Abstract:
A circuit for powering of a Light Emitting Diode (LED) string has a switching power converter. A brightness control circuit is coupled to the switching power converter to allow a duration of a conductive state of the power converter to exceed a duration of a conductive state of the LED string for maintaining a current magnitude in the LED string constant.

Description:
RELATED APPLICATION 
       [0001]    The present patent application is a Continuation of U.S. patent application Ser. No. 12/564,176, filed Sep. 9, 2009, in the name of the same inventors listed above, and entitled, “LED DRIVER WITH EXTENDED DIMMING RANGE AND METHOD FOR ACHIEVING THE SAME” which is further related to U.S. Provisional Application Ser. No. 61/168,985, filed Apr. 14, 2009, in the name of the same inventors listed above, and entitled, “LED DRIVER WITH EXTENDED DIMMING RANGE AND METHOD FOR ACHIEVING THE SAME”. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to a Light Emitting Diode (LED) driver and, more specifically, to an LED driver having an extended dimming range. 
         [0003]    Recent developments of high-brightness light emitting diodes (LED) have opened new horizons in lighting. Highly efficient and reliable LED lighting continuously wins recognition in various areas of general lighting, especially in areas where cost of maintenance is a concern. 
         [0004]    A wide dynamic range of the LED brightness control becomes important in many applications, such as automobiles, avionics and television. In some cases it is needed due to large variation in the ambient light, in others it allows to improve the contrast ratio of a display. Due to the color and chromaticity properties of LED&#39;s, it is beneficial to control brightness of an LED through pulse width modulation of the current in it, while maintaining the current magnitude at a fixed level. This LED brightness control method is commonly referred to as the PWM dimming. 
         [0005]    Presently, the brightness control range of current circuits is limited to the minimum on time of a switch needed to maintain the current magnitude in the LED string. When the output pulse width of a generator becomes shorter than the on-time of the switch needed for the current sense voltage to reach the error voltage level, the control over the LED string current is lost, and the current drops out of regulation. This limit is more restrictive, when an inductor is operated in continuous conduction mode (CCM), since a longer time is needed for it to develop its steady-state current. 
         [0006]    Therefore, it would be desirable to provide a circuit and method that overcomes the above problems. 
       SUMMARY 
       [0007]    A circuit for powering of a Light Emitting Diode (LED) string has a switching power converter. A brightness control circuit is coupled to the switching power converter to allow a duration of a conductive state of the power converter to exceed a duration of a conductive state of the LED string for maintaining a current magnitude in the LED string constant. 
         [0008]    A method of achieving wide dimming range in an LED driver of a boost type having an inductor and a current control feedback comprising: storing a state of a current control feedback upon a falling edge of the PWM signal; and disabling switching of the LED driver after the falling edge of the PWM signal and upon an inductor meeting a reference corresponding to a stored state of a current control feedback. 
         [0009]    The features, functions, and advantages can be achieved independently in various embodiments of the disclosure or may be combined in yet other embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Embodiments of the disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0011]      FIG. 1  shows one example of a PWM dimming scheme in a prior art LED driver of the boost type; 
           [0012]      FIG. 2  shows an LED driver of the boost type employing a modified PWM dimming control scheme of the present invention, which overcomes the above limitation of the minimum dimming duty ratio; 
           [0013]      FIG. 3  is a chart illustrating waveforms during operation of the circuit of  FIG. 2 ; and 
           [0014]      FIG. 4  is a chart illustrating waveforms during operation of the circuit of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    A boost converter is one DC/DC converter topology commonly used to drive a string of LEDs. In the prior art, PWM dimming techniques are used that allow controlling the LED brightness in a boost converter within reasonably wide limits. Referring now to  FIG. 1 , one example of a PWM dimming scheme in a prior art LED driver of the boost type is shown. The boost converter power train (hereinafter boost converter) in the  FIG. 1  includes an inductor  103  receiving input power from an input voltage source  101  via a power switch  102 , and delivering power to an output filter capacitor  106  and an LED string  107  via a rectifier diode  105 . 
         [0016]    The brightness control circuit of the boost converter of  FIG. 1  includes a PWM switch  108  receiving a brightness control signal from a PWM pulse generator, the PWM switch  108  periodically disconnecting the LED string  107  from the output of the boost converter when the output of the PWM pulse generator  100  is low. The brightness control circuit also includes an LED current sense element  109 ; an error amplifier  110  having a reference IREF and a compensator network  112 ; a hold switch  111  for disconnecting the compensator network  112  from the output of the error amplifier  110  when the output of the PWM pulse generator  100  is low; a peak current sense element  104  for detecting peak current in the inductor  103 ; a current sense comparator  115  for comparing the output of the current sense element  104  with an error voltage at the compensator network  112 , and for generating a reset signal when the error voltage is exceeded; a PWM latch turning the power switch  102  on upon receiving a clock signal  117 , and turning the switch  112  off upon receiving the reset signal; a logic gate  118  for inhibiting the turn on of the switch  102  when the output of the PWM pulse generator  100  is low. 
         [0017]    The brightness control range of the circuit of  FIG. 1  is limited to the minimum on time of the switch  102  needed to maintain the current magnitude in the LED string  107 . When the output pulse width of the generator  100  becomes shorter than the on-time of the switch  102  needed for the current sense  104  voltage to reach the error voltage level, the control over the LED string current is lost, and the current drops out of regulation. This limit is more restrictive, when the inductor  103  is operated in continuous conduction mode (CCM), since a longer time is needed for it to develop its steady-state current. 
         [0018]    Referring now to  FIG. 2 , an LED driver  130  of the boost type employing a modified PWM dimming control scheme of the present invention is shown. The LED driver  200  of  FIG. 2  overcomes the above limitation of the minimum dimming duty ratio. 
         [0019]    The LED driver of  FIG. 2  includes an inductor  103  receiving input power from an input voltage source  101  via a power switch  102 , and delivering power to an output filter capacitor  106  and an LED string  107  via a rectifier diode  105 . 
         [0020]    Like in  FIG. 1 , a brightness control circuit  132  of the boost converter  130  of  FIG. 2  includes a PWM switch  108  which is coupled to the LED string  107 . The PWM switch  108  receives a brightness control signal from a PWM pulse generator  100 . The PWM switch  108  periodically disconnects the LED string  107  from the output of the boost converter when the output of the PWM pulse generator  100  is low. 
         [0021]    The brightness control circuit  202  further includes an LED current sense element  109  coupled to the PWM switch  108 . An error amplifier  110  has a first input coupled to the LED current sense element  109 . A second input of the error amplifier  110  is coupled to a reference IREF. The output of the error amplifier  110  is coupled to a hold switch  111 . The hold switch  111  is used for disconnecting a compensator network  112  from the output of the error amplifier  110  when the output of the PWM pulse generator  100  is low. 
         [0022]    A peak current sense element  104  is coupled to the power switch  102 . The peak current sense element is used for detecting peak current in the inductor  103 . A current sense comparator  115  has a first input coupled to the peak current sense element  104  and a second input coupled to the compensator network  112 . The current sense comparator  115  is used for comparing the output of the current sense element  104  with an error voltage at the compensator network  112  and for generating a reset signal when the error voltage is exceeded. A PWM latch  116  has a reset input coupled to the output of the current sense comparator  115  and a set input coupled to a clock signal  117 . The PWM latch  116  turns the power switch  102  on upon receiving a clock signal  117 , and turning the switch  112  off upon receiving the reset signal. A logic gate  118  is used for inhibiting the turn on of the switch  102  when the output of the PWM pulse generator  100  is low. 
         [0023]    In  FIG. 2 , a logic block  120  is used for maintaining the power switch  102  in the conductive state until the signal of the current sense element  104  exceeds the error voltage at the compensator network  112 , regardless of the PWM pulse generator  100  state. 
         [0024]    In accordance with one embodiment, the logic block  120  comprises a logic gate  113  and a D-type flip-flop  114 . The gate  113  has a first input coupled to the output of the current sense comparator  115  and a second input coupled to the PWM pulse generator  100 . The output of the logic gate  113  is coupled to a clock input of the D-type flip-flop  114 . In the embodiment shown in  FIG. 2 , the logic gate  113  is an OR gate. 
         [0025]    The D input of the D-type flip-flop  114  is coupled to the PWM pulse generator  100 . The Q output of the D-type flip-flop  114  is coupled to a first input of the logic gate  118 . The second input of the logic gate  118  is coupled to the output of the PWM latch  116 . 
         [0026]    Referring now to  FIG. 3 ,  FIG. 3  illustrates operation of the circuit of  FIG. 2 . The rising edge of the PWM signal  200  from the generator  100  propagates through the logic gate  113 , and the D-type flip-flop  114  stores a logic-high state. This high output state of the D-type flip-flop  114  enables turn-on of the power switch  102  through the logic gate  118 . The beginning pulse of the clock signal  117  represented by the waveform  217  is synchronized with the rising edge of the PWM signal  200 . At the falling edge of the PWM signal  200 , the switching of the power switch  102  will continue until the current in the inductor  103  represented by the waveform  203  reaches the reference  212  reflecting the error voltage at the compensator  112 . At this moment, the flip-flop  114  receives a signal from the comparator  115  through the logic gate  113 , and the output of the flip-flop  114  stores the logic-low state of the PWM signal generator  100 . Therefore, the actual turn-off transition of the boost converter occurs after a delay ΔT. Thus, the circuit depicted in  FIG. 2  is able to maintain the current control loop closed even when the PWM dimming signal  200  pulse width is shorter than one switching cycle of the boost converter. 
         [0027]      FIG. 4  shows the corresponding waveforms similar to the ones of  FIG. 3 . Upon the rising edge of the signal  200 , the inductor current  203  must reach the reference  212  at least once, before switching of the switch  102  is disabled. The clock signal  117  may be kept running, or it may be stopped after the delay ΔT, as long as it is synchronized with the rising edge in every cycle of the waveform  200 . 
         [0028]    Referring to  FIGS. 2-4 , a method of operation is disclosed that achieves a wide dimming range in the LED driver  140  of the boost type having an inductor  103  and a current control feedback. First, one should synchronize switching of the boost converter with the rising edge of the PWM signal  200  from the generator  100 . Next, the state of the current control feedback upon the falling edge of the PWM signal  200  is stored. The LED load  107  is disconnected from the output of the boost converter upon the falling edge of the PWM signal  200 . Switching of the boost converter is disabled after the falling edge of the PWM signal  200 , but not until the inductor  103  meets a reference corresponding to the stored state of the current control feedback. 
         [0029]    While embodiments of the disclosure have been described in terms of various specific embodiments, those skilled in the art will recognize that the embodiments of the disclosure can be practiced with modifications within the spirit and scope of the claims.