Patent Publication Number: US-10314131-B1

Title: LED driver with brightness control and driving method thereof

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of priority to Taiwan Patent Application No. 107131146, filed on Sep. 5, 2018. The entire content of the above identified application is incorporated herein by reference. 
     Some references, which may include patents, patent applications and various publications, may be cited and discussed in the description of this disclosure. The citation and/or discussion of such references is provided merely to clarify the description of the present disclosure and is not an admission that any such reference is “prior art” to the disclosure described herein. All references cited and discussed in this specification are incorporated herein by reference in their entireties and to the same extent as if each reference was individually incorporated by reference. 
     FIELD OF THE DISCLOSURE 
     The present disclosure provides a light-emitting diode (LED) driver and a driving method thereof, and in particular, to an LED driver with brightness control and a driving method thereof. 
     BACKGROUND OF THE DISCLOSURE 
     LEDs have been massively produced at present, and most of the LEDs are used for lighting and display. A plurality of LEDs can be connected in series to form one or more LED strings, and an LED driver drives the LED string to emit light. A conventional LED driver has varied structures, one of which is shown in  FIG. 1 . An LED driver  10  is coupled to an LED string  50 , and drives the LED string  50  according to image brightness information Sbr. The LED driver  10  may control an LED current IL flowing through the LED string  50  according to different image brightness information Sbr (that is, different image brightness information Sbr corresponds to a different LED current IL), so as to control the brightness of the LED string  50 . Because the brightness is controlled within a relatively wide operating current range, the image brightness information Sbr needs to be programmed also within a relatively wide range. 
     As shown in  FIG. 1 , the LED driver  10  includes an LED controller  11 , a current source  13 , a first current mirror  15 , a second current mirror  17 , and a drive transistor  19 . The LED controller  11  receives the image brightness information Sbr, generates a digital code signal Code according to the image brightness information Sbr, and transmits the digital code signal Code to the current source  13 , to adjust a reference current Iref flowing through the current source  13 . Further, if the digital code signal Code is 8-bit data, the LED controller  11  converts the image brightness information Sbr into an 8-bit digital code signal Code, to adjust the reference current Iref flowing through the current source  13 . 
     The first current mirror  15  generates a first current I 1  according to the reference current Iref and a first rate K 1  of the first current mirror  15 . Subsequently, the second current mirror  17  generates the LED current IL flowing through the LED string  50  according to the first current I 1  and a second rate K 2  of the second current mirror  17 . In addition, the LED controller  11  generates a pulse-width modulation (PWM) signal according to the image brightness information Sbr, to turn on/off the drive transistor  19 . In this way, the LED string  50  is driven, and the brightness of the LED string  50  is controlled according to the LED current IL. It should be noted that, the LED current IL is equal to a product obtained by multiplying the reference current Iref, the first rate K 1 , and the second rate K 2  together, where the first rate K 1  multiplied by the second rate K 2  is a constant. 
     Therefore, the first rate K 1  and the second rate K 2  are nonadjustable constants conventionally, and the brightness (corresponding to the LED current IL) is controlled within a relatively wide operating current range (for example, from a small current 20 mA to a large current 200 mA). Therefore, the image brightness information Sbr needs to be programmed also within a relatively wide range. However, if running within a relatively wide operating current range, the conventional LED driver  10  is unable to correctly maintain the LED current IL in a preset variation range. 
       FIG. 2  simulates a variation of the LED current IL in the case where the conventional LED driver  10  runs within a relatively wide operating current range (that is, from a small current 20 mA to a large current 200 mA). As shown in  FIG. 2 , the curves CV 1  and CV 2  separately show a result obtained through a Monte Carlo method performed for different numbers of times. In the operating current range from the small current 20 mA to the large current 200 mA, the variation of the LED current IL gradually decreases. Therefore, if the preset variation range is set from −2% to +2%, the variations shown by the curves CV 1  and CV 2  cannot be maintained in the preset variation range all the time. A conventional solution is to correct the variation of the LED current IL in sections according to the simulation diagram of  FIG. 2 . As shown in  FIG. 2 , an operator divides the whole operating current range (that is, from 20 mA to 200 mA) into four sections A, B, C, and D according to the result shown by the actual simulation diagram, and then adjusts the variation in each section of A to D, such that the adjusted variation is maintained in the preset variation range. However, the conventional solution increases the test time and cost, and the operator cannot correctly decide an adjustment amount for each section, causing an unsatisfactory effect after the adjustment. 
     SUMMARY OF THE DISCLOSURE 
     In order to reduce the test time and cost and avoid the operator from deciding a wrong adjustment amount, an objective of the present disclosure is to provide an LED driver with brightness control and a driving method thereof, so as to solve the foregoing problems. 
     An embodiment of the present disclosure provides an LED driver with brightness control, which is used to reduce the loss of an LED current flowing through an LED string in an operating current range. The LED driver includes a first current mirror, a second current mirror, and an LED controller. The first current mirror is coupled to a current source, and generates a first current according to a reference current generated by the current source, where the first current is the reference current multiplied by a first rate. The second current mirror is coupled to the first current mirror via a first transistor switch, is coupled to the LED string via a drive transistor, and generates an LED current flowing through the LED string according to the first current, where the LED current is the first current multiplied by a second rate. The LED controller is coupled to the current source, the first current mirror, and the second current mirror. The LED controller receives image brightness information; generates a first parameter, a second parameter, a digital signal, a control signal, and a PWM signal according to the image brightness information; and drives the LED string according to the PWM signal and the control signal. The first current mirror adjusts the first rate according to the first parameter. The second current mirror adjusts the second rate according to the second parameter. The LED controller adjusts the reference current according to the digital signal multiplied by a third rate, where a product obtained by multiplying the first rate, the second rate, and the third rate together is a fixed value. 
     An embodiment of the present disclosure provides an LED driving method with brightness control, which is applicable to an LED driver. The LED driver is coupled to an LED string, and is used to reduce the loss of an LED current flowing through the LED string in an operating current range. The LED driving method includes the following steps: step (A): receiving image brightness information, and generating a first parameter, a second parameter, a digital signal, and a PWM signal according to the image brightness information; step (B): adjusting a first rate according to the first parameter, adjusting a second rate according to the second parameter, and adjusting a reference current according to the digital signal multiplied by a third rate, where the reference current is generated by a current source, and a product obtained by multiplying the first rate, the second rate, and the third rate together is a fixed value; step (C): adjusting the reference current according to the first rate to generate a first current, and adjusting the first current according to the second rate to generate an LED current flowing through the LED string, where the first current is the reference current multiplied by the first rate, and the LED current is the first current multiplied by the second rate; and step (D): driving the LED string according to the PWM signal and the control signal. 
     To sum up, the LED driver with brightness control and the driving method thereof in the present disclosure adjust a first rate of a first current mirror, a second rate of a second current mirror, and a reference current of a current source according to the brightness to be presented (related to image brightness information) to adaptively adjust an LED current flowing through an LED string, thereby reducing the loss of the LED current during operation over an operating current range. Besides, the LED driver with brightness control and the driving method thereof in the present disclosure do not require that the operator adjusts the variation of the LED current in different operating current ranges in advance, thereby reducing the test time and cost and avoiding the operator from deciding a wrong adjustment amount. 
     In order to further understand features and technical content of the present disclosure, reference is made to the following detailed descriptions and drawings related to the present disclosure. However, the accompanying drawings are merely used to describe the present disclosure, but not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional LED driver; 
         FIG. 2  is a diagram showing a conventional relationship between an LED current and a variation during operation over an operating current range; 
         FIG. 3A  is a schematic diagram of an LED driver in an embodiment of the present disclosure; 
         FIG. 3B  is a schematic diagram of an LED driver in another embodiment of the present disclosure; 
         FIG. 4A  is a diagram showing a relationship between a first rate, a second rate, and an LED current in an embodiment of the present disclosure; 
         FIG. 4B  is a diagram showing a relationship between a digital signal and an LED current in an embodiment of the present disclosure; 
         FIG. 4C  is a diagram showing a relationship between a drain voltage of a second current mirror and an LED current in an embodiment of the present disclosure, the drain voltage being controlled according to a control signal; 
         FIG. 5  is a diagram showing a relationship between a conventional LED driver and an LED driver of the present disclosure; 
         FIG. 6  is a flowchart of an LED driving method according to an embodiment of the present disclosure; and 
         FIG. 7  is a schematic diagram of an LED driver in another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     In the following, the present disclosure will be described in detail by way of illustration of various exemplary embodiments of the present disclosure with reference to the drawings. However, the concept of the present inventive may be embodied in many different forms and should not be construed as being limited to the illustrative embodiments set forth herein. In addition, the same reference numerals in the drawings may be used to indicate similar elements. 
     An LED driver with brightness control and a driving method thereof provided by the embodiments of the present disclosure adjust an LED current flowing through an LED string according to the brightness to be presented (related to image brightness information) and a relationship between a first rate of a first current mirror, a second rate of a second current mirror, and a third rate for adjusting a reference current (the relationship indicates that a product obtained by multiplying the first rate, the second rate, and the third rate together is a fixed value). Furthermore, as the LED current gradually increases from a small current to a large current (that is, the reference current gradually increases (related to the image brightness information)), the first rate gradually decreases while the second rate gradually increases; or the first rate is fixed, the third rate gradually decreases while the second rate gradually increases, to adaptively reduce the loss of the LED current during operation over an operating current range. In addition, the LED driver and the driving method thereof can further divide the operating current range into multiple operating sections and adjust a variation of the LED current in sections, thus reducing circuit computation. Therefore, the LED driver and the driving method thereof in the present disclosure do not require that an operator adjusts the variation of the LED current in different operating current ranges in advance, thereby reducing the test time and cost and avoiding the operator from deciding a wrong adjustment amount. The following further describes the LED driver with brightness control and the driving method thereof disclosed by the present disclosure. 
     First, reference is made to  FIG. 3A , which is a schematic diagram of an LED driver according to an embodiment of the present disclosure. As shown in  FIG. 3A , the LED driver  100  is coupled to an LED string  500  and drives the LED string  500  according to image brightness information Sbr, to reduce the loss of an LED current IL flowing through the LED string  500  in an operating current range. The LED driver  100  can control the LED current IL according to different image brightness information Sbr (that is, different image brightness information Sbr corresponds to a different LED current IL), to control the brightness of the LED string  500 . 
     The LED driver  100  includes an LED controller  110 , a current source  130 , a first current mirror  150 , a second current mirror  170 , and a drive transistor  190 . The LED controller  110  is coupled to the current source  130 , the first current mirror  150 , and the second current mirror  170 . The first current mirror  150  is coupled to the current source  130 , and generates a first current I 1  according to a reference current Iref generated by the current source  130 . Based on an internal element structure of the first current mirror  150 , a proportional relationship is formed between the reference current Iref and the first current I 1 , and the first current I 1  is the reference current Iref multiplied by a first rate K 1 . 
     The second current mirror  170  is directly coupled to the first current mirror  150 , and coupled to the LED string  500  via the drive transistor  190 . In this embodiment, the drive transistor  190  may be a P-type transistor, N-type transistor, or another transistor with a switch function, but the present disclosure is not limited thereto. The second current mirror  170  generates the LED current IL flowing through the LED string  500  according to the received first current I 1 . Based on an internal element structure of the second current mirror  170 , a proportional relationship is formed between the LED current IL and the first current I 1 , and the LED current IL is the first current I 1  multiplied by a second rate K 2 . 
     Therefore, a relationship between the LED current IL and the reference current Iref is: an LED peak current IL=the reference current Iref×the first rate K 1 ×the second rate K 2 . The internal element structure of the first current mirror  150  which implements that the first current I 1  is the reference current Iref multiplied by the first rate K 1 , and the internal element structure of the second current mirror  170  which implements that the LED current IL is the first current I 1  multiplied by the second rate K 2  are known by persons with ordinary skill in the art, so the details are not described herein again. 
     The LED controller  110  is coupled to the current source  130 , the first current mirror  150 , and the second current mirror  170 . The LED controller  110  receives image brightness information Sbr, and generates a first parameter Gf 1 , a second parameter Gf 2 , a digital signal Code, and a PWM signal according to the image brightness information Sbr. The LED controller  110  drives the LED string  500  according to the PWM signal. In this embodiment, when the LED controller  110  generates a high-level PWM signal in a duty cycle, the LED controller  110  turns on PWM for a transistor to drive the LED string  500 . On the contrary, when the LED controller  110  generates a low-level PWM signal, the LED controller  110  turns off PWM for the transistor to stop driving the LED string  500 . Therefore, the LED controller  110  may turn on/off the drive transistor  190  according to the PWM signal, to further drive the LED string  500 . In addition, the LED controller  110  transmits the first current I 1  to the second current mirror  170 , to provide the LED current IL flowing through the LED string  500 . 
     The LED controller  110  receives the image brightness information Sbr; generates a digital signal Code, for example, a 4-bit or 8-bit digital signal Code, according to the image brightness information Sbr; and then adjusts the digital signal Code to obtain the digital signal Code multiplied by a third rate K 3  (that is, K 3 ×Code). Furthermore, the LED controller  110  has a gain adjuster  120 . The gain adjuster  120  receives and adjusts the digital signal Code to generate the digital signal Code multiplied by the third rate K 3 . Referring to  FIG. 3A  again, the first current mirror  150  adjusts the first rate K 1  of the first current mirror  150  according to the first parameter Gf 1 , and the second current mirror  170  adjusts the second rate K 2  of the second current mirror  170  according to the second parameter Gf 2 . The LED controller  110  adjusts the reference current Iref of the current source  130  according to the digital signal Code multiplied by the third rate K 3 . 
     It should be noted that, the product obtained by multiplying the first rate K 1 , the second rate K 2  and the third rate K 3  together is a fixed value. For example, based on a particular piece of image brightness information Sbr, the first rate K 1  is 4, the second rate K 2  is 500, and the third rate K 3  is 1. However, based on another piece of image brightness information Sbr, the first rate K 1  is 4, the second rate K 2  is 250, and the third rate K 3  is 2. Therefore, during design of the first parameter Gf 1 , the second parameter Gf 2 , and the third rate K 3 , the second parameter Gf 2  needs to be equal to the first parameter Gf 1  multiplied by the third rate K 3  (that is, Gf 2 =Gf 1 ×K 3 ), such that the LED current IL generated by the second current mirror  170  and flowing through the LED string  500  can be maintained in a preset variation range (for example, from −2% to +2%). 
     Preferably, in the operating current range of the LED driver  100 , when the LED current IL gradually increases from a small current to a large current (that is, when the value of the image brightness information Sbr gradually increases or the brightness is gradually raised), the first current mirror  150  reduces the first rate K 1  according to the decreasing first parameter Gf 1 , while the second current mirror  170  raises the second rate K 2  according to the decreasing second parameter Gf 2 . In this embodiment, the reduced first rate K 1  is denoted by K 1 ×Gf 1 . The raised second rate K 2  is denoted by K 2 /Gf 2 . 
     Reference is made to  FIG. 3B , which is a schematic diagram of an LED driver in another embodiment of the present disclosure. The part same as that in  FIG. 3A  is not described herein again, and the following merely describes different features shown in  FIG. 3B . Compared with that shown in  FIG. 3A , the LED driver  100  in  FIG. 3B  further includes a first transistor  160 . The second current mirror  170  is coupled to the first current mirror  150  via the first transistor  160 . The first transistor  160  may be a P-type transistor, a N-type transistor, or another transistor with a switch function, but the present disclosure is not limited thereto. 
     A gate of the first transistor  160  is connected to the LED controller  110 , a drain of the first transistor  160  is connected to a source of the first current mirror  150  and a gate of the second current mirror  170 , and a source of the first transistor  160  is connected to the LED controller  110  and a drain of the second current mirror  170 . The LED controller  110  generates a control signal Vg to drive the first transistor  160 , and then feeds back a source voltage Vb 2  of the first transistor  160  to the LED controller  110 , to control a drain voltage Vb 2  of the second current mirror  170 . 
     In the operating current range of the LED driver  100 , when the LED current IL gradually increases from a small current to a large current (that is, when the value of the image brightness information Sbr gradually increases or the brightness is gradually raised), the control signal Vg drives the first transistor  160 , to gradually increase the drain voltage Vb 2  of the second current mirror  170 , such that the LED current IL generated by the second current mirror  170  and flowing through the LED string  500  can be maintained in a preset variation range (for example, from −2% to +2%). 
     In other embodiments, the image brightness information Sbr may be divided into several numeric intervals. The LED controller  110  decreases the second rate K 2  and increases the first rate K 1  sequentially according to the magnitude of numeric values in these numeric intervals. The numeric values in these numeric intervals are in direct proportion to the values of the LED current IL. For example, as shown in  FIG. 4A  to  FIG. 4C , the image brightness information Sbr is divided into two numeric intervals which are a first numeric interval (correspondingly, 20 mA≤LED current IL≤125 mA) and a second numeric interval (correspondingly, LED current IL&gt;125 mA). As shown in  FIG. 4A , the first rate K 1  and the second rate K 2  corresponding to the first numeric interval are respectively 8 and 250, and the first rate K 1  and the second rate K 2  corresponding to the second numeric interval are respectively 4 and 500. As shown in  FIG. 4B , the digital signal Code and the LED current IL (related to the image brightness information Sbr) meet a linear relationship. As shown in  FIG. 4C , the drain voltage Vb 2  of the second current mirror  170  corresponding to the first numeric interval is 0.1V, and the drain voltage Vb 2  of the second current mirror  170  corresponding to the second numeric interval is 0.2V. 
     Therefore, when receiving image brightness information Sbr representing the first numeric interval, the LED controller  110  matches the first rate K 1  and the second rate K 2  with 4 and 500 respectively according to the relationship diagram of  FIG. 4A , matches the image brightness information Sbr with a particular digital signal Code according to the relationship diagram of  FIG. 4B , and matches the drain voltage Vb 2  of the second current mirror  170  with 0.1V according to the relationship diagram of  FIG. 4C . The LED controller  110  then generates an LED current IL according to the foregoing numeric values to drive the LED string  500 . Similarly, when receiving image brightness information Sbr representing the second numeric interval, the LED controller  110  finds the matched values in the same manner, and generates the LED current IL to drive the LED string  500 . 
     It can be known from the above that, because the image brightness information Sbr is divided into two numeric intervals, the first rate K 1 , the second rate K 2 , and the third rate K 3  can be adjusted only twice. Thus, the LED controller  110  does not need to adjust the first parameter Gf 1 , the second parameter Gf 2 , and the third rate K 3  at any time as the image brightness information Sbr is changed, thereby reducing circuit computation. It should be noted that, more numeric intervals of the image brightness information Sbr indicate a smaller variation of the LED current IL generated by the LED controller  110 , and a smoother LED current IL in the whole operating current range. 
     Afterwards, reference is made to  FIG. 5 , which is a diagram showing a relationship between a conventional LED driver and an LED driver of the present disclosure. The curve S 1  (a solid line) simulates a variation of an LED current IL in the case where the conventional LED driver  10  runs in an operating current range of 20 mA to 200 mA. In the curve S 1 , the first rate K 1  is 4, the second rate K 2  is 500, and the third rate K 3  is 1. The curve S 2  (a dotted line) simulates a variation of an LED current IL in the case where the LED driver  100  runs in an operating current range of 20 mA to 200 mA. In the curve S 2 , the image brightness information Sbr is divided into two numeric intervals, as shown in  FIG. 4A  to  FIG. 4C . The first rate K 1 , the second rate K 2  and the third rate K 3  that correspond to the first numeric value is 8, 250, and 1 respectively. The first rate K 1 , the second rate K 2  and the third rate K 3  that correspond to the second numeric value is 4, 500, and 1 respectively. 
     Therefore, as shown in  FIG. 5 , in the operating current range of 20 mA&lt;IL≤125 mA, a variation (related to the LED driver  100  of the present disclosure) shown by the curve S 2  is lower than that (related to the conventional LED driver  10 ) shown by the curve S 1 . In the operating current range of IL&gt;125 mA, a variation shown by the curve S 2  is equal to that shown by the curve S 1 . Thus, compared with the conventional LED driver  10 , the LED driver  100  of the present disclosure can adaptively reduce the loss of an LED current IL in an operating current range. 
     From the foregoing embodiment, the present disclosure concludes an LED driving method, which is applicable to the LED driver  100  with brightness control described in the foregoing embodiment. Reference is made to  FIG. 3B  and  FIG. 6  together. First, the LED driver  100  receives image brightness information Sbr; generates a first parameter Gf 1 , a second parameter Gf 2 , a digital signal Code, a control signal Vg, a current mirror signal (including the source voltage Vb 2  of the first transistor  160  and the drain voltage Vb 2  of the second current mirror  170  that are described above), and a PWM signal according to the image brightness information Sbr (step S 610 ). 
     In other embodiments, the image brightness information Sbr may be divided into several numeric intervals, and numeric values in these numeric intervals are in direct proportion to the values of the LED current IL. In this step S 610 , the LED driver  100  may decrease the first parameter Gf 1  sequentially according to the magnitude of the numeric values in these numeric intervals, to reduce a first rate K 1 ; and decrease the second parameter Gf 2  sequentially to raise a second rate K 2 . 
     Subsequently, the LED driver  100  adjusts the first rate K 1  according to the first parameter Gf 1 , adjusts the second rate K 2  according to the second parameter Gf 2 , and adjusts a reference current Iref according to the digital signal Code multiplied by a third rate K 3 , the reference current Iref being generated by a current source (step S 620 ). A product obtained by multiplying the first rate K 1 , the second rate K 2 , and the third rate K 3  together is a fixed value. Furthermore, the LED driver  100  reduces the first rate K 1  according to the decreasing first parameter Gf 1 , while raises the second rate K 2  according to the decreasing second parameter Gf 2 . 
     Then, the LED driver  100  adjusts the reference current Iref according to the first rate K 1 , to generate a first current I 1 ; and adjusts the first current I 1  according to the second rate K 2 , to generate an LED current IL flowing through the LED string  500  (step S 630 ). The first current I 1  is the reference current Iref multiplied by the first rate K 1 , and the LED current IL is the first current I 1  multiplied by the second rate K 2 . 
     Finally, the LED driver  100  drives the LED string  500  according to the PWM signal, the control signal Vg, and the current mirror signal (including the source voltage Vb 2  of the first transistor  160  and the drain voltage Vb 2  of the second current mirror  170  that are described above) (step S 640 ). Implementations of steps S 610  to S 640  have been roughly explained in the foregoing embodiment, so details are not described herein again. 
     Reference is made to  FIG. 7 , which is a schematic diagram of an LED driver according to another embodiment of the present disclosure. Compared with the LED driver  100  in the foregoing embodiment, the LED driver  200  of this embodiment is coupled to multiple LED strings which are a first LED string  600  and a second LED string  700 ; drives the first LED string  600  and the second LED string  700  according to image brightness information Sbr, to reduce the loss of an LED current IL 1  flowing through the first LED string  600  and the loss of an LED current IL 2  flowing through the second LED string  700  in an operating current range. The LED driver  200  can control the LED currents IL 1  and IL 2  according to different image brightness information Sbr, to control the brightness of the first LED string  600  and the second LED string  700 . The LED driver  200  includes an LED controller  210 , a current source  230 , a first current mirror  250 , a first transistor  260 , a second current mirror  270 , a drive transistor  290 , a third transistor  360 , a third current mirror  370 , and a drive transistor  390 . The LED controller  210  generates a first parameter Gf 1 , a second parameter Gf 2 , a digital signal Code, a first control signal Vg 1 , a first PWM signal PWM 1 , a third parameter Gf 3 , a second control signal Vg 2 , and a second PWM signal PWM 2  according to the image brightness information Sbr, to control the foregoing elements, so as to drive the first LED string  600  and the second LED string  700 . 
     A structural relationship and implementations related to the current source  230 , the first current mirror  250 , the first transistor  260 , the second current mirror  270 , and the drive transistor  290  are roughly identical with those related to the current source  130 , the first current mirror  150 , the first transistor  160 , the second current mirror  170 , and the drive transistor  190  in the foregoing embodiment, so the details are not described herein again. In addition, a structural relationship and implementations related to the third transistor  360 , the third current mirror  370 , and the drive transistor  390  are roughly identical with those related to the first transistor  160 , the second current mirror  170 , and the drive transistor  190  in the foregoing embodiment, so the details are not described herein again. 
     Therefore, the LED driver  200  can simultaneously control the first LED string  600  and the second LED string  700  (that is, multiple LED strings) according to the image brightness information Sbr, to adaptively reduce the loss of the LED currents IL 1  and IL 2  during operation over the operating current range. 
     To sum up, the LED driver with brightness control and the driving method thereof provided by the embodiments of the present disclosure adjust an LED current flowing through an LED string according to the brightness to be presented (related to image brightness information) and a relationship between a first rate of a first current mirror, a second rate of a second current mirror, and a third rate for adjusting a reference current (the relationship indicates that a product obtained by multiplying the first rate, the second rate, and the third rate together is a fixed value). Therefore, when an LED current gradually increases from a small current to a large current in an operating current range, the first rate gradually decreases while the second rate gradually increases, to adaptively reduce the loss of the LED current during operation over the operating current range. Therefore, the LED driver and the driving method thereof in the present disclosure do not require that the operator adjusts the variation of the LED current in different operating current ranges in advance, thereby reducing the test time and cost and avoiding the operator from deciding a wrong adjustment amount. 
     The above merely describes the embodiments of the present disclosure, and is not intended to limit the scope of present disclosure.