Patent Publication Number: US-7902762-B2

Title: System and method for driving LED with high efficiency in power consumption

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to driving light-emitting diode (LED), and more particularly to system and method for driving the LED with high efficiency in power consumption. 
     2. Description of the Prior Art 
     The light-emitting diode (LED) is widely used in a variety of electronic devices for diverse purposes. For example, the LEDs may be utilized in the backlight module of a liquid crystal display (LCD) to provide backlight, or may provide flash light in a charge-couple device (CCD) camera. In practice, the LED is, however, temperature dependent, or, in other words, the characteristics of the LED vary according to its temperature.  FIG. 1  shows an LED and its equivalent circuit. The LED is equivalently made of a voltage source connected in series with a negative-temperature-coefficient (NTC) resistor. The resistance of the NTC resistor falls with increasing temperature caused by the current flowing through the LED, and vice versa. Accordingly, the voltage potential across the anode and the cathode electrode of the LED (or the forward voltage VF) decreases with increasing temperature by a constant current flowing through, and vice versa. 
     There are two conventional methods for driving the LED or LEDs: the constant-voltage driving method and the constant-current driving method. In the conventional constant-voltage driving method, the anode electrode of the LED controllably receives a constant-voltage supply. As discussed above, the current flowing through the LED will vary even though the anode electrode receives the constant voltage. Consequently, the LED suffers varying driving current, and thus its associated illuminance. Furthermore, the LED in the conventional constant-voltage driving method is typically connected in series with a current-limiting resistor, which disadvantageously consumes precious power. 
     In the conventional constant-current driving method, the driving current through the LED is controllably constant. Although the LED driving current (and its associated illuminance) in the conventional constant-current method does not vary with respect to the fluctuating forward voltage VF, the LED, however, is connected in series with a current-sensing resistor, which disadvantageously consumes precious power. 
     For the foregoing reasons that either conventional constant-voltage or constant-current driving method wastefully consumes power, a need has arisen to propose a novel driving scheme with increased efficiency in power consumption, while maintaining constant driving current. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide system and method for driving the LED with constant current and illuminance, and with increased efficiency in power consumption. 
     According to the embodiment, the driving system includes a constant-current mode circuit for providing a constant current to the LED, and a constant-voltage mode circuit for providing a constant voltage to the LED. A switch is utilized to switch between the constant-current mode circuit and the constant-voltage mode circuit to assert constant-current mode and constant-voltage mode respectively. Accordingly, the forward voltage of the LED could be maintained constant, and the efficiency in power consumption could be substantially increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an LED and its equivalent circuit; 
         FIG. 2  illustrates a LED driving system according to one embodiment of the present invention; 
         FIG. 3A  illustrates the LED driving system of  FIG. 2  in constant-current mode; 
         FIG. 3B  illustrates the LED driving system of  FIG. 2  in constant-voltage mode; 
         FIG. 4  shows the duty cycles between the constant-current mode ( FIG. 3A ) and the constant-voltage mode ( FIG. 3B ); and 
         FIG. 5  shows the flow diagram of the LED driving system of  FIGS. 2-3B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates a LED driving system  10  according to one embodiment of the present invention. Although one LED is demonstrated in the embodiment, a person skilled in the pertinent art will appreciate that more than one LED may be adapted, and the LEDs may be connected in either series or parallel. In the embodiment, a regulator  12  continuously switches the transistor Q 2  on and off in turn such that the supply voltage Vin stores energy in the inductor L 1  when the transistor Q 2  is turned on, and the stored energy is delivered to the LED D 1  at the output node Vout when the transistor Q 2  is turned off. The rectifying diode D 2  is used to prevent the current from being returned from the output node Vout back to the supply voltage Vin. The switching duty cycle of the regulator  12  varies according to the output of an error comparator  18 . For example, the switching duty cycle increases when the output of the error comparator  18  increases, indicating that the LED output voltage or current decreases; and vice versa. 
     According to the embodiment, the LED driving system  10  includes a current sensing resistor R 3 , which is connected, in series, between the cathode electrode of the LED D 1  and the ground. The LED driving system  10  also includes a voltage divider R 1 -R 2 , which is connected between the anode electrode (or the output node) of the LED D 1  and the ground. The error comparator  18  is coupled to compare a reference voltage (at the non-inverting end) and an input voltage (at the inverting end). The reference voltage and the input voltage are different in different modes, and will be described in details later. A controller  13 , as will also be described later, is utilized to control and regulate the operation of the LED driving system  10 . The controller  13  may be implemented by hardware circuitry, software program, or their combination. Further, the controller  13  may, in practice, be subdivided into connected or unconnected functional blocks. 
     In the operation, the LED driving system  10  is operated in two modes in turn, that is, the constant-current (CC) mode and the constant-voltage (CV) mode. The switching between these two modes is schematically implemented by a switch SW, which is controlled by the controller  13 . The constant-current mode is asserted when the connections a 1 -a 2  and b 1 -b 2  are made, as shown in  FIG. 3A . Otherwise, the constant-voltage is asserted when the connection a 2 -b 1  and floating b 2  are made, as shown in  FIG. 3B . The duty cycles of the constant-current (CC) mode and the constant-voltage (CV) mode are schematically exemplified in  FIG. 4 , in which the CC period is substantially shorter than the CV period. For example, the CC period may be a few mini second (ms) while the CV period may last a couple of minutes or longer. 
     Specifically speaking, in the constant-current mode as illustrated in the system diagram  FIG. 3A  and flow diagram  FIG. 5 , the controller  13  turns off the transistor Q 1  (step  51 ), followed by acquiring the dividing voltage V 1  at node d (block  14  and step  52 ) that is derived by the voltage divider R 1 -R 2  across between the output node Vout and the ground. The acquired voltage V 1  may, in the embodiment, be converted into the digital form by an analog-to-digital converter (ADC) and then temporarily stored in the controller  13  for the following operations. The voltage at node c (or the voltage potential across the current sensing resistor R 3 ) is controllably maintained at a predetermined reference voltage Vref (block  16 ) by way of the error comparator  18 . According to basic circuit law,
 
 V 1=( R 2/( R 1 +R 2))* V out
 
or
 
 V out=( V 1/ R 2)*( R 1+ R 2)
 
     Therefore, the forward voltage VF across the LED D 1  could be derived, by the controller  13 , as follows (step  53 ):
 
 VF=V out− V ref=( V 1 /R 2)*( R 1 +R 2)− V ref
 
     Subsequently, the LED driving system  10  enters into the constant-voltage (CV) mode (commanded, for example, by the controller  13 ) as illustrated in the system diagram  FIG. 3B  and flow diagram  FIG. 5 . The controller  13  turns on the transistor Q 1  (step  54 ), therefore connecting the cathode electrode of the LED D 1  to the ground and thus bypassing the resistor R 3 . In other words, no current now flows through the resistor R 3 , and thus no power is consumed in the resistor R 3  in the CV mode. Subsequently, in step  55 , the reference voltage  16  to the error comparator  18  is changed, by the controller  13 , to a new reference voltage V 1 −Vref*R 2 /(R 1 +R 2 ). In the embodiment, the controller  13  provides the new reference voltage in analog form by a digital-to-analog converter (DAC). Consequently, the voltage at the node d thus approaches towards the new reference voltage V 1 −Vref*R 2 /(R 1 +R 2 ). According to basic circuit law,
 
 V 1− V ref* R 2/( R 1 +R 2)=( R 2/( R 1 +R 2))* V out
 
or
 
 V out=( V 1− V ref* R 2/( R 1 +R 2))*(( R 1 +R 2)/ R 2)=( V 1 /R 2)*( R 1 +R 2)− V ref= VF  
 
     Accordingly, the forward voltage VF of the LED D 1  is maintained at the constant voltage VF. It is particularly noted that the resistor R 3  no longer acts as a current-limiting resistor in the constant-voltage mode, and thus no power is consumed by the resistor R 3  in this CV mode. By increasing the duty cycle of the CV mode ( FIG. 4 ) as larger as possible, the efficiency in power consumption could be substantially increased compared to either the conventional constant-current driving method or the constant-voltage driving method. 
     Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.