voltage V CSON , such that LED strings S 1 ˜S 4  emit light continuously and stably. In the opposite, when the dimming signal S DIM  is “0” in logic, LED controller  14  constantly turns OFF the power switch  28  in the booster  12 , and all the driving currents I 1   LED ˜I 4   LED  are to be 0 A, such that LED strings S 1 ˜S 4  do not emit light. This kind of dimming control is generally called PWM dimming. Here in this specification, a dimming ON period T DIM-ON  refers to the period of time when the dimming signal S DIM  is “1”, and a dimming OFF period T DIM-OFF  to the period of time when the dimming signal S DIM  is “0”. A dimming duty cycle, the ratio of one dimming ON period T DIM-ON  to one cycle time of the dimming signal S DIM , is a factor substantially corresponding to the brightness of the LED strings S 1 ˜S 4 . Dimming linearity refers to the correlation between the brightness of a light source and the dimming duty cycle. Perfect dimming linearity means the brightness of a light source is entirely proportional to the dimming duty cycle, and is always a dream that designers of lighting apparatuses or lighting controllers desire to achieve. 
       SUMMARY 
       [0008]    Embodiments of the present invention provide a control circuit capable of controlling the light dimming of a light emitting device. A current driver is in response to a dimming signal and selectively causes a driving current flowing through the light emitting device. The dimming signal is capable of defining a dimming ON period and a dimming OFF period. A pulse-width modulator generates a PWM signal to control a power switch, in order to build up a driving voltage at a power node of the light emitting device. The PWM signal is generated in response to a compensation signal. A feedback circuit, based upon a feedback voltage from the light emitting device, drives a compensation capacitor to generate the compensation signal. A decoupling circuit defines a decoupling period at the start of the dimming ON period and causes the feedback circuit not driving the capacitor during the decoupling period. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted. 
           [0010]    The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0011]      FIG. 1  demonstrates a LED driver  10  in the art, which drives four LED strings S 1 ˜S 4  in parallel; 
           [0012]      FIG. 2  shows a conventional LED controller; 
           [0013]      FIG. 3  shows waveforms possibly resulted from the circuits demonstrated in  FIGS. 1 and 2 ; 
           [0014]      FIG. 4  demonstrates a LED controller according to one embodiment of the invention; 
           [0015]      FIG. 5  shows waveforms for signals when the LED controller of  FIG. 4  replaces the LED controller in  FIG. 1 ; 
           [0016]      FIG. 6  exemplifies a transconductor; 
           [0017]      FIG. 7  demonstrates another LED controller according to embodiments of the invention; and 
           [0018]      FIG. 8  shows waveforms for signals when the LED controller of  FIG. 7  replaces the LED controller in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 3  shows waveforms possibly resulted from the circuits demonstrated in  FIGS. 1 and 2 , and has, from top to bottom, the dimming signal S DIM , the minimum feedback voltage V FBMIN , the compensation voltage V COM , the control voltage V 1   GAT  at the control node GAT 1  in the constant current driver CC 1 , the current sense voltage V 1   CS , and the PWM signal S DRV . 
         [0020]    A short dimming ON period T DIN-ON  could worsen dimming linearity or cause flickering to the LED strings S 1 ˜S 4  in  FIG. 1  due to the limited driving ability of the constant current driver CC 1 . 
         [0021]    Please refer to  FIG. 3 . It is a common practice that, during the dimming OFF period T DIM-OFF , the current sense voltage V 1   CS  is 0V and the compensation voltage V COM  is held unchanged, staying as what it was at the end of a previous dimming ON period T DIM-ON , as demonstrated in the dimming OFF period T DIM-OFF  before t 0  when the compensation voltage is about a value V COMON . As there is no current going through the LED strings S 1 ˜S 4  during the dimming OFF period T DIM-OFF , the voltage drop across each LED string is 0V, and the minimum feedback voltage V FB-MIN  will be about the same as the driving power voltage V OUT , which could be as high as several tens volt. 
         [0022]    At time t 0 , the dimming signal S DIM  turns to “1” from “0” and a dimming ON time T DIM-ON  starts. The operational amplifier  30  starts to pull up the control voltage V 1   GAT , in hopes of raising the current sense voltage V 1   CS  to the predetermined voltage V CSON  as soon as possible. The control node GAT 1  always has a parasitic capacitive load, however, which could be considerably large due to the existence of an external power transistor controlling the driving current I 1   LED , and often causes the control voltage V 1   GAT  to ramp up slowly. Only after the control voltage V 1   GAT  exceeds a certain threshold, the current sense voltage V 1   CS  starts slowly approaching to the predetermined voltage V CSON , in order to make the driving current I 1   LED  to stay at its steady state during the dimming ON period. The predetermined voltage V CSON  is about 0.4V for example. 
         [0023]    As long as the driving current I 1   LED  increases, the minimum feedback voltage V FBMIN  drops. Nevertheless, at time t 0  when the driving current I 1   LED  is still 0 A, the minimum feedback voltage V FBMIN  is so high and exceeds the reference voltage V REF , such that the transconductor  22  in the feedback loop discharges the compensation capacitor  23  and abruptly pulls down the compensation voltage V COM . As known in the art, a low compensation voltage V COM  induces the PWM signal S DRV  with a small duty cycle, as demonstrated by the PWM signal S DRV  in the period from time t 0  to t 1 . 
         [0024]    At time t 1 , the minimum feedback voltage V FBMIN  has dropped below the reference voltage V REF , and the compensation voltage V COM  starts to rise and approach the value V COMON , which is the steady value for the compensation voltage V COM  during the dimming ON period T DIM-ON . 
         [0025]    At time t 2 , a dimming ON period T DIM-ON  ends and a dimming OFF period T DIM-OFF  follows. Apparently from the period between times t 1  and t 2 , for a dimming ON period T DIM-ON , the minimum feedback voltage V FBMIN  finally stabilizes to equal to the reference voltage V REF , and the current sense voltage V 1   CS  to the predetermined voltage V CSON , which corresponds to a steady value of the driving current I 1   LED ˜I 4   LED . 
         [0026]    The compensation voltage V COM  is far below the value V COMON  during the starting period from t 0  to t 1 . As the compensation voltage V COM  decides the power transferred to the LED strings S 1 ˜S 4 , the power delivered during this starting period is much less than what the LED strings S 1 ˜S 4  need during their steady condition. The current sense voltage V 1   CS , as a result, ramps up slowly and it inevitably takes a relatively long time for the current sense voltage V 1   CS  to reach the predetermined voltage V CSON . This phenomenon implies that the LED string S 1 ˜S 4  might not have been fully driven before the beginning of the dimming OFF time T DIM-OFF  if the dimming ON time T DIM-ON  is very short, and poor dimming linearity is expected. Furthermore, this phenomenon could cause unfriendly flickering when the dimming ON time T DIM-ON  is short. 
         [0027]      FIG. 4  demonstrates a LED controller  60  according to one embodiment of the invention, which controls the light emitting of 4 LED strings S 1 ˜S 4 . The invention is not limited to however. One embodiment of the invention controls only one LED string, and another might control more than four LED strings. The LED controller  60  has some parts similar or the same with some parts of the LED controller  14  in  FIG. 2 , and details of these parts are omitted herein for brevity because they are comprehensible to persons in the art. 
         [0028]    Unlike the LED controller  14  in  FIG. 2 , the LED controller  60  has an additional decoupling circuit  62  connected to the EN node of the transconductor  64 . The decoupling circuit  62  provides enabling signal S EN  in response to the dimming signal S DIM . If the enabling signal S EN  is “1” in logic, the transconductor  64 , which is a kinde of a feedback circuit, charges or discharges the compensation capacitor  23  based on the difference between the minimum feedback voltage V FBMIN  and the reference voltage V REF . In the opposite, when the enabling signal S EN  is “0” in logic, the output of the transconductor  64  becomes high impedance and the compensation voltage V COM  is held to have the same value as it was just before the enabling signal turned to “0”. 
         [0029]    In one embodiment, the decoupling circuit  62  has a rising-edge-triggered pulse generator  66  and a logic gate. The rising-edge-triggered pulse generator  66  provides a pulse with a pulse width when the dimming signal S DIM  is having a rising edge, and this pulse width defines a decoupling period T FORCE , which starts at the beginning of the dimming ON period T DIM-ON . This pulse width could be fixed to be 10 micro seconds, or two switch cycle times of the PWM signal S DRV , for example. Derivable from  FIG. 4 , the enabling signal S EN  is “0” during both the dimming OFF period T DIM-OFF  and the decoupling period T FORCE , and is “1” during the dimming ON period T DIM-ON  except the decoupling period T FORCE . 
         [0030]      FIG. 5  shows waveforms for signals when the LED controller  60  of  FIG. 4  replaces the LED controller  14  in  FIG. 1 . From top to bottom, the waveforms in  FIG. 5  are the dimming signal S DIM , the minimum feedback voltage V FBMIN , and the pulse signal S PLS , the control voltage V 1   GAT , the current sense voltage V 1   CS , and the PWM signal S DRV . 
         [0031]    As shown in  FIG. 5 , during the decoupling period T FORCE , which is a beginning period of time within the dimming ON period T DIM-ON , the compensation voltage V COM  is held to be the value V COMON , as it was at the end of the previous dimming ON period T DIM-ON , because the output of the transconductor  64  is in high impedance, not driving the compensation capacitor  23 . What is shown in the beginning period of the dimming ON period T DIM-ON  in  FIG. 5  is very different with the same period of time in  FIG. 3 , which shows the compensation voltage V COM  dropping quickly below the value V COMON  at the beginning of the dimming ON period T DIM-ON . The compensation voltage V COM  in  FIG. 5  is able to make the PWM signal S DRV  have a high duty cycle at the beginning of the dimming ON period T DIM-ON , and, at the same time, immediately causes the booster  12  to transfer or deliver the relatively-high power which the LED strings S 1 ˜S 4  require for steadily emitting light during a dimming ON period T DIM-ON . It is predictable that the current sense voltage V 1   CS  and the minimum feedback voltage V FBMIN , as shown in  FIG. 5 , both soon reach to their steady values, respectively, which are the predetermined voltage V CSON  and the reference voltage V REF . 
         [0032]    After the decoupling period T FORCE , the enabling signal S EN  becomes “1” in logic, and the transconductor  64  starts driving the compensation capacitor  23  in response to the difference between the minimum feedback voltage V FBMIN  and the reference voltage V REF  until the start of the dimming OFF period T DIM-OFF . 
         [0033]    In comparison with what is shown in  FIG. 3 , the dimming ON period T DIM-ON  in  FIG. 5  starts with the current sense voltage V 1   CS  and the minimum feedback voltage V FBMIN  both approaching to their steady values in a relatively quick rate. Accordingly, if the LED controller  60  in  FIG. 1  is replaced by the LED control  14 , dimming linearity is probably improved and the flickering to the LED strings S 1 ˜S 4  might be eliminated. 
         [0034]      FIG. 6  exemplifies the transconductor  64 , which includes a switch  68  and a transconductor  70 . When the switch  68  performs an open circuit, the transconductor  70  is disconnected from the compensation capacitor driven by the transconductor  64 , and the output of the transconductor  64  is in high impedance. When the switch  68  performs a short circuit, the transconductor  70  generates output current to the output node of the transconductor  64  based on the difference between the minimum feedback voltage V FBMIN  and the reference voltage V REF .  FIG. 6  is not intended to limit the embodiment of the transconductor  64 , however. Based on the teaching in this specification, circuit designers could develop other kinds of transconductor having functions or characteristics similar or the same with the transconductor  64 . 
         [0035]      FIG. 7  demonstrates another LED controller  80  according to embodiments of the invention. Unlike the decoupling circuit  62  in  FIG. 4 , the decoupling circuit  82  in  FIG. 7  defines a decoupling period T FORCE  whose length is not a constant all the time, because, in  FIG. 7 , the end of the decoupling period T FORCE  is in response to the current sense voltages V 1   CS ˜V 4   CS . 
         [0036]    The decoupling circuit  82  includes a maximum selector  84 , a comparator  86 , and a logic gate  88 . Maximum selector  84  provides a maximum current sense voltage V CSMAX  based on the maximum among the current sense voltages V 1   CS ˜V 4   CS . Derivable from the decoupling circuit  82 , when the dimming signal S DIM  turns into “1” from “0” to claim the start of a dimming ON period T DIM-ON , the enabling signal S EN  remains at “0” because all the current sense voltages V 1   CS ˜V 4   CS  are still at about 0V. Only if the maximum among them rises to a certain level such that the maximum current sense voltage V CSMAX  exceeds the predetermined reference voltage V CS-OK , then the comparator  86  outputs “1” in logic to make the enabling signal S EN  becoming “1”. In other words, the moment when the dimming signal S DIM  turns into “1” from “0” determines the start of the decoupling period T FORCE , but it is the maximum among the current sense voltages V 1   CS ˜V 4   CS  who determines the end of the decoupling period T FORCE . 
         [0037]    In one embodiment, when the maximum among the current sense voltages V 1   CS ˜V 4   CS  exceeds the predetermined reference voltage V CS ˜V OK , the decoupling circuit claims the conclusion of the decoupling period T FORCE , where the predetermined reference voltage V CS-OK  is close to, but less than the predetermined voltage V CSON , the steady value that all the current sense voltages V 1   CS ˜V 4   CS  approach for lightening the LED strings S 1 ˜S 4 . For instance, the predetermined voltage V CSON  is about 0.4V and the predetermined reference voltage V CS-OK  0.3V. The predetermined reference voltage V CS-OK  corresponds to a predetermined driving current I CS-OK . When at least one of the driving currents I 1   LED ˜I 4   LED  exceeds the predetermined driving current I CS-OK , the driving currents I 1   LED ˜I 4   LED  should be very close to their steady values and the decoupling circuit ends the decoupling period T FORCE . 
         [0038]      FIG. 8  shows waveforms for signals when the LED controller  80  of  FIG. 7  replaces the LED controller  14  in  FIG. 1 . From top to bottom, the waveforms in  FIG. 5  are the dimming signal S DIM , the minimum feedback voltage V FBMIN , and the enabling signal S EN , the compensation signal V COM , the control voltage V 1   GAT , the current sense voltage V 1   CS , and the PWM signal S DRV . As shown in  FIG. 8 , the decoupling period T FORCE  ends when the current sense voltage V 1   CS  exceeds the predetermined reference voltage V CS-OK . Similar with  FIG. 5 , the decoupling period T FORCE  in  FIG. 8  also causes the current sense voltage V 1   CS  to rise faster and the minimum feedback voltage V FBMIN  to fall quicker. Accordingly, better dimming linearity could be expected and flickering to LED strings might be eliminated by the LED controller  80  replacing the LED controller  14  in  FIG. 1 . 
         [0039]    During a decoupling period T FORCE , whether it is defined by the decoupling circuit  62  in  FIG. 4  or the decoupling circuit  82  in  FIG. 7 , the control loop fed to the booster  12  is broken and the booster  12  is blindly forced to deliver certain power to the common power node of the LED strings S 1 ˜S 4 . In case that, due to some unknown reasons, the compensation voltage V COM  drifts high away from the value V COMON  and the decoupling circuit  62  in  FIG. 4  defines a over-long decoupling period T FORCE , the booster  12  might build up an over-high voltage at the common power node of the LED strings S 1 ˜S 4 , causing damage or risk. Comparatively, the decoupling circuit  82  could prevent this over-high voltage by ending the decoupling period T FORCE  at the moment when one of the current sense voltages V 1   CS ˜V 4   CS  is more than the predetermined reference voltage V CS-OK , or, in other words, almost reaches its steady value, such that the control loop to the booster  12  is timely resumed to stabilize the voltage at the common power node. 
         [0040]    Embodiments of the invention introduce at the beginning of a dimming ON period a decoupling period, during which a decoupling circuit stops a transconductor driving a compensation capacitor, and makes the compensation capacitor hold a compensation voltage, such that a booster is forced to deliver certain power to drive LED strings. It is believed that embodiments of the invention could result in better dimming linearity and avoid the problem of flickering. 
         [0041]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.