FIELD OF THE INVENTION 
   The present invention is related generally to a driver for driving Light-Emitting Diode (LED) and more particularly, to a LED driver using a depletion mode transistor to serve as a current source. 
   BACKGROUND OF THE INVENTION 
   LEDs are so easy and cost-effective to use that they are widely applied in various portable apparatus. However, when several LEDs are used for providing a light source, a control circuit is requested for all the LEDs to have uniform brightness. The brightness of a LED is proportional to the driving current flowing through the LED, and therefore the control of the driving current for a LED is the control of the brightness of the LED. Generally, cost is an important concern when providing a current source for driving a LED. 
     FIG. 1  shows a conventional LED driver  100  comprising several light sources, LEDs  104  and  106 , LEDs  108  and  110 , LEDs  112  and  114 , LEDs  116  and  118 , and LEDs  120  and  122 , coupled together in parallel, and each light source thereof is coupled in series with a resistor  102 . When coupled with a supply voltage VCC, based on the Ohm&#39;s law, the LED driver  100  will have the light sources each flowing with a driving current 
                   I   n     =       Vcc   -     Vd   n       R             [     EQ   ⁢     -     ⁢   1     ]               
where n=1, 2, 3, 4, or 5, Vd n  is the forward-bias voltage across the respective light source, and R is the resistance of the resistor  12 . From the equation EQ-1, the currents I 1 -I 5  supplied for the LEDs  104 - 122  are dependent on the respective forward-bias voltages Vd n . The forward-bias voltages Vd n  varies with the size of the LED, the process to manufacture the LED, and the temperature of the LED in the respective light source, it is therefore hard for the currents I 1 -I 5  to be uniform.
 
   In U.S. Pat. No. 5,025,204 to Su, a current mirror using resistor ratios in CMOS process is provided by using an additional control circuit (i.e., operational amplifiers) to unify the driving currents flowing through several light sources. However, the driving current flowing through a light source is dependent on the battery voltage supplied to the light source, and the additional control circuit will increase the cost. Proposed by U.S. Pat. No. 6,538,394 to Volk et al., current source methods and apparatus for light emitting diodes further employ an adjustable power supply for providing a stable supply voltage to generate driving currents for each of several light sources, in addition to a control circuit to maintain the driving currents of the light sources uniform. This circuit also requires more cost. 
   Therefore, it is desired a low-cost LED driver. 
   SUMMARY OF THE INVENTION 
   One object of the present invention is to provide a LED driver using a depletion mode transistor to serve as a current source. 
   Another object of the present invention is to provide a low-cost LED driver. 
   Yet another object of the present invention is to provide a high efficiency LED driver. 
   According to the present invention, a LED driver comprises a depletion mode transistor that is self-biased for providing a driving current to drive at least one LED. For the driving current is generated by the self-biased depletion mode transistor, the driving current is independent on the supply voltage coupled to the at least one LED, and therefore additional voltage regulator and control circuit are not necessary, thereby reducing the cost. The depletion mode transistor may be a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) or a Junction Field-Effect Transistor (JFET). If the depletion mode transistor is a JFET, the cost may be further reduced by integrating the JFET with the LEDs on a same chip, since a JFET process could be combined in a LED process. Moreover, the LED driver will have a higher efficiency due to the smaller conductive resistance of the JFET. In another embodiment, a switch is further comprised in the LED driver to switch the driving current, and the switch is switched by a controller using (Pulse-Width Modulation) PWM or Pulse-Frequency Modulation (PFM) for modulating the average of the driving current, and thereby modulating the brightness of the LEDs. The switch may be also integrated with the current source and LEDs on a same chip. 


   
     BRIEF DESCRIPTION OF DRAWINGS 
     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a conventional LED driver; 
       FIG. 2  shows a first embodiment of a LED driver according to the present invention; 
       FIG. 3  shows a second embodiment of a LED driver according to the present invention; 
       FIG. 4  shows a third embodiment of a LED driver according to the present invention; 
       FIG. 5  shows a fourth embodiment of a LED driver according to the present invention; 
       FIG. 6  shows a fifth embodiment of a LED driver according to the present invention; 
       FIG. 7  shows a sixth embodiment of a LED driver according to the present invention; 
       FIG. 8  shows a seventh embodiment of a LED driver according to the present invention; and 
       FIG. 9  shows an eighth embodiment of a LED driver according to the present invention. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a LED driver  200  according to the present invention, which comprises three light sources  202 ,  204 , and  206  coupled together in parallel between a supply voltage VCC and ground GND. Each of the light sources  202 - 206  includes two LEDs, and is coupled in series with a depletion mode NMOS transistor  208  that behaves as a current source. Each of the depletion mode NMOS transistors  208  has its own gate and source coupled together to ground GND, and its drain coupled to a respective one of the light sources  202 - 206 , such that a driving current I for the LEDs in the respective one of the light sources  202 - 206  is generated due to the self-bias of the depletion mode NMOS transistor  208 . It is well known for a MOS transistor that
   I ∝( V   GS   −V ) 2   [EQ-2] 
where V GS  is the voltage difference between the gate and source of the depletion mode NMOS transistor  208 , and V t  is the threshold voltage for the depletion mode NMOS transistor  208  to be conductive. Since the gate and source of the depletion mode NMOS transistor  208  are coupled together, the voltage difference therebetween is zero, i.e., V GS =0, and therefore the equation EQ-2 becomes
 I∝V t   2   [EQ-3] 
The threshold voltage V t  is dependent on the semiconductor process of manufacturing the depletion mode NMOS transistor  208 , and therefore all the driving currents I&#39;s for the light sources  202 - 206  in the LED driver  200  will be the same only that all the depletion mode NMOS transistors  208  are manufactured by a same process. Due to the same magnitude of the driving currents I&#39;s, the light sources  202 - 206  will have a uniform brightness. If the depletion mode NMOS transistor  208  is designed to have a threshold voltage V t  smaller than −0.5V, i.e., its absolute value |V t |&gt;0.5, the device size of the depletion mode NMOS transistor  208  and the variation of the driving current I will be reduced. In other embodiment, the depletion mode NMOS transistor  208  may be replaced with a JFET. A JFET process could be combined in a LED process, and therefore the JFETs for serving as the current sources may be integrated with the LEDs in the light sources  202 - 206  on a same chip to further reduce the cost. Moreover, a JFET has a smaller conductive resistance, it may improve the efficiency of the LED driver  200 .
 
     FIG. 3  shows a LED driver  300  according to the present invention, which also comprises the light sources  202 ,  204 , and  206 , and the depletion mode NMOS transistors  208 , as in the LED driver  200  of  FIG. 2 . However, to turn the light sources  202 ,  204 , and  206  on and off, three switches SW 1 , SW 2 , and SW 3  are inserted between a respective one of the depletion mode NMOS transistors  208  and ground GND, and a controller  302  is coupled to the switches SW 1 , SW 2 , and SW 3  to switch them individually. The controller  302  may comprises a PWM or PFM controller for brightness modulation of the light sources  202 ,  204 , and  206  by modulating the average of the driving current I. 
   As shown in  FIG. 4 , the LED driver  300  of  FIG. 3  may be modified to be another one  400 , in which the switches SW 1 , SW 2 , and SW 3  are inserted between a respective one of the depletion mode NMOS transistors  208  and a respective one of the light sources  202 - 206 . 
   Another modification of the LED driver  300  of  FIG. 3  is shown in  FIG. 5 . In a LED driver  500 , the switches SW 1 , SW 2 , and SW 3  are inserted between a respective one of the light sources  202 - 206  and the supply voltage VCC. 
   Again, the depletion mode NMOS transistors  208  in the LED driver  300 ,  400 , and  500  may be replaced with JFETs, such that the JFETs for serving the current sources may be integrated with the LEDs in the light sources  202 - 206  and the switches SW 1 -SW 3  in a same chip. 
   In a LED driver  600  shown in  FIG. 6 , three light sources  602 ,  604 , and  606  are coupled together in parallel between a supply voltage VCC and ground GND, and each of the light sources  602 - 606  includes two LEDs and is coupled in series with a depletion mode PMOS transistor  608  behaving as a current source and generating a driving current I by self-bias for a respective one of the light sources  602 - 606 . For the self-bias, each of the depletion mode PMOS transistors  608  has its own gate and source coupled together to a respective one of the light sources  602 - 606 , and its drain is grounded. In other embodiment, the depletion mode PMOS transistor  608  may be replaced with a JFET. For a JFET process could be combined in a LED process, the JFETs for serving as the current sources may be integrated with the LEDs in the light sources  602 - 606  on a same chip to further reduce the cost. Moreover, a JFET has a smaller conductive resistance, it may improve the efficiency of the LED driver  600 . 
   As shown in  FIG. 7 , a LED driver  700  further comprises three switches SW 1 , SW 2 , and SW 3  inserted between a respective one of the depletion mode PMOS transistors  608  and ground GND to turn the light sources  602 - 606  on and off, and a controller  302  coupled to the switches SW 1 , SW 2 , and SW 3  to switch them individually. The controller  302  may comprises a PWM or PFM controller for brightness modulation of the light sources  602 - 606  by modulating the average of the driving current I. 
   In a modification  800  shown in  FIG. 8 , the switches SW 1 , SW 2 , and SW 3  are inserted between a respective one of the depletion mode PMOS transistors  608  and a respective one of the light sources  602 - 606 . 
   Another modification  900  shown in  FIG. 9  has the switches SW 1 , SW 2 , and SW 3  inserted between a respective one of the light sources  602 - 606  and the supply voltage VCC. 
   Similarly, those depletion mode PMOS transistors  608  in the LED driver  700 ,  800 , and  900  may be replaced with JFETs, such that the JFETs for serving the current sources may be integrated with the LEDs in the light sources  602 - 606  and the switches SW 1 -SW 3  in a same chip. 
   While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.
In a LED driver using a depletion mode transistor to serve as a current source, the depletion mode transistor is self-biased for providing a driving current to drive at least one LED, thereby requesting no additional control circuit to control the depletion mode transistor. The driving current is independent on the supply voltage coupled to the at least one LED, thereby requesting no additional voltage regulator, reducing the circuit size, and lowering the cost.