Abstract:
A linear voltage regulator is provided for providing an output voltage to a load. In a preferred embodiment, the linear voltage regulator includes: a pass element for receiving an input voltage and providing an output voltage to a load, the pass element being adapted to be controlling by a controlling voltage; two resistors connected to each other in series for receiving the output voltage and providing a voltage reference; and a feedback circuit for receiving the voltage reference and providing the controlling voltage to the pass element. The linear voltage regulator is capable of providing a steady output voltage to the load, and a cost thereof being down at the same time.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     Relevant subject matter is disclosed in co-pending U.S. patent application Ser. No. ______ entitled “LINEAR VOLTAGE REGULATOR”, which is assigned to the same assignee with this application. 
     
    
     BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to voltage regulators, and particularly to a linear voltage regulator for providing a regulated voltage to a load mounted on a motherboard.  
         [0004]     2. General Background  
         [0005]     Linear voltage regulators are widely used to supply power to electronic devices, such as to a load on a motherboard of a computer. Such linear voltage regulators are available in a wide variety of configurations for many different applications.  
         [0006]     Referring to  FIG. 9 , a first typical linear voltage regulator includes a resistive voltage divider  1 , a feedback circuit  2 , and a regulating circuit  3 . The resistive voltage divider  1  includes a resistor R 1  and a resistor R 2  connected to each other in series between a system voltage and a ground. A node between the resistor R 1  and the resistor R 2  provides a voltage reference Vref to the feedback circuit  2 . The feedback circuit  2  includes an amplifier  22 . The amplifier  22  has a non-inverting input terminal, an inverting input terminal, and an output terminal. The non-inverting input terminal is connected to the node of the resistive voltage divider  1  for receiving the voltage reference Vref. The inverting input terminal receives an output voltage Vout. The output terminal is connected to the regulating circuit  3  to provide a controlling voltage for controlling the regulating circuit  3 . The regulating circuit  3  includes a metal-oxide-semiconductor field-effect transistor (MOSFET) Q 1 . The MOSFET Q 1  includes a gate, a source, and a drain. The gate is connected to the output terminal of the amplifier  22  for receiving the controlling voltage. The drain is connected to a system voltage Vcc. The source is connected to a load for providing the output voltage Vout.  
         [0007]     When the output voltage Vout suddenly increases, the controlling voltage decreases correspondingly. As a result, a voltage UGS (not shown in  FIG. 10 ) between the gate and the source decreases. The decrease of the voltage UGS induces a reduction of a current through the MOSFET Q 1 . Therefore the output voltage Vout drops to a same level as before the sudden increase thereof. Contrarily, when the output voltage Vout suddenly decreases, the controlling voltage increases correspondingly. Then the voltage U GS  increases. The increase of the voltage U GS  induces an increasing of the current through the MOSFET Q 1 . Therefore the output voltage Vout climbs to a same level as before the sudden decrease thereof.  
         [0008]     Referring to  FIG. 10 , a second typical linear voltage regulator includes a first transistor Q 2 , a second transistor Q 3 , and resistors R 3  and R 4 . The resistors R 3  and the resistor R 4  are connected to each other in series between an output voltage Vout and a ground. A node between the resistors R 3  and R 4  provides a voltage reference Vref to the transistor Q 2 . The transistor Q 2  as a feedback circuit feeds the output voltage Vout back to the transistor Q 3 . The transistor Q 3  as a regulating circuit provides the output voltage Vout to a load.  
         [0009]     However, a cost of the first typical linear voltage regulator is high because of employing the amplifier as the feedback circuit. The voltage reference Vref of the second typical linear voltage regulator is not steady because of employing the bipolar transistor as the feedback circuit.  
         [0010]     What is needed, therefore, is a linear voltage regulator which is able to provide a steady current to a load and has a low cost.  
       SUMMARY  
       [0011]     A linear voltage regulator is provided for providing an output voltage to a load. In a preferred embodiment, the linear voltage regulator includes: a pass element for receiving an input voltage and providing an output voltage to a load, the pass element being adapted to be controlling by a controlling voltage; two resistors connected to each other in series for receiving the output voltage and providing a voltage reference; and a feedback circuit for receiving the voltage reference and providing the controlling voltage to the pass element. Since the feedback circuit employs a three-terminal adjustable shunt regulator, a cost of the linear voltage regulator is lower than that of the first typical linear voltage regulator. Because the three-terminal adjustable shunt regulator can provides a steadier voltage reference than a bipolar can, the embodiment of the invention can provides a steadier output voltage than the second typical linear voltage regulator illustrated in the background.  
         [0012]     The linear voltage regulator is capable of providing a steady output voltage to the load, and a cost thereof being down at the same time.  
         [0013]     Other advantages and novel features will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a circuit diagram of a linear voltage regulator of a preferred embodiment of the present invention;  
         [0015]      FIG. 2-6  are various embodiments of the pass element comprising two or three bipolar transistors;  
         [0016]      FIG. 7-8  are various embodiments of the pass element comprising two MOSFETs;  
         [0017]      FIG. 9  is a circuit diagram of a first typical linear voltage regulator; and  
         [0018]      FIG. 10  is a circuit diagram of a second typical linear voltage regulator. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0019]     Referring to  FIG. 1 , in a preferred embodiment of the present invention, a linear voltage regulator includes a feedback circuit  10 , a pass element  20 , a resistive voltage divider  30 . The feedback circuit  10  is a three-terminal adjustable shunt regulator in the embodiment. The feedback circuit  10  includes a first terminal  10 - 1  (shown as “1” in the circuit  10 ), a second terminal  10 - 2  (shown as “2” in the circuit  10 ), and a third terminal  10 - 3  (shown as “3” in the circuit  10 ). The first terminal  10 - 1  receives a voltage reference Vref. The second terminal  10 - 2  is grounded. The third terminal  10 - 3  provides a controlling voltage V 2  to the pass element  20  for controlling the pass element  20 , and is coupled to a system voltage V 1  via a resistor R 6 . The pass element  20  is a metal-oxide-semiconductor field-effect transistor Q 4  in the embodiment. The pass element  20  includes a gate as a controlling terminal, a drain as an input terminal, and a source as an output terminal. The gate is connected to the third terminal  10 - 3  for receiving the controlling voltage V 2 , and is coupled to the system voltage V 1  via the resistor R 6 . The drain receives an input voltage Vin. The source provides an output voltage Vout to a load RL. The resistive voltage divider  30  includes a resistor R 7  and a resistor R 8  connected to each other in series between the output voltage and a ground. A node between the resistor R 7  and the resistor R 8  provides a voltage reference Vref to the first terminal  11  of the feedback circuit  10 .  
         [0020]     When the output voltage Vout suddenly increases, the voltage reference Vref increases correspondingly. Then the controlling voltage V 2  decreases. A voltage Δ U GS  between the gate and the source decreases than before the sudden increase thereof. The decrease of the voltage Δ U GS  induces a decrease of the output voltage Vout. Therefore the output voltage Vout drops to a same level as before the sudden increase thereof.  
         [0021]     Contrarily, when the output voltage Vout suddenly becomes lower, the voltage reference Vref becomes lower correspondingly. Then the controlling voltage V 2  becomes higher. The voltage Δ U GS  becomes higher than before the sudden increase thereof. The increase of the voltage Δ U GS  induces an increase of the output voltage Vout. Therefore the output voltage Vout climbs to a same level as before the sudden increase thereof.  
         [0022]     As shown in  FIG. 2 , the pass element  20  can include a PNP bipolar transistor Q 5 , and a PNP bipolar transistor Q 6 . An emitter of the PNP bipolar transistor Q 5  is connected to a base of the PNP bipolar transistor Q 6 . Collectors of the PNP bipolar transistor Q 5  and PNP bipolar transistor Q 6  are connected to each other as the input terminal, and receive the input voltage Vin. A base of the PNP bipolar transistor Q 5  as the controlling terminal receives the controlling voltage V 2 . An emitter of the PNP bipolar transistor Q 6  as the output terminal provides the output voltage Vout.  
         [0023]     As shown in  FIG. 3 , the pass element  20  can include an NPN bipolar transistor Q 7 , and an NPN bipolar transistor Q 8 . An emitter of the NPN bipolar transistor Q 7  is connected to a base of the NPN bipolar transistor Q 8 . Collectors of the NPN bipolar transistor Q 7  and NPN bipolar transistor Q 8  are connected to each other as the input terminal, and receive the input voltage Vin. A base of the NPN bipolar transistor Q 7  as the controlling terminal receives the controlling voltage V 2 . An emitter of the NPN bipolar transistor Q 8  as the output terminal provides the output voltage Vout.  
         [0024]     As shown in  FIG. 4 , the pass element  20  can include an NPN bipolar transistor Q 9 , and a PNP bipolar transistor Q 10 . A collector of the NPN bipolar transistor Q 9  is connected to a base of the PNP bipolar transistor Q 10 . An emitter of the NPN bipolar transistor Q 9  and a collector of the PNP bipolar transistor Q 10  are connected to each other as the input terminal, and receive the input voltage Vin. A base of the NPN bipolar transistor Q 9  as the controlling terminal receives the controlling voltage V 2 . An emitter of the PNP bipolar transistor Q 10  as the output terminal provides the output voltage Vout.  
         [0025]     As shown in  FIG. 5 , the pass element  20  can include a PNP bipolar transistor Q 11 , and an NPN bipolar transistor Q 12 . A collector of the PNP bipolar transistor Q 11  is connected to a base of the NPN bipolar transistor Q 12 . An emitter of the PNP bipolar transistor QIl and a collector of the NPN bipolar transistor Q 12  are connected to each other as the input terminal, and receive the input voltage Vin. A base of the PNP bipolar transistor Q 11  as the controlling terminal receives the controlling voltage V 2 . An emitter of the NPN bipolar transistor Q 12  as the output terminal provides the output voltage Vout.  
         [0026]     As shown in  FIG. 6 , the pass element  20  can include a PNP bipolar transistor Q 13 , an NPN bipolar transistor Q 14 , and an NPN bipolar transistor Q 15 . A collector of the PNP bipolar transistor Q 13  is connected to a base of the NPN bipolar transistor Q 14 . An emitter of the NPN bipolar transistor Q 14  is connected to a base of the NPN bipolar transistor Q 15 . An emitter of the PNP bipolar transistor Q 13 , a collector of the NPN bipolar transistor Q 14 , and a collector of the NPN bipolar transistor Q 15  are connected to each other as the input terminal, and receive the input voltage Vin. A base of the PNP bipolar transistor Q 13  as the controlling terminal receives the controlling voltage V 2 . An emitter of the NPN bipolar transistor Q 15  as the output terminal provides the output voltage Vout.  
         [0027]     As shown in  FIG. 7 , the pass element  20  can include a N-channel MOSFET Q 16 , and a N-channel MOSFET Q 17 . Gates of the N-channel MOSFET Q 16  and N-channel MOSFET Q 17  are connected to each other as the controlling terminal, and receive the controlling voltage V 2 . Drains of the N-channel MOSFET Q 16  and N-channel MOSFET Q 17  are connected to each other as the input terminal, and receive the input voltage Vin. Sources of the N-channel MOSFET Q 16  and N-channel MOSFET Q 17  are connected to each other as the output terminal, and provide the output voltage Vout.  
         [0028]     As shown in  FIG. 8 , the pass element  20  can include a P-channel MOSFET Q 18 , and an N-channel MOSFET Q 19 . A drain of the P-channel MOSFET Q 18  is connected to a gate of the N-channel MOSFET. A gate of the P-channel MOSFET Q 18  as the controlling terminal receives the controlling voltage V 2 . A source of the P-channel MOSFET Q 18  and a drain of the N-channel MOSFET Q 19  are connected to each other as the input terminal, and receive the input voltage Vin. A source of the N-channel MOSFET Q 19  as the output terminal provides the output voltage Vout.  
         [0029]     In the illustrated embodiments, because that the feedback circuit  10  employs the three-terminal adjustable shunt regulator, a cost is lower than that of the first typical linear voltage regulator illustrated in the background. At the same time, because the three-terminal adjustable shunt regulator can provide a steadier voltage reference than a bipolar can, the embodiments of the invention can provide a steadier output voltage than the second typical linear voltage regulator illustrated in the background.  
         [0030]     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.