Abstract:
A current source comprises a current driver comprising a current generator and a first resistor serially coupled at a first node, a level shift unit located between the first node and a second node to generate a rated voltage difference between the second and the first nodes, and a voltage regulator device having an input terminal coupled to the second node and an output terminal coupled to a control terminal of the current generator. The voltage regulator device maintains the voltage level of the second node at a first voltage reference by modifying the voltage level of the control terminal. Along with the variation of the voltage level of the control terminal, a supply current generated by the current source for a load is varied to modify the voltage level of the second node to the first voltage level. The control loop stabilizes the supply current value.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to current sources and more particularly to controllable large current sources supplying large current to high power electronic devices. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 1  shows a conventional current source  100 , which generates a constant supply current I o . The conventional current source  100  comprises a TLV431 regulator  102  and a transistor  104  operating as a current generator. The TLV431 regulator  102  maintains the voltage value of node  106  at a first voltage level V ref . A constant current I 1  flows through a first resistor R 1 , wherein I 1 =V ref /R 1 . In transistor  104 , the emitter current approximates the collector current. A reference input terminal of the TLV431 regulator  102  is coupled to node  106  and has high input impedance. The supply current I o , therefore, approximates the constant current I 1 (I o ≈I 1 ). If the supply current I o  deviates from the constant current I 1  (wherein I 1 =V ref /R 1 ), the voltage level of node  106  deviates correspondingly from the first voltage level V ref . The voltage deviation is detected and adjusted by the TLV431 regulator  102 . As shown in  FIG. 1 , node  110  is connected to a control terminal of the current generator implemented by the transistor  104 . The control terminal is the base terminal of the transistor  104 . The supply current I o  is determined by the voltage value of the control terminal  110 . Along with the variation in the supply current I o , the voltage value of node  106  can be adjustable. The TLV431 regulator  102  adjusts the voltage level of the control terminal  110  and adjusts the base current of the transistor  104  to control the supply current I o  to maintain the voltage value of node  106  at the first voltage value V ref , and therefore the supply current I o  can maintain at the constant value V ref /R 1 . 
         [0005]    With the conventional current source  100 , however, the supply current I o  is insufficient for high power electronic devices. For example, when the first voltage level (V ref ) of the TLV431 regulator  102  is 1.24V, the first resistor R 1  is set to 1.24 Ohm to generate a supply current I o  of 1 Amp for a load  108 . The power consumption of the first resistor R 1  is 1.24 W (evaluated from P=I·V=1A·1.24V=1.24W). Currently, 1.24 W is considerably large for a chip. In general, the conventional current source  100  is designed to generate a supply current less than 500 mA. A current source generates large supply current for high power electronic devices such as direct current motors, power LEDs, or energy generators and others is thus called for. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    Novel current sources are provided to generate large supply current. The magnitude of the supply current is controllable and the supply current can be set as a pulse wave. 
         [0007]    An exemplary embodiment of a current source comprises a current driver, a level shift unit and a voltage regulator device. The current driver comprises a current generator and a first resistor which are coupled in series via a first node. The current generator comprises a control terminal, and generates a supply current for a load. The first node is coupled to a second node via the level shift unit. The level shift unit generates a rated voltage difference between the first and the second nodes. The input terminal and the output terminal of the voltage regulator device are coupled to the second node and the control terminal, respectively. The voltage level of the control terminal of the current generator is adjusted by the voltage regulator device to maintain the voltage level of the second node at a first voltage level. 
         [0008]    The level shift unit comprises a constant current source and a second resistor. The second resistor is coupled between the first and the second nodes. The constant current from the constant current source flows through the second resistor and generates a constant voltage across the second resistor. The voltage regulator device may be implemented by a voltage regulator chip having an input terminal and a cathode terminal respectively coupled to the second node and the control terminal. In another exemplary embodiment, the level shift unit further comprises a third resistor and a variable voltage source. The third resistor is coupled between the output terminal of the variable voltage source and the second node. The rated voltage difference between the first and the second nodes varies with the output voltage level of the variable voltage source. The supply current decreases with increasing output voltage of the variable voltage source. 
         [0009]    In another exemplary embodiment, the level shift unit comprises a second resistor, a third resistor, and a variable voltage source. The first node is coupled to the second node via the second resistor. The third resistor is coupled between the output terminal of the variable voltage source and the second node. A rated voltage difference, generated by the level shift unit, is maintained between the first and the second nodes. The rated voltage difference varies with the output voltage level of the variable voltage source. The supply current decreases with increasing output voltage of the variable voltage source. 
         [0010]    In another exemplary embodiment, the current source further comprises a current source switch coupled to the control terminal. The current source switch can shut down the current source by coupling the control terminal to a second voltage level. If the current source switch shuts down the current source intermittently, the supply current is a pulse wave. The current source switch comprises a pulse voltage source and a switch. When the output of the pulse voltage source is at a first level, the control terminal is coupled to the second voltage level by the switch, and the current source is shut down. When the output of the pulse voltage source is at a second level, the switch ceases coupling the control terminal to the second voltage level, the current source generates the supply current normally. The switch comprises a fourth resistor, a fifth resistor and a transistor. The fourth resistor is coupled between the output terminal of the pulse voltage source and the base of the transistor. The fifth resistor is coupled between the base and the emitter of the transistor. The collector and the emitter of the transistor are coupled to the control terminal and the second voltage level, respectively. 
         [0011]    In another exemplary embodiment, the current source further comprises a diode and a sixth resistor. The anode and the cathode of the diode are coupled to the output of the voltage regulator device and the control terminal, respectively. The cathode of the diode is coupled to ground via the sixth resistor. The diode ensures the voltage level of the output of the regulator is in a correct region. 
         [0012]    In another exemplary embodiment, the current generator of the current driver may be a transistor or a Darlington circuit. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0014]      FIG. 1  shows a conventional current source; 
           [0015]      FIG. 2  shows an embodiment of the invention; 
           [0016]      FIG. 3  shows another embodiment of the invention; 
           [0017]      FIG. 4  shows another embodiment of the invention; 
           [0018]      FIG. 5  shows another embodiment of the invention; 
           [0019]      FIG. 6  shows another embodiment of the invention; and 
           [0020]      FIG. 7  shows another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0022]      FIG. 2  shows an embodiment of the invention. The current source  200  comprises a first node  210 , a second node  216 , a current driver  202 , a level shift unit  204  and voltage regulator device  206 . The current driver  202  comprises a current generator  208  and a first resistor R 1 . In the embodiment, the current generator  208  is implemented by a transistor Q 1 . The current generator  208  is coupled to the first resistor R 1  in series via the first node  210 . According to the voltage difference between a control terminal  212  (the base of transistor Q 1 ) and the first node  210  (the emitter of transistor Q 1 ), the transistor Q 1  generates a supply current I o  for a load  214 . The level shift unit  204  is coupled between the first node  210  and the second node  216  to generate a rated voltage difference therebetween. The voltage level of the first node  210  is lower than that of the second node  216 . In the embodiment, a TLV431 regulator IC 1  is implemented as the voltage regulator device  206 . The reference input terminal and the cathode of the TLV431 regulator IC 1  are the input terminal and the output terminal of the voltage regulator device  206 , respectively. The voltage regulator device  206  may be implemented by other chips such as TS 431(ST), LMV431(NS), RC431A(Fairchild), APL431L(ANPEC), AT431(Aimtron), CAT431L(Catalyst) and others. 
         [0023]    The reference terminal and the cathode of the TLV431 regulator IC 1  are coupled to the second node  216  and the control terminal  212 , respectively. If the voltage level of the second node  216  deviates from a first voltage level V ref , the TLV431 regulator IC 1  adjusts the voltage level of the control terminal  212  to change the supply current I o . The voltage level of the first node  210  varies with the supply current I o . The control loop can maintain the voltage level of the first node at the first voltage level V ref , and the supply current I o  is maintained at a constant value. The level shift unit  204  comprises a constant current source I G  and a second resistor R 2 . The first node  210  is coupled to the second node  216  by the second resistor R 2 . The magnitude of the constant current source I G  and the second resistor R 2  are defined by the user. The constant current I G  flows through the second resistor R 2  and generates a constant voltage difference V R2 (I G ) across the second resistor R 2 . When the current supply  200  is in stable, the voltage level of the first node is a constant value of V ref -V R2 (I G ), and the supply current I o  is constant. When the constant current I G  is 0.94 mA and the second resistor R 2  is 1 KOhm, the rated voltage between the second and the first nodes  216  and  210  is 0.94V. When the first voltage level V ref  is 1.24V, the voltage level of the first node is 0.3V (1.24V−0.92V). If the supply current I o  is 1 A, the first resistor R 1  approximates 0.3 Ohm. The power consumption of the first resistor R 1  approximates 0.3 Watt (P=I·V). The power consumption of the first resistor R 1  of the current source  200  is much lower than that of the conventional current source  100  (which requires 1.24 W to generate a supply current of 1 A). The novel current source can generate high supply current for high power application. The level shift unit  204  may be implemented by other devices which can maintain the voltage level of the first node  210  at a value lower than the first voltage level V ref  and decrease the power consumption of the first resistor R 1 . 
         [0024]      FIG. 3  shows another embodiment of the invention. The difference between the current sources  200  and  300  is the level shift unit. In  FIG. 3 , the level shift unit  304  comprises a second resistor R 2 , a constant current source I G , a third resistor R 3 , and a variable voltage source S v . There is a rated voltage difference between the second and the First nodes. The constant I G  flows through the second resistor R 2  and generates a constant voltage difference V R2 (I G ) across the second resistor R 2 . The variable voltage source S v  generates a current I v , (S v -V ref )/R 3 , through the third resistor R 3 . The current I v  generates a voltage difference V R2 (I v ), varying with the output voltage level of the variable voltage source S v , across the second resistor R 2 . The rated voltage difference between the second and the first nodes is (V R2 (I G )+V R2 (I v )). The variable voltage source S v  controls the rated voltage difference to control the voltage level of the first node  310 . The voltage level of the first node  310  is V ref -(V R2 (I G )+V R2 (I v )). When the output voltage of the variable voltage source S v  exceeds the first voltage level V ref , V R2 (I v ) is positive and the rated voltage difference (V R2 (I G )+V R2 (I v )) exceeds V R2 (I G ), the voltage level of the first node is lower than V ref -V R2 (I G ). When the output voltage of the variable voltage source S v  is lower than the first voltage level V ref , V R2 (I v ) is negative and the rated voltage difference (V R2 (I G )+V R2 (I v )) is lower than V R2 (I G ), the voltage level of the first node exceeds V ref -V R2 (I G ). The higher the output voltage of the variable voltage source S v , the lower the voltage level of the first node  310  and the lower the supply current I o . In the embodiment, the voltage level of the first node  310  may exceed the first reference voltage level V ref  if the output voltage of the variable voltage source S v  is too small. In such a situation, the third resistor R 3  has to be far larger than the second resistor R 2  to prevent the voltage level of the first node  310  from exceeding the first reference voltage level V ref . In general, we select the third resistor R 3  is about 10 times than the second resistor R 2 . 
         [0025]      FIG. 4  shows another embodiment of the invention. The difference between the current sources  300  and  400  is the level shift unit. In  FIG. 4 , the level shift unit  404  comprises a second resistor R 2 , a third resistor R 3 , and a variable voltage source S v . The current I v  through the third resistor R 3  is (S v -V ref )/R 3 . The current I v  generates a rated voltage difference V R2 (I v ) across the third resistor R 3 . The voltage level of the first node  410  varies with the rated voltage difference V R2 (I v ) which varies with the output voltage of the variable voltage source S v . When the output voltage of the variable voltage source S v  exceeds the first voltage level V ref , the voltage level of the first node  410  is lower than that of the second node. When the output voltage of the variable voltage source S v  is lower than the first voltage level V ref , the voltage level of the first node  410  exceeds that of the second node. The magnitude of the supply current I o  can be controlled by the variable voltage source S v . The supply current I o  decreases with increasing output voltage level of the variable voltage source S v . 
         [0026]      FIG. 5  shows another embodiment of the invention. Unlike that shown in  FIG. 2 , the current source  500  here further comprises a current source switch  518  which is coupled to the control terminal  512 . The current source switch  518  can couple the control terminal  512  to a second voltage level (such as ground) to shut down transistor Q 1  to stop the supply current I o  and shut down the current source  500 . The current source switch  518  can control the supply current I o  to be a pulse wave by intermittently coupling the control terminal  512  to ground. The current source switch  518  comprises a pulse voltage source S p  and a switch  520 . The switch  520  comprises a fourth resistor R 4 , a fifth resistor R 5 , and a transistor Q 2 . The fourth resistor R 4  is coupled between the output of the pulse voltage source S p  and the base of the transistor Q 2 . The fifth resistor R 5  is coupled between the base and the emitter of the transistor Q 2 . The collector and the emitter of the transistor Q 2  are coupled to the control terminal  512  and ground, respectively. When the output of the pulse voltage source S p  is at a first level (a high voltage level), the transistor Q 2  is turned on and the control terminal  512  is coupled to ground via the transistor Q 2 , and current source  500  is shut down. When the output of the pulse voltage source S p  is at a second level (a low voltage level), the transistor Q 2  is turned off. The current source  500  can normally generate the supply current I o . The current source switch  518  can also be introduced to the current sources  300  and  400  to generate supply current in pulse form. Any embodiment of the invention can adopt the current source switch  518 . The current sources comprising the current source switch  518  coupled at the control terminal to turn on/off the current source or to generate a supply current in a pulse form are in the scope of the disclosure. 
         [0027]    The voltage difference between the cathode and the anode of the TLV431 regulator IC 1  must exceed a minimum operating voltage to ensure the correct operation of the TLV431 regulator IC 1 .  FIG. 6  shows another embodiment of the invention. Unlike current source  200 , the current source  600  here further comprises a diode D 1  and a sixth resistor R 6 . The anode and the cathode of the TLV431 regulator IC 1  are coupled to the cathode of the TLV431 regulator IC 1  ( 622 ) and the control terminal  612 , respectively. When the transistor Q 1  is conducting, the voltage difference provided by the diode D 1 , the base-emitter of the transistor Q 1  and the first resistor R 1  must exceed the minimum operating voltage to ensure the correct operation of the TLV431 regulator IC 1 . The technique disclosed in  FIG. 6  can be applied to other embodiments of the invention to ensure correct operation of the voltage regulator device. 
         [0028]      FIG. 7  shows another embodiment of the invention. Unlike current source  200 , the current generator of the current source  700  is here implemented by a Darlington circuit. The current generator of all embodiments of the invention can be replaced by the Darlington circuit or any circuit having similar function. 
         [0029]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded to the broadest interpretation so as to encompass all such modifications and similar arrangements.