Patent Publication Number: US-8525424-B2

Title: Circuitry and method for driving LED display

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to devices and methods used for light emitting diode (LED) displays, and more particularly to circuitries and methods for driving the LED display. 
     BACKGROUND 
     A time-multiplexing LED matrix display comprises one or more arrays of LEDs. One of the performance characteristics of a LED matrix display is the time it takes for an LED to light up when its select signal turns on. Since the components in an LED matrix display have capacitance that needs to be charged before the LED becomes lit, the time delay could be significant. 
       FIG. 1  shows a generalized form of LED array in a time-multiplexing LED matrix display. The LEDs are arranged in an N by M fashion, with M channels (columns) of current source intersecting with N scan lines (rows). At any given time, there may be zero to M channels being supplied with a current while only one scan line being selected for any current channel. In order to light an LED at the intersection of column i and row j (i.e., LED  111  in  FIG. 1 ), the current source has to charge all the capacitors on the N by M array affecting this LED. For LED  111 , such capacitors include the wire capacitor C W    100 , capacitor C L  for each of the LEDs in column i  101 , . . . ,  10 N, and switch capacitor C S  for each of the switches  121  to  12 N, excluding  12   j.    
     The effect of all the capacitors can be presented as an equivalent capacitor C i  for LED(i,j). For illustrative purposes, assuming the capacitance for all the LEDs (C L ) are identical and the capacitance for all the switches (C S ) are identical, C i  can be expressed as follows:
 
 C   i   =C   W   +C   L +( N− 1)* C   S   *C   L /( C   S   +C   L )
 
     LEDs arranged in manners other than those in  FIG. 1  would have similar time delays since the components (e.g., the LEDs, wires, switches) have capacitance. The present disclosure provides devices and means to shorten the time delay caused by charging the affecting capacitors. 
     SUMMARY OF INVENTION 
     The current disclosure provides a circuit for driving an LED display panel including a plurality of light emitters being arranged in an array having a plurality of columns and rows. The driving circuit is electrically coupled with the array of the light emitters. The driving circuit comprises a selection circuit for selecting a first light emitter from the plurality of light emitters, a pre-charging circuit for charging an equivalent capacitor of the display panel with respect to the selected first light emitter, and a power circuit for supplying power to the first light emitter after the first light emitter is selected. The power circuit comprises a power source, and a current control mechanism coupled with the power source. The current control mechanism is configured to supply a first current to the power source when a driving current through the first light emitter is less than or equal to a first threshold value. The current control mechanism is also configured to supply a second current to the power source when the driving current is greater than the first threshold value and less than a second threshold value. The first current is greater than the second current. 
     In one embodiment, the pre-charging circuit comprises a first transistor, a first switch electrically coupled with the first transistor, and first logic gate electrically coupled with the first switch. The logic gate comprises an AND gate. 
     In one embodiment, the power circuit comprises second logic gate, a second transistor electrically coupled with the second logic gate, a second switch electrically coupled with the second transistor, a first resistor, and a second resistor, wherein the second switch is switchable among the first and second resistors. 
     The current disclosure also provides a method for driving a display panel including an array of light emitters. The method comprises charging an equivalent capacitor of the array before a first light emitter is selected from the array of light emitters and after a previously selected light emitter is unselected, selecting the first light emitter, and applying a driving voltage to the first light emitter using a power source after the first light emitter is selected, so as to induce a driving current through the first light emitter, wherein applying the driving voltage comprises, when the driving current is less than or equal to a first threshold value, applying a first current to the power source, so as to quickly turn on the power source, and when the driving current is greater than the first threshold value and less than a second threshold value, applying a second current to the power source, so as to minimize current overshoot , wherein the first current is greater than the second current. 
    
    
     
       DESCRIPTIONS OF DRAWINGS 
       The teachings of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  is a diagram of an LED array. 
         FIG. 2  is a timing diagram for scan line driving signals. 
         FIG. 3  is a circuit according to one embodiment of the current disclosure. 
         FIG. 4  is a diagram illustrating the behaviors of various signals in one embodiment of the current disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     The Figures (FIG.) and the following description relate to the embodiments of the present disclosure by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed inventions. 
     Reference will now be made in detail to several embodiments of the present disclosure(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein. 
       FIG. 2  shows a timing diagram of scan line switch selection. SW j  denotes an active high signal of scan line j selection when the switch at row j is turned ON. There is a short period of time between two consecutive active high signals. This time gap is called dead time. 
       FIG. 3  is an operation diagram of an LED fast rising driver circuit of the present disclosure. Component  313  represents the equivalent capacitor load described above. 
     According to one embodiment of the current disclosure, the fast rising driver circuit comprises two modules  300  and  301 . Module  300  comprises an NMOS  302 , an AND gate  303  and a switch  304 . It connects the logic “1” voltage, POWER or VDD, at one end and the anode of LED  311  at the other end. 
     Module  300  has three inputs—VR i , a dead time signal, and a signal for “Next PWM not equal to 0”. VR i  is a reference voltage specifically set for LED  311 . The dead time signal is a logic signal deducted or derived from SW j  as shown in  FIG. 2 . It is only active between two sequential active select signals SW j . The third signal “Next PWM not equal to 0” is a logic signal deducted or derived from the PWM (pulse width modulation) signal. A more detailed description about the PWM signal can be found in U.S. application Ser. No. 13/237,960, filed Sep. 21, 2011, which is incorporated herein by reference in its entirety. The “Next PWM not equal to 0” signal is only active when the next PWM signal is not equal to logic zero. 
     In one embodiment, Module  300  takes effect during dead time. Switch  312  is OFF during dead time so that no current flows through LED  311 . On the other hand, if the two signals at gate  303  become true and drive the gate  303  to output a true signal, the switch  304  is turned ON. VR i  is set at a value so that the current through NMOS  302  charges the capacitor  313  and pull up the LED anode side voltage to slightly lower than VF 0 . That is, the value of (VR i −Vth) is slightly lower than the LED inflection voltage VF 0 . Vth is the threshold voltage of NMOS  302 , while VF 0  equals the forwarding voltage at the inflection point of the LED. The value of VF 0  for a red LED ranges from, for example, 1.6V to 2.6V. The value of VF 0  for a green or blue LED ranges from, for example, 2.6V to 3.8V. 
     NMOS  302  becomes ON when switch  304  is ON. This allows the current to flow from VDD through NMOS  302  to charge the capacitive load of the LED display, represented by the equivalent capacitor  313 . Module  300  stops contributing voltage or current after the dead time is over as gate  303  outputs false and switch  304  is turned OFF. 
     Accordingly, the anode side voltage is raised to a value close to but less than the LED VF 0  during the dead time by the operation of Module  300 . For example, the anode side voltage can be raised to a value that is 0.2V or less below VF 0 . Consequently, when the next PWM selection cycle comes, the time it takes to charge the capacitor  313  to the LED VF 0  is short. As shown in  FIG. 4 , the current through NMOS  302  (I MNi ) quickly increases after the dead time starts, while V Anode  also rises, approaching VF 0 . 
     Module  301  takes effect after the dead time is over. Module  301  comprises two resistors  305  and  306 , a switch  307 , an NMOS  308 , and an AND gate  309 . One end of Module  301  connects to the gate input of PMOS  310  and the other end connects to the logic “0”, GROUND or VSS. There are two logic signal inputs to the AND gate  309 , one is “PWM equals high” and the other is related to I MPi . The logic signal “PWM equals high” is deducted or derived from the PWM signal for chancel i. It becomes high when the PWM signal becomes high and it becomes low when the PWM signal becomes low. The other logic signal becomes high when the LED current I MPi  from PMOS  310  is less than a threshold value. This threshold value can be set at any value between 0 and the target LED current, for example, between 30% to 98%, or between 50% to 95%, or between 75% to 95%, or between 85% to 95% of the target LED current. In the embodiment of  FIG. 3 , this threshold value is set at 90%. In some embodiments, the target LED current is the electrical current through an LED such that the LED can emit light of the maximum possible intensity without damaging the LED. 
     Accordingly, Module  301  operates when the PWM signal is high and when the LED current I MPi  from PMOS  310  is less than 90% of the target LED current value. In such instances, the AND gate  309  outputs a logical true signal to the gate input at NMOS  308 . This gate input turns NMOS  308  ON, which in turn pulls down the gate voltage of PMOS  310  to turn it ON. Once PMOS  310  is ON, the current source starts driving current to LED  311  and charging the capacitor  313 . 
     As shown in  FIG. 4 , the voltage at the LED anode side, V Anode , begins to rise during the dead time and continues to rise after the dead time, as the capacitor  313  is being charged by the operation of Module  301 . When the V Anode  exceeds VF 0 , LED  311  starts to have current. The LED current (I LEDij ) continues to rise gradually until it reaches a steady level—the target LED current, e.g., at 5 mA. “t p ” is the time from when PWM turns high to when LED  311  starts to have a current. During the time of t p , there is no current going through the LED. “t r ” is the time it takes for the LED current to rise from one value to another, e.g., from just above zero to 90% of the target LED current in this embodiment. 
     In the embodiment shown in  FIG. 3 , Module  301  further uses two resistors  305  and  306  to minimize current overshoot. When I MPi  is zero, the smaller resistor  305  is used to allow a bigger current to pull down the gate input of PMOS  310 . When I MPi  is larger than zero but less than 90% of the target LED current, the bigger resistor  306  is selected by switching switch  307 , which lowers the current and thus limits the current overshoot at PMOS  310 . When the I MPi , is larger than 90% of the target LED current, none of the resistors is selected. Instead, a mirror current source, which comprises a reference current source (I ref ) and PMOS  315 , maintains the PMOS  310  output current substantially at the target LED current, for example, within ±3% of the target LED current. Note that switch  314  is OFF when PWM is high, disconnecting the mirror current source from PMOS  310 . 
     Therefore, the embodiment shown in  FIG. 3  uses two stages of charging: the first stage when the LED current is zero and the second stage when the LED current is larger than zero but less than 90% of the target value. In another embodiment, a single stage of charging may be employed when the LED current ranges from zero to 100% of its target value, for example, by implementing only one or none of the resistors  305  and  306 . In still other embodiments of this disclosure, more than two stages of charging may be implemented by adding additional charging stages when the LED current is between zero and the target value. For example, adding one charging stage at each current range when the LED current is between zero to 30%, between 30% to 60%, and between 60% to 90% of the LED target current results in a total four charging stages. The number of AND gates and resistors used in Module  301  should increase accordingly. 
     When the PWM signal becomes low and gate  309  output is low, NMOS  308  turns OFF. There is no current to the gate input for PMOS  310 . The gate input of PMOS  310  is then pulled close to VDD. This keeps PMOS  310  OFF and no current will reach the anode side of LED  311  through PMOS  310 . 
     Therefore, embodiments of driver design disclosed herein allow the voltage at the anode side of LED  311  to be pre-charged to close to VF 0  by the operation of Module  300 ; so that after the PWM signal goes high, it can be quickly charged to VF 0 . As shown in  FIG. 4 , t p  is much shorter than the normal charge time when the capacitor is charged from ground up to VF 0 . Furthermore, Module  301  acts to pull down the gate voltage of PMOS  310  quickly, which reduces the value of t r . As a result, the total of t p  and t r  is short, for example, less than 10 ns. 
     In some embodiments of the current disclosure, the time period of pre-charging may be longer or shorter than the dead time. The start or finish of the pre-charging period may not be aligned with the start or finish of the dead time. Signals other than those derived from PMW or the dead time signal may be used to initiate or terminate the pre-charging period. 
     The methods and circuits disclosed herein can be used to drive LEDs arranged in a common anode or in a common cathode fashion, which are described in more details in pending U.S. application Ser. No. 13/237,960. They can also be used, for example, in the fast charge circuits  304 ,  305 , and  306  as disclosed in U.S. application Ser. No. 13/237,960. 
     Many modifications and other embodiments of the disclosure will come to the mind of one skilled in the art having the benefit of the teaching presented in the forgoing descriptions and the associated drawings. For example, the driver IC can be used to drive an LED array in either common cathode or common anode configuration. Elements in the LED array can be single color LEDs or RGB units or any other forms of LEDs available. The driver IC can be scaled up or scaled down to drive LED arrays of various sizes. Multiple driver ICs may be employed to drive a plurality of LED arrays in a LED display system. The components in the driver can either be integrated on a single chip or on more than one chip or on the PCB board. Such variations are within the scope of this disclosure. It is to be understood that the disclosure is not to be limited to the specific embodiments disclosed, and that the modifications and embodiments are intended to be included within the scope of the dependent claims.