Abstract:
Systems and methods for smoothing out visual effects of Light Emitting Diodes (LEDs). An example circuit includes one or more LEDs, a hysteretic controller circuit, and a linearization circuit. The hysteretic controller circuit supplies current to the LEDs when a Pulse Width Modulation (PWM) signal is in a first state and the linearization circuit drives the current supplied by the hysteretic controller circuit to an off state when the PWM signal transitions to a second state.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     When backlighting Liquid Crystal Displays (LCDs) at a low luminance level using a recirculating current type controller, the brightness with respect to Pulse Width Modulation (PWM), while monotonically increasing, exhibits flat regions that make control difficult. The flat regions are created by the use of the recirculating current. The placement of these flat regions can vary from unit to unit.  
         [0002]     The recirculating current method is the most efficient method to power a Light Emitting Diode (LED). The current is ramped up through an inductor, storing energy, when a switch is closed. When the switch is opened, the energy stored in the inductor is recirculated through the LEDs (typically with a free-wheeling diode). If termination occurs when the switch is already off, the level of brightness is the same whether the point of termination is just after the switch turned off, or just before it is about to turn back on; thus exhibiting flat regions of brightness.  
         [0003]     Therefore, there exists a need to linearize the flat regions to smooth the visual effects especially in dimming situations.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     The present invention provides systems and methods for smoothing the visual effects of Light Emitting Diodes (LEDs). An example circuit formed in accordance with an embodiment with the present invention includes one or more LEDs, a hysteretic controller circuit, and a linearization circuit. The hysteretic controller circuit supplies current to the LEDs when a Pulse Width Modulation (PWM) signal is in a first state and the linearization circuit drives the current supplied by the hysteretic controller circuit to an off state when the PWM signal transitions to a second state.  
         [0005]     In one aspect of the invention, the linearization circuit includes an N channel MOSFET transistor. The linearization circuit includes an inverter for inverting the PWM signal and sending the inverted PWM signal to a gate of the transistor. The hysteretic controller circuit includes an inductor and the N channel MOSFET transistor includes a drain that is connected between one end of the inductor and an anode of the one or more LEDs. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0006]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0007]      FIG. 1  illustrates a block diagram of a device formed in accordance with an embodiment of the present invention;  
         [0008]      FIG. 2  illustrates a detailed circuit diagram of the device shown in  FIG. 1 ;  
         [0009]      FIG. 3  illustrates a graph of current through an LED that reacts in accordance with an embodiment of the present invention relative to a pulse width modulation signal; and  
         [0010]      FIG. 4  illustrates a graph of examples of voltages across a sense resistor, indicative of LED current, in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]      FIG. 1  illustrates a block diagram of an example circuit  20  for hysteretically driving one or more Light Emitting Diodes (LEDs)  26  while employing low-end linearization. The circuit  20  includes a hysteretic controller circuit  28  that controls the current to one or more LEDs  26  according a received Pulse Width Modulation (PWM) signal and a backlight bias voltage (VIN). The backlight bias voltage is preferably controlled by a user operating a switch, which allows the user to set a dimming value for the LEDs  26  with the PWM signal. An example of the hysteretic controller circuit  28  is described in U.S. patent application Ser. No. 11/069,298, filed Mar. 1, 2005 and Ser. No. 11/181,815 filed Jul. 15, 2005, the contents of which are hereby incorporated by reference.  
         [0012]     The circuit  20  also includes a linearization circuit  30  that provides better control of the brightness level especially at low luminance values. The linearization circuit  30  operates in accordance with the received PWM signal.  
         [0013]      FIG. 2  illustrates an example circuit diagram of the circuit  20  from  FIG. 1 . The contents and the function of the components of the hysteretic controller circuit  28  are described by example in the co-pending applications described above. The linearization circuit  30  includes an N channel MOSFET transistor  40  that has a drain that is coupled between an inductor L 1  of the hysteretic controller circuit  28  and an anode of the LED  26 . The source of the transistor  40  is connected to ground and the gate of the transistor  40  is connected to a first capacitor  42 , a first end of a resistor  44 , and the anode of diode  46 . A second side of the capacitor  42  is coupled to ground. The cathode of the diode  46  is coupled to second side of the resistor  44  and to an output of an inverter  48 . The inverter  48  receives the PWM signal at its input and receives a supply voltage. The supply voltage is also connected to a first end of a capacitor  50  and a second end of the capacitor  50  is connected to ground.  
         [0014]     When the PWM signal is high, the hysteretic controller circuit  28  turns a transistor Q 1  on and off to provide a current (I LED ) through the LEDs  26  such as is shown in  FIG. 3 . When the transistor Q 1  is off, the controller circuit  28  recirculates the current in the inductor L 1  through the LEDs  26  using diode D 1 . When the PWM signal goes low, the transistor Q 1  is inhibited from turning back on and the transistor  40  turns on. When the transistor  40  turns on a path to ground is provided for the recirculating current of the inductor L 1 . Thus, the LEDs  26  turn off immediately, instead of when the energy in the inductor L 1  is dissipated through them. The resistor  44  and capacitor  42  are provided to create a soft turn on for the transistor  40  for reducing Electromagnetic Interference (EMI) and prevent shoot-through should transistor Q 1  be on at the instant the PWM signal goes low. The diode  46  is provided to create a fast turn off of the transistor  40  so as to minimize or eliminate any potential shoot-through effects with both transistors  40  and Q 1  on at the same time.  
         [0015]      FIG. 3  illustrates a time graph when the LEDs  26  are in a dimmed mode of operation. In this dimmed mode of operation, the PWM signal  100  cycles on and off over a short cycle. In this example, the PWM signal  100  is on for approximately 25 μS. During the time in which the PWM signal  100  is high, or in this example, 5 volts, the hysteretic controller circuit  28  produces an LED current  102  as shown. When the PWM signal  100  goes low at T 2 , the linearization circuit  30  causes the LED current to instantaneously or near instantaneously shunt to zero. In another embodiment, the linearization circuit  30  is designed to shunt the energy of the inductor L 1  when the PWM signal  100  cycles to an off state.  
         [0016]      FIG. 4  illustrates another timing diagram illustrating a PWM signal  10  and a plurality of graphs of voltage VR56 across the sense resistor  56  of the hysteretic controller circuit  28  when different values for the resistor  44  are used. The larger the value that is used for the resistor  44 , the longer it will take for the voltage VR56 to drop to zero across the resistor  56 . In this example, it will take approximately 300 nanoseconds (nS) for the voltage V R56  to drop to zero when the value of the resistor  44  is equal to 250 ohms from the time the PWM signal  110  goes low.  
         [0017]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.