Abstract:
A driving system used for light emitting diodes relating to a controllable driver which can detect the voltage desire from application and adjust driving voltage (V app1 -V app2 ) automatically in order to reach a steady driving current. Additionally, users can adjust the setting of driving current and the output value of DC voltage source for application with different voltage and current requirements through a control interface. The over-temperature and over-current protections (e.g. cutting off driving current or setting the upper limit of driving current) are also included in the system for prevention of possible harms. In the system, a driving system is also disclosed for integration of all the mentioned functions but no need of massive space and can be used in LED lighting system or LED backlight system for constant power emitting.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention generally relates to a light emitting diode (LED) driver, and more specifically relates to a driving control technology of auto voltage adjustment for keeping steady driving current. The present invention is operable to drive high power LEDs (e.g. lighting LED and backlight LED). 
         [0003]    2. Description of the Prior Art 
         [0004]    In traditionally industrial producing, there are two methods for a constant current driver: one is constant-voltage method to clamp driving current by regulating a setting voltage; the other is constant-current method to clamp driving current by regulating a current source. As the constant-voltage method shown in  FIG. 2A  (appliance example in this figure is LED), an LED controller  210  regulates an external voltage V DD  to an output voltage V LED  in order to drive a current I LED  on the LED, and further sets a voltage drop V set  cross a resister R connected with the LED in series as well as clamps current on the resister R (also I LED  on the LED). As the constant-current method shown in  FIG. 2B , an external driving voltage V LED  directly applies on an LED to generate a driving current I LED , and an LED controller regulates a reference current by applying an external voltage V DD  on a setting resister R set  as well as clamps current I LED  on the LED. However, for some delicate appliances (e.g. high power lighting and backlight LEDs), a rise on temperature during emitting, voltage fluctuation, and variation of the LED emitting property from producing will change the steady driving current beyond the settings. 
         [0005]    Furthermore, using a current mirror to clamp the driving current by a reference current source is anther constant-current method. As shown in  FIG. 6A , two current mirrors  611 , 612  with same magnification ratio 1:N are integrated together to clamp the driving current I LED =N*I ref  by a reference current I ref . Nevertheless, the chip space for two current mirror with 1:N magnification ratio is too large; and the current clamping is not strong enough to manager possible electrical characters change (e.g. effective resister, I-V curve, and chemical and physical characters change) and the followed voltage desire. Too complicate situations are still hard to manager for the traditional constant current drivers. 
       SUMMARY OF THE INVENTION 
       [0006]    The main objective of this invention is to provide a controllable driver and a driving system with an excellent stability for LEDs. Except a constant-current technology, a particular technology of voltage adjuster in the present invention can auto adjust appliance&#39;s driving voltage to fit different requirements for keeping steady driving current even in facing the possible rise on temperature or voltage desire by certain situations. 
         [0007]    A driving system according to the present invention can integrate all the said functions successfully but no need of too much chip space especially for light mobile specification. 
         [0008]    Further, with the particular technology of voltage adjuster, an additional adjustable-voltage source combined with a user control interface expands the driving system&#39;s applicability. Users can select different driving voltages to fit diverse appliance requirements through the control interface. Additionally, an over-temperature and an over-current protection are equipped in the present invention to prevent harms from over-temperature and over-current happenings especially in high power lighting and backlight LEDs. 
         [0009]    The present invention will be apparent after reading the detailed description of the preferred embodiments hereinafter in reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A  is a circuit diagram of a driving system according to an embodiment of the present invention (the appliance example is lighting or backlight LED in this figure, but it can be replaced by other appliances). 
           [0011]      FIG. 1B  is a circuit diagram of an adjustable-voltage source according to an embodiment of the present invention. 
           [0012]      FIG. 2A  is a traditional constant-voltage driving system (prior art). 
           [0013]      FIG. 2B  is a traditional constant-current driving system (prior art). 
           [0014]      FIG. 3  is a circuit diagram of a current controller according to an embodiment of the present invention (the appliance example is lighting or backlight LED in this figure, but it can be replaced by other appliances). 
           [0015]      FIG. 4A  is a variation diagram for driving current vs. system temperature during an over-temperature protection in the present invention. 
           [0016]      FIG. 4B  is a variation diagram for driving current during an over-current protection in the present invention. 
           [0017]      FIG. 4C  is a variation diagram for driving current during the other over-current protection in the present invention. 
           [0018]      FIG. 5  is a schematic diagram of a controllable driver according to an embodiment of the present invention. 
           [0019]      FIG. 6A  is a circuit diagram of a traditional current mirror (prior art) 
           [0020]      FIG. 6B  is a circuit diagram of an improved current mirror according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    As an embodiment of the invention, a controllable driver  500  to drive a stead current from high voltage V app1    501  to low voltage V app2    502  in an application  520  comprises: (a) a DC voltage input  510  for DC voltage Vo supply; (b) a 1st field effect transistor (FET)  131  as a voltage adjustor to adjust voltage differential (V app1 -V app2 ) on the appliance for voltage desire from the steady driving current I app  requirement by changing its drain-to-source voltage differential; (c) a controller  530  to control gate voltage of the 1st FET; and (d) a current controller  140  to clamp the steady driving current as setting. The controller  532  can is operable to detect voltage variation of the appliance and send negative feedback voltage to gate of the 1 st  FET in order to auto adjust drain-to-source voltage differential of the 1 st  FET and compensate the said voltage desire for keeping steady driving current. By this negative feedback circuit, the controllable driver in this invention can automatically adjust the proper driving voltage to maintain the steady driving current with excellent stability event in facing large voltage fluctuation, effective resistor variation, hardly objective and subjective situations and so on. 
         [0022]    Further, an adjustable-voltage source  110  coupled with this invention is operable to take an external voltage source V DD  and supply the said DC voltage V o  to the DC voltage input. Furthermore, a controllable interface  160  is operable to take user commands for changing the driving system&#39;s setting. With the adjustable-voltage source and the controllable interface, the range of adjustable voltage in this invention becomes more flexible to fit most part of appliance. 
         [0023]    Moreover, an over-temperature protection and an over-current protection on circuit  534  of gate voltage of the 1 st  FET or on the current control are operable to cut-off driving current or set the upper limit of driving current at over-temperature and over-current conditions to remain the controllable driver&#39;s normal operation. A temperature sensor  552  and a current monitor  553  are operable to be included in this invention to strengthen the over-temperature and over-current protections. 
         [0024]    A driving system to drive a steady current on an appliance mainly comprises six parts: (a) a DC voltage input  510  for DC voltage V o  supply; (b) an output for appliance  501  to supply high voltage V app1  to the appliance; (c) an input for appliance  502  to supply low voltage V app2  to the appliance; (d) a 1 st  field effect transistor (FET)  131  as a voltage adjuster; (e) a 1 st  operation amplifier (OpAmp)  132  operable to detect voltage variation of the appliance and send negative feedback voltage to gate of the 1 st  FET in order to auto adjust drain-to-source voltage differential of the 1 st  FET and compensate voltage desire for keeping steady driving current; and (f) a current controller  140  to clamp the steady driving current as setting. 
         [0025]    The driving system according to the present invention, wherein the current controller as shown in  FIG. 3  (appliance example in this figure is LED) between the appliance and ground takes the driving current from the appliance and also comprises: (a) a reference current source  147  to output a steady reference current I ref ; and (b) a current mirror  145  with magnification ratio 1:N to clamp the steady driving current as I ref *N by magnifying the reference current I ref . As shown in  FIG. 6B , this current mirror  145  also comprises: a 2 nd  OpAmp  144  to precisely clamp the magnification ratio 1:N of the current mirror, wherein its positive input and output of the reference current source are on the same voltage; its output and common-gate of the current mirror are on the same voltage; and its negative input and positive input of the 1 st  OpAmp  132  are on the same voltage (V set2 =V set1 ) Compared with the bulky traditional current mirror (please see  FIG. 6A ), this current mirror  145  is smaller. Furthermore, the reference current source also comprises: (a) a 3 rd  OpAmp  141  wherein its positive input is connected to an energy gap reference voltage, and its negative input and its output are on the same voltage to form a negative feedback circuit; (b) a 2 nd  FET  142  on negative feedback circuit of the 3 rd  OpAmp where its gate and output of the 3 rd  OpAmp are on the same voltage, its source and negative input of the 3 rd  OpAmp are on the same voltage, and negative input voltage of the 3 rd  OpAmp is clamped and varied by positive input voltage of the 3 rd  OpAmp; (c) a resister R set3  between negative input of the 3 rd  OpAmp and ground for current I set3  generation through the 2 nd  FET; and (d) a p channel current mirror  143  to take current I set3  of 2 nd  FET on one side and output the said reference current I ref  on the other side. 
         [0026]    In order to compensate the said voltage desire on time, the 1 st  FET has a connection between its source and the output for appliance and a connection between its drain and the DC voltage input in order to adjust voltage differential between the said DC voltage V o  and the output for appliance voltage V app1 ; likewise the 1 st  OpAmp has an input voltage through its negative input from the input for appliance voltage V app2  and output the said negative feedback voltage to gate of the 1 st  FET. Similarly, the 1 st  FET has a connection between its drain and the input for appliance and a connection between its source and negative input of the 1 st  OpAmp in order to adjust voltage differential between the input for appliance voltage V app2  and negative input voltage of the 1 st  OpAmp; likewise the 1 st  OpAmp output the said negative feedback voltage to gate of the 1 st  FET. In both circuit, they can auto adjust drain-to-source voltage differential of the 1 st  FET to compensate the said voltage desire for keeping steady driving current. Further, a capacitance between source or drain of the 1 st  FET and ground is operable to adjust source or drain voltage of the 1 st  FET. 
         [0027]    For over-temperature and over-current situations in most high power appliance, the present invention is operable to equip: a temperature sensor to detect system temperature T sys , cut off the driving current as an over-temperature protection when T sys &gt;T 1 , and reset for normal operation when system temperature is back to safe operation temperature T sys &lt;T 2  (as arrows in  FIG. 4A ); and a current monitor to monitor the driving current and cut off the driving current (as shown in  FIG. 4B ) or keep the driving current on an upper limit (as shown in  FIG. 4C ) as an over-current protection to prevent terrible harms for appliances. The over-temperature and over-current protections can be appendixed on gate voltage circuit  534  of the 1 st  FET or on output circuit of the 2 nd  OpAmp  144  but no need of an extra circuit for them. 
         [0028]    The driving system according to the present invention can be associated with an adjustable-voltage source  110  comprising: a DC-DC converter  111  or a voltage regulator or an AC-DC converter to rise/lower and rectify an external voltage source V DD  for output of the DC voltage V o  as shown in  FIG. 1B . Further, combining a voltage selection circuit  112  or an analog switch and digital control circuit can help the adjustable-voltage source to change the value of V o  more functionally by taking voltage selection signal  113  and switching a proper voltage circuit for feedback voltage  114  on circuit between the said external voltage source V DD  and the DC voltage V o . Furthermore, a controllable interface  160  is operable to take user commands for changing the voltage circuit in the adjustable-voltage source through a voltage controller  551 . The DC-DC converter  111  or the voltage regulator or the AC-DC converter is operable to have low drop-out function in order to avoid voltage dissipation from low input voltage. Moreover, a plurality of charge pumps can also be comprised to rise or lower voltage. With the above operable circuits, the range of adjustable voltage in the present invention becomes more flexible to fit most part of appliance. 
         [0029]    Accordingly, as disclosed by the above description and accompanying drawings, the present invention surely can accomplish its objective to provide a controllable driver and a driving system with excellent stability for LEDs, and may be put into industrial use especially for mass product. 
         [0030]    It should be understood that various modifications and variations could be made from the teaching disclosed above by the persons familiar in the art, without departing the spirit of the present invention.