Patent Publication Number: US-11032886-B2

Title: Light emitting diode driver and driving method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of U.S. Provisional application Ser. No. 62/881,335, filed on Jul. 31, 2019. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND 
     Field of the Invention 
     The invention relates to a light-emitting diode (LED) driver and a driving method thereof and more particularly, to an LED driver capable of adjusting a calculation frequency of brightness codes and a driving method thereof. 
     Description of Related Art 
     Generally speaking, a back-light source of a display panel is composed of light emitting diodes (LEDs). In a technical field related to an LED driving technique, brightness of the LEDs is usually determined by a duty cycle of a pulse width modulation (PWM) signal.  FIG. 1  is a schematic block diagram illustrating a light emitting diode (LED) driving system. Referring to  FIG. 1 , an LED driving system  100  includes an LED driver  110 . The LED driver  110  includes a brightness code calculator  111  and a driving current generation circuit  113 . The brightness code calculator  111  is in charge of calculating a duty cycle of a pulse width modulation (PWM) signal that is sent in. Specifically, the brightness code calculator  111  may calculate the duty cycle that is a ratio of an active time (in which the PWM signal is at an active voltage level, such as high voltage level) to the period of the PWM signal  101  and thereby generate a plurality of brightness codes  112  according to the calculated duty cycle. The driving current generation circuit  113  outputs corresponding driving currents to LEDs respectively according to the plurality of brightness codes  112 . 
     In a normal operation, the brightness code calculator  111  continuously generates the brightness codes  112 , so as to instantly update the brightness required by the LED strings  120 . However, when an abnormality phenomenon occurs to the PWM signal  101  received by the brightness code calculation circuit  111  (e.g., a signal phase change caused by switching different display cards), it is directly reflected to the brightness codes  112 , which leads to unstable brightness of the LED strings  120  and causes visual flickers. In addition, since the brightness code calculator  111  has to keep calculating the duty cycle of the PWM signal and generate the brightness codes, the LED driver  110  may produce a great amount of power consumption. 
     SUMMARY 
     The application is capable of adjusting an execution frequency of a calculation operation of a brightness code calculation circuit of a light emitting diode (LED) driver to prevent an issue of brightness flickers from occurring to LEDs caused by an abnormal state. 
     The application provides an LED driver and a driving method thereof. The LED driver includes a brightness code calculation circuit, a driving current generation circuit and a control circuit. The brightness code calculation circuit is configured to to receive a pulse width modulation (PWM) signal and perform a calculation operation comprising calculating a duty cycle of the PWM signal with respect to a period of the PWM signal and generating a brightness code according to the duty cycle of the PWM signal. The driving current generation circuit is coupled to the brightness code calculation circuit, and configured to generate a driving current according to the brightness code. The control circuit is coupled to the brightness code calculation circuit, and configured to detect a plurality of brightness codes generated by the brightness code calculation circuit repeatedly performing the calculation operation and determine whether or not to adjust an execution frequency of the calculation operation of the brightness code calculation circuit according to the brightness codes. 
     In an embodiment of the application, the adjustment of the execution frequency of the calculation operation of the brightness code calculation circuit is determined by generating a control signal sent to the brightness code calculation circuit, in response to a comparison result indicating that the plurality of brightness codes is the same. 
     In an embodiment of the application, the control circuit is a dynamic window modulator configured to adjust the execution frequency of the calculation operation of the brightness code calculation circuit according to at least one of the control signal, the PWM signal and the plurality of brightness codes. 
     In an embodiment of the application, the brightness code calculation circuit includes a calculation circuit and a memory circuit. The calculation circuit is configured to calculate the duty cycle of the PWM signal with respect to the period of the PWM signal and generate the brightness code according to the duty cycle. The memory circuit is configured to store the brightness code. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic block diagram illustrating a light emitting diode (LED) driving system. 
         FIG. 2  is a schematic diagram of an LED driver according to an embodiment of the application. 
         FIG. 3  is a schematic block diagram of an LED driving system according to an embodiment of the application. 
         FIG. 4  is a schematic block diagram of an LED driving system according to an embodiment of the application. 
         FIG. 5  is a flowchart illustrating of an LED driving method according to an embodiment of the application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to the exemplary embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 2  is a schematic block diagram of an LED driving system according to an embodiment of the application. Referring to  FIG. 2 , an LED driving system  200  includes an LED driver  210  and a load apparatus including one or more LED strings  220 . The LED driver  210  may generate a driving current to drive the LED string  220 . The LED driver  210  includes a brightness code calculation circuit  211 , a driving current generation circuit  213  and a dynamic window modulator  214 . 
     The brightness code calculation circuit  211  receives a pulse width modulation (PWM) signal  201  and performs a calculation operation, including to calculate a duty cycle of the PWM signal  201  with respect to a period of the PWM signal and to generate a brightness code  212  corresponding to the calculated duty cycle. Specifically, the duty cycle of the PWM signal  201  is a ratio (e.g., 20%) of an active time, in which the PWM signal  201  is at an active voltage level such as a high voltage level, to the period of the PWM signal  201 . The brightness code calculation circuit  211  may continuously or intermittently execute the calculation operation to obtain a plurality of brightness codes  212  with respect to a plurality of periods (which may be consecutive or not) of the PWM signal  201 . Continuously executing the calculation operation and intermittently executing the calculation operation are regarded as repeatedly executing the calculation operation. Furthermore, the brightness code calculation circuit  211  may also be divided in to a calculation circuit  2111  and a memory circuit  2112 . The calculation circuit  2111  is configured to execute the calculation operation described above. The memory circuit  2112  may store the brightness code  212  generated by the calculation circuit  2111  executing the calculation operation and may update the stored brightness code  212  every time when the calculation circuit  2111  generates the brightness code. 
     The driving current generation circuit  213  may generate a driving current according to each of the brightness codes  212  generated by the brightness code calculation circuit  211  to drive the LED string  220 . A first control signal s may be sent from outside the LED driver  210  to the brightness code calculation circuit  211  and the dynamic window modulator  214 . When the first control signal s is at a first voltage level (e.g., a low voltage level), the brightness code calculation circuit  211  may continuously execute the calculation operation on the PWM signals  201  and continuously update the brightness code  212 , which ensures that the LED string  220  is capable of instantly presenting the brightness as required. When the first control signal s is at a second voltage level (e.g., a high voltage level), the brightness code calculation circuit  211  may intermittently execute calculation operation on the PWM signals  201  based on an execution frequency, and the dynamic window modulator  214  may detect a plurality of brightness codes  212  and generate a control signal  215  according to the plurality of brightness codes  212 . The control signal  215  is sent to the brightness code calculation circuit  211  for adjusting the execution frequency of the calculation operation. 
     Specifically, the dynamic window modulator  214  may be configured to compare the brightness codes  212  to examine whether the brightness codes  212  are the same or not, and thereby determine whether or not to adjust the execution frequency of the calculation operation of the brightness code calculation circuit  211  according to a comparison result indicating the brightness codes are the same or not. The control signal  215  sent to the brightness code calculation circuit  211  is utilized for indicating whether or not to adjust the execution frequency of the calculation operation. In response to the comparison result indicating that the brightness codes are the same, the dynamic window modulator  214  generates the control signal  215  sent to the brightness code calculation circuit  211  to adjust the execution frequency of the calculation operation of the brightness code calculation circuit  211 . In response to the comparison result indicating that the brightness codes are not the same, the dynamic window modulator  214  generates the control signal  215  sent to the brightness code calculation circuit  211  to not to adjust the execution frequency of the calculation operation of the brightness code calculation circuit  211 . In another aspect, intermittently executing the calculation operation is intermittently suspending the calculation operation. The brightness code generation circuit  211  may suspend the calculation operation during some periods of an abnormal PWM signal and not generate the brightness codes. In this circumstance, the brightness code  212  stored by the memory circuit  2112  is the brightness code calculated before the occurrence of the abnormal PWM signal. In this way, the driving current generation circuit  213  may output the driving current according to a previous brightness code  212  to prevent the issue of brightness flickers. For example, when a computer system (where the LED driver  210  is used) equipped with two graphic cards performs graphic card switching, the PWM signal input to the LED driver  210  before and after graphic card switching may be not synchronized and is regarded as an abnormal PWM signal, and the LED strings may generate flickers. By configuring the first control signal s to be at a voltage level that controls the brightness code generation circuit  211  to intermittently execute calculation operation, the periods of abnormal PWM signal can be ignored and the driving current may be more stable. 
     In brief, the application may determine to intermittently execute/suspend the calculation operation of the brightness code calculation circuit  211  through the first control signal s to maintain brightness uniformity of the LED string  220 . Furthermore, the brightness code calculation circuit  211  may resume the calculation operation when the first control signal s is converted from the second voltage level to the first voltage level. Or, alternatively, the brightness code calculation circuit  211  may suspend the calculation operation of the brightness code calculation circuit  211  within a predetermined time period and resume the aforementioned calculation operation after the predetermined time period is passed. Two embodiments are provided to describe details related to adjusting the calculation operation of the brightness code calculation circuit  211  by the dynamic window modulator  214  according to the PWM signal  201  or the plurality of brightness codes  212 . 
       FIG. 3  is a schematic block diagram of an LED driving system according to an embodiment of the application. Referring to  FIG. 3 , an LED driving system  300  includes an LED driver  310  and one or more LED strings  320 . The LED driver  310  may generate a driving current to drive each of the LED strings  320 . The LED driver  310  includes a brightness code calculation circuit  311 , a driving current generation circuit  313  and a dynamic window modulator. The brightness code calculation circuit  311  may be further divided in to a calculation circuit  3111  and a memory circuit  3112 . Details related to the brightness code calculation circuit  311 , the calculation circuit  3111 , the memory circuit  3112 , the driving current generation circuit  313  and the LED string  320  may be inferred with reference to the description contents related to same elements in the embodiment illustrated in  FIG. 2  and thus, will not be repeated. 
     In the present embodiment, the dynamic window modulator may be implemented by a pattern generation circuit  314 . The pattern generation circuit  314  may detect a plurality of brightness codes calculated by the brightness code calculation circuit  311  to determine whether a plurality of continuous brightness codes  312  have the same value. When N brightness codes  312  detected by the pattern generation circuit  314  have the same value, an execution frequency of the calculation operation of the brightness code calculation circuit  311  is reduced from one calculation per period to one calculation per two periods. Furthermore, when the number of the brightness codes  312  having the same value is accumulated to 2N, the execution frequency of the calculation operation of the brightness code calculation circuit  311  is again reduced from one calculation per two periods to one calculation per three or more periods. N is an integer greater than or equal to 2. In another embodiment, the pattern generation circuit  314  may also detect the plurality of brightness codes  312  within a predetermined time period to reduce the execution frequency of the calculation operation of the brightness code calculation circuit  311  in a condition that the plurality of brightness codes  312  have the same value within the predetermined time period. 
     In this way, in the condition that the PWM signal  301  temporarily does not change (i.e., the plurality of brightness codes  312  have the same value), the number of times of the calculation of the brightness code calculation circuit  311  may be reduced, thereby indirectly preventing the issue of brightness flickers from occurring to the LED string  320  due to the abnormal state as well as achieving an effect of reducing static power consumption at the same time. 
       FIG. 4  is a schematic block diagram of an LED driving system according to an embodiment of the application. Referring to  FIG. 4 , an LED driving system  400  includes an LED driver  410  and one or more LED strings  420 . The LED driver  410  may generate a driving current to drive each of the LED strings  420 . The LED driver  410  includes a brightness code calculation circuit  411 , a driving current generation circuit  413  and a dynamic window modulator. The brightness code calculation circuit  411  may be further divided in to a calculation circuit  4111  and a memory circuit  4112 . Details related to the brightness code calculation circuit  411 , the calculation circuit  4111 , the memory circuit  4112 , the driving current generation circuit  413  and the LED string  420  may be inferred with reference to the description contents related to same elements in the embodiment illustrated in  FIG. 2  and thus, will not be repeated. 
     In the present embodiment, the dynamic window modulator  214  may be implemented by a counting circuit  414 . The counting circuit  414  may receive a PWM signal  401  and the first control signal s to determine whether to reduce an execution frequency of the brightness code calculation circuit  411  according to the PWM signal  401 . The counting circuit  414  may generate a second control signal  415  in response to the first control signal s at a high voltage level, to control the brightness code calculation circuit  411  to stop the calculation operation. Furthermore, the counting circuit  414  may control the brightness code calculation circuit  411  to resume the calculation operation after counting to a predetermined number of periods of the PWM signals. Alternately, the counting circuit  414  may detect the PWM signal  401  or the corresponding brightness code  412  to check whether a duty cycle of the PWM signal  401  in a predetermined number of continuous periods are the same. The counting circuit  414  may generate a second control signal  415  in response to that the duty cycle of the PWM signal  401  in a predetermined number of continuous periods are the same and reduce the execution frequency of the brightness code calculation circuit  411  through the second control signal  415 . Similarly, the execution frequency of the brightness code calculation circuit  411  may be continuously reduced, without being reduced to 0. In this way, in a condition that the PWM signal  401  temporarily does not change (i.e., the plurality of brightness codes have the same value), the number of times of the calculation of the brightness code calculation circuit  411  may be reduced, thereby indirectly preventing the issue of brightness flickers from occurring to the LED string  420  due to the abnormal state as well as achieving an effect of reducing power consumption at the same time. 
     It should be noted that even though the first control signal s is not illustrated in  FIG. 3 , the application is not intent to limit the pattern generation circuit  314  of the embodiment illustrated in  FIG. 3  from being capable of simultaneously receiving the first control signal s. 
       FIG. 5  is a flowchart illustrating of an LED driving method according to an embodiment of the application. Referring to  FIG. 5 , the LED driving method may include: receiving a PWM signal and performing a calculation operation comprising calculating a duty cycle of the PWM signal with respect to a period of the PWM signal and generating a brightness code according to the duty cycle of the PWM signal (step S 510 ); generating a driving current according to the brightness code (step S 520 ); detecting a plurality of brightness codes generated by the brightness code calculation circuit repeatedly performing the calculation operation (step S 530 ); and determining whether or not to adjust an execution frequency of the calculation operation according to the plurality of brightness codes (step S 540 ). More detailed steps may be inferred with reference to the embodiments illustrated in  FIG. 2  to  FIG. 4  and thus, will not be repeated. 
     In light of the foregoing, the execution frequency of the calculation operation of the brightness codes can be reduced according to whether the PWM signal is in the stable state in the application. Furthermore, the calculation operation of the brightness codes can be stopped according to the first control signal. Through the technical means described above, the application can directly or indirectly prevent the issue of brightness flickers from occurring to the LEDs due to the abnormal state. In addition, the effect of reducing static power consumption can be achieved. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.