Abstract:
A power supply circuit for a load has a first differential amplifier circuit, a first voltage-current conversion circuit, a second voltage-current conversion circuit, a reference voltage circuit, a second differential amplifier circuit, and an overheat protection circuit. The power supply circuit is constructed as an integrated circuit. A current detector circuit and bipolar transistors are attached externally to the integrated circuit. The power supply circuit can increase its output current with the reliability ensured, thus becoming able to supply a wide range of output currents.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and incorporates herein by reference Japanese Patent Application No. 2004-40203 filed on Feb. 17, 2004.  
       FIELD OF THE INVENTION  
       [0002]     This invention relates to a power supply circuit of a series regulator system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Conventionally, as a power supply circuit which can maintain its output voltage constant against variation of a load, a series regulator system is disclosed in JP-A-2001-337729. This series regulator is comprised of a booster transistor, a driver transistor for driving the booster transistor, and a differential amplifier. The differential amplifier compares an output voltage of the series regulator with a reference voltage and controls the booster transistor according to a difference between the two voltages through the driver transistor. The power supply circuit can thus ensure a constant output voltage.  
         [0004]     From the point of view of circuit design, when an output current flowing through this booster transistor is small, it is possible to integrate the booster transistor, the driving transistor and the differential amplifier in an IC (integrated circuit) monolithically. However, in the case where the output current is large, generation of heat within the booster transistor becomes large. Hence it is not possible to integrate the booster transistor in the IC monolithically.  
         [0005]     Therefore, it becomes necessary that the driver transistor and the differential amplifier are integrated in an IC monolithically, and an external booster transistor is attached to this IC as a discrete device.  
         [0006]     In recent years, an air bag system has developed into various kinds ranging from a small one that has only front seat air bags (for driver&#39;s seat and passenger&#39;s seat) to a large one that has rear seat air bags, side air bags, knee air bags, etc. additionally. With such diversification of the air bag systems, it has become necessary for power supply circuits of ECU (electronic control unit) forming the air bag systems to deliver large output currents as well as those of small output currents. When the above series regulator is used as a power supply circuit of ECU for an air bag system, two configurations are proposed.  
         [0007]     In one configuration, a plurality of ICs are provided according to output current magnitudes. In the other configuration, a single IC capable of delivering a large output current needed is used to meet output currents of any magnitudes needed.  
         [0008]     However, if a plurality of ICs each corresponding to an output current of a given magnitude are to be provided, a considerable amount of costs is required for development of these ICs. On the other hand, if a single IC capable of delivering output currents of any magnitudes needed is used for output currents of any magnitudes, the IC becomes relatively expensive in power supply circuits of small magnitudes as compared to an IC optimized for the current of that magnitude. Furthermore, this circuit configuration fails to realize a sophisticated protection capability which conforms to an output current magnitude.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an object of this invention to provide an improved power supply circuit which is less expensive and supports a wide range of output currents while ensuring reliability.  
         [0010]     A power supply circuit according to the present invention comprises, a first differential amplifier circuit, a first voltage-current conversion circuit, a second voltage-current conversion circuit, a reference voltage circuit and a second differential amplifier circuit. Those circuits are constructed monolithically as an integrated circuit. This integrated circuit is connectable to an external current supply path to increase a current to drive a load.  
         [0011]     The first differential amplifier circuit compares a first voltage corresponding to an output voltage applied to the load with a first reference voltage and generates a first difference voltage. The first voltage-current conversion circuit converts the first difference voltage to a first current and supplies the first current to the load. The second voltage-current conversion circuit converts the first difference voltage to a second current. The reference voltage circuit generates a second reference voltage from the second current. The second differential amplifier circuit compares a second voltage proportional to a magnitude of an external current supplied to the load through the external current path different from a current path of the first current with the second reference voltage, and generates a second difference voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
         [0013]      FIG. 1  is a circuit diagram of a power supply circuit in a first embodiment of the present invention;  
         [0014]      FIG. 2  is a circuit diagram of a power supply circuit in a second embodiment of the present invention; and  
         [0015]      FIG. 3  is a circuit diagram of a power supply circuit in a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     In the following embodiments, a power supply circuit is placed in, for example, an ECU for an air bag and used as a power supply circuit which generates an output voltage of 5 V used to drive a circuit from 12 V output voltage of a vehicle storage battery.  
       First Embodiment  
       [0017]     Referring to  FIG. 1 , a power supply circuit  1  is made up of an IC  8  which is comprised of a first differential amplifier circuit  2 , a first voltage-current conversion circuit  3 , a second voltage-current conversion circuit  4 , a reference voltage circuit  5 , a second differential amplifier circuit  6 , and an overheat protection circuit  7 .  
         [0018]     The first differential amplifier circuit  2  is comprised of resistors  20   a  and  20   b , a differential amplifier  21 , and a reference supply  22 . The resistor  20   a  and the resistor  20   b  are connected in series. One end of this serially connected resistors  20   a  and  20   b  is connected to a load  10  via an output terminal VCC of the IC  8 , and the other end of the same is connected to a vehicle chassis via a terminal GND of the IC  8 .  
         [0019]     An inverting input terminal of the differential amplifier  21  is connected to the junction between the resistor  20   a  and the resistor  20   b , and a non-inverting input terminal of the same is connected to a positive electrode terminal of the reference supply  22 . A negative electrode terminal of the reference supply  22  is grounded to the vehicle chassis via the terminal GND. An output terminal of the differential amplifier  21  is connected to the first voltage-current conversion circuit  3 .  
         [0020]     The first voltage-current conversion circuit  3  is constructed with two current mirror circuits.  
         [0021]     A first current mirror circuit is comprised of bipolar transistors  31   a ,  31   b , and  40  and resistors  32   a ,  32   b , and  41 . The bases of the bipolar transistors  31   a ,  31   b , and  40  are all connected to the collector of the bipolar transistor  31   a , the emitters of the same are connected to one ends of the resistors  32   a ,  32   b , and  41 , respectively. The other ends of the resistor  32   a ,  32   b , and  41  are grounded to the vehicle chassis all via the terminal GND. Resistance values of the resistors  32   a ,  32   b , and  41  are set such that the ratio of collector currents of the bipolar transistors  31   a ,  31   b , and  40  becomes a given ratio, for example, 1:10:10.  
         [0022]     A second current mirror circuit is comprised of field effect transistors  33   a ,  33   b  and resistors  34   a ,  34   b . The gates of the field effect transistors  33   a  and  33   b  are both connected to the drain of the field effect transistor  33   a , and the sources of the same are connected to one ends of the resistors  34   a  and  34   b , respectively. The other ends of the resistors  34   a  and  34   b  are both connected to the positive electrode terminal of the battery  9  via an input terminal VK. The mirror ratio (size ratio) of the field effect transistors  33   a  and  33   b  is set such that the ratio of their drain currents assumes a given value, for example, 1:50.  
         [0023]     The first voltage-current conversion circuit  3  is comprised of resistors  30 ,  32   a ,  32   b ,  34   a , and  34   b , the bipolar transistors  31   a  and  31   b , and the field effect transistors  33   a  and  33   b . One end of the resistor  30  is connected to the output terminal of the differential amplifier  21 , and the other end of the same is connected to the collector of the bipolar transistor  31   a , respectively. The collector of the bipolar transistor  31   b  is connected to the drain of the field effect transistor  33   a . The drain of the field effect transistor  33   b  is connected to the load  10  via the output terminal VCC.  
         [0024]     The second voltage-current conversion circuit  4  has the first current mirror circuit described above internally, and is comprised of resistors  30 ,  32   a , and  41  and the bipolar transistors  31   a  and  40 . The collector of the bipolar transistor  40  is connected to the reference voltage circuit  5 .  
         [0025]     The reference voltage circuit  5  is made up of a resistor  50 . One end of the resistor  50  is connected to the collector of the bipolar transistor  40 , and the other end of the same is connected to the battery  9  via the input terminal VK, respectively.  
         [0026]     The second differential amplifier circuit  6  is comprised of a differential amplifier  60  and a resistor  61 . A non-inverting input terminal of the differential amplifier  60  is connected to a junction between the collector of the bipolar transistor  40  and the resistor  50 , and an inverting input terminal of the same is connected to a terminal IS to which a current detector circuit needed when increasing the output current is connected and one end of the resistor  61 , respectively. The other end of the resistor  61  is grounded to the vehicle chassis via the terminal GND. The output terminal of the differential amplifier  60  is connected to a terminal OUT to which a transistor needed when increasing the output current is connected.  
         [0027]     The overheat protection circuit  7  is made up of a bipolar transistor  70 . The collector of the bipolar transistor  70  is connected to the collector of the bipolar transistor  31   a , and the emitter of the same is grounded to the vehicle chassis via the terminal GND. Moreover, the base of the bipolar transistor  70  is connected to an overheat detector circuit (not shown) which is installed in the IC  8  and detects overheat inside the IC  8 .  
         [0028]     In the first embodiment, when the output voltage of the battery  9  is fed to the input terminal VK of the IC  8 , the power supply circuit  1  will start its operation. The output voltage of the power supply circuit  1  is outputted from the output terminal VCC of the IC  8  to drive the load  10 . This output voltage is divided into voltages by the resistor  20   a  and the resistor  20   b.    
         [0029]     One of the divided voltages is applied to the inverting input terminal of the differential amplifier  21 , which compares the divided voltage with a voltage (a first reference voltage) of the reference supply  22  which is connected to the non-inverting input terminal. The differential amplifier  21  applies a voltage which is in proportion to the difference between the two voltages to the resistor  30  of the first voltage-current conversion circuit  3 .  
         [0030]     As a result, a collector current which is in proportion to the difference between the output voltage of the power supply circuit  1  and the voltage of the reference supply flows in the bipolar transistor  31   a  through the resistor  30 . This collector current of the bipolar transistor  31   a  is made to pass through the bipolar transistor  31   b  which, together with the bipolar transistor  31   a , forms the first current mirror circuit and enter the second current mirror circuit.  
         [0031]     The field effect transistor  33   b  forming the second current mirror circuit maintains its output voltage constant, and supplies a drain current (a first output current), which is 500 times larger than the collector current of the bipolar transistor  31   a , to the load via the output terminal VCC. For example, when the differential amplifier circuit  2  and the resistor  30  of the first voltage-current conversion circuit  3  are set such that a current of up to 200 pA flows in the bipolar transistor  31   a , the field effect transistor  33   b  can supply an output current of up to 100 mA to the load  10  via the output terminal VCC. Moreover, a collector current, which is 10 times larger than the collector current of the bipolar transistor  31   a , flows in the bipolar transistor  40  forming the first current mirror circuit.  
         [0032]     This collector current of the bipolar transistor  40  is converted to a voltage (a second reference voltage) by the resistor  50  forming the reference voltage circuit  5 . This voltage is fed to the non-inverting input terminal of the differential amplifier  60 . However, no circuit is connected to the output terminal of the differential amplifier  60  at all. Consequently the differential amplifier circuit  60  does not affect the output of the power supply circuit  1 .  
         [0033]     When the inside of the IC  8  is overheated to, for example, 150° C. or more, the overheat detector circuit detects this overheating and applies a voltage to the base of the bipolar transistor  70  of the overheat protection circuit  7 . Then, a base current flows in the bipolar transistor  70 , and turns on the bipolar transistor  70 . The turn-on of the bipolar transistor  70  cuts off the collector currents of the bipolar transistors  31   a  and  40 . Further, the cut-off of the collector current of the bipolar transistor  31   a  cuts off the drain current of the field effect transistor  33   b  as well. Consequently, the power supply circuit  1  suspends its output.  
         [0034]     According to the first embodiment, the power supply circuit  1  can ensure its output voltage by the field effect transistor  33   b  forming the first voltage-current conversion circuit  3 , and can supply a small output current, for example, 100 mA to the load  10 . In addition, since the power supply circuit  1  can be constructed in the form of the IC  8 , the cost can be reduced.  
         [0035]     It should be noted that the power supply circuit  1  is constructed with the first voltage-current conversion circuit  3  including the current mirror circuit. Therefore, the power supply circuit  1  cannot supply a current which exceeds a maximum output current, for example, 100 mA determined by circuit parameters to the load  10 . Thus, when the power supply circuit  1  becomes overloaded, the output voltage of the power supply circuit  1  drops, and the overload can be detected by a supply voltage drop detector circuit or the like installed in the load  10  connected thereto, i.e., the ECU.  
         [0036]     When the inside of the IC  8  becomes overheated to, for example, 150° C. or more, the output of the power supply circuit  1  can be suspended by the overheat protection circuit  7 . Consequently, the power supply circuit can be constructed as a very reliable device. Moreover, the power supply circuit  1  can perform voltage-current conversion certainly by constructing the first voltage-current conversion circuit  3  and the second voltage-current conversion circuit  4  each in the form of a current mirror circuit.  
       Second Embodiment  
       [0037]     In the second embodiment, the same components as those of the first embodiment are designated by similar references to give the explanation.  
         [0038]     As shown in  FIG. 2 , the power supply circuit  1  is constructed by attaching a current detector circuit  11  and a bipolar transistor  12  (main transistor) externally to the IC  8  including the first differential amplifier circuit  2 , the first voltage-current conversion circuit  3 , the second voltage-current conversion circuit  4 , the reference voltage circuit  5 , the second differential amplifier circuit  6 , and the overheat protection circuit  7 .  
         [0039]     The current detector circuit  11  is made up of a resistor  110 . One end of the resistor  110  is connected to the battery  9 , and the other end of the same is connected to both the terminal IS of the IC  8  and the emitter of the bipolar transistor  12 , respectively. The base of the bipolar transistor  12  is connected to the terminal OUT of the IC  8 , and the collector of the same is connected to the load  10  via the output terminal VCC, respectively.  
         [0040]     In the second embodiment, a collector current, which is 10 times larger than the collector current of the bipolar transistor  31   a  flows in the bipolar transistor  40  forming the first current mirror circuit. This collector current of the bipolar transistor  40  is converted to a voltage (the second reference voltage) by the resistor  50  forming the reference voltage circuit  5 , and fed to the non-inverting input terminal of the differential amplifier  60 . A collector current (second output current) of the bipolar transistor  12  flows in the resistor  110  forming the current detector circuit  11 . This collector current of the bipolar transistor  12  is converted to a voltage by the resistor  110  and fed to an inverting input terminal of the differential amplifier  60 .  
         [0041]     The differential amplifier  60  controls the bipolar transistor  12  by applying a voltage to the base of the bipolar transistor  12  so that a voltage which the current detector circuit  11  generates becomes equal to a voltage which the reference voltage circuit  5  generates. By this operation, the bipolar transistor  12  maintains its output voltage constant, and supplies a collector current to the load  10  so that the voltage which the current detector circuit  11  generates becomes equal to the voltage which the reference voltage circuit  5  generates.  
         [0042]     For example, when the differential amplifier circuit  2  and the resistor  30  of the first voltage-current conversion circuit  3  are set such that a current of up to 200 μA flows in the bipolar transistor  31   a , the bipolar transistor  40  can supply a current of up to 2 mA to the reference supply circuit  5 .  
         [0043]     At this time, for example, when the resistor  50  of the reference supply circuit  5  is 200 Ω and the resistor  110  of the current detector circuit  11  is 2 Ω, since the voltage which the reference supply circuit  5  generates is 0.4 V at the maximum, the bipolar transistor  12  can supply its output current of up to 200 mA to the load  10 . As a result, the power supply circuit  1  can supply an output current of up to 300 mA, which is a sum of the drain current of the field effect transistor  33   b  and the collector current of the bipolar transistor  12 , to the load  10 .  
         [0044]     According to the second embodiment, the power supply circuit  1  can ensure the output voltage by attaching the resistor  110  forming the current detector circuit  11  and the bipolar transistor  12  externally to the IC  8 , and can increase its output current of up to 300 mA as described above.  
         [0045]     In case a disconnection occurs in a path from the terminal IS of the IC  8  to both the resistor  110  and the emitter of the bipolar transistor  12 , or in a path from the terminal OUT to the base of the bipolar transistor  12 , the bipolar transistor  12  turns off and the power supply circuit  1  becomes overloaded. Then, the output voltage of the power supply circuit  1  drops, and overload can be detected on the load  10  side. Therefore, the power supply circuit can be made very reliable.  
       Third Embodiment  
       [0046]     In the third embodiment, the same components as those of the above embodiments are designated by similar references.  
         [0047]     As shown in  FIG. 3 , the power supply circuit  1  is constructed by attaching the current detector circuit  11  and bipolar transistors  12   a  and  12   b  (main transistors) externally to the IC  8  including the first differential amplifier circuit  2 , the first voltage-current conversion circuit  3 , the second voltage-current conversion circuit  4 , the reference voltage circuit  5 , the second differential amplifier circuit  6 , and the overheat protection circuit  7 .  
         [0048]     The current detector circuit  11  is made up of resistors  110   a  and  110   b . One end of the resistor  110   a  is connected to the battery  9 , and the other end of the same is connected to both the terminal IS of the IC  8  and the emitter of the bipolar transistor  12   a , respectively. The base of the bipolar transistor  12   a  is connected to the terminal OUT of the IC  8 , and the collector of the same is connected to the load  10  via the output terminal VCC, respectively.  
         [0049]     One end of the resistor  110   b  is connected to the battery  9 , and the other end of the same is connected to the emitter of the bipolar transistor  12   b , respectively. The base of the bipolar transistor  12   b  is connected to the terminal OUT of the IC  8 , and the collector of the same is connected to the load  10  via the output terminal VCC, respectively.  
         [0050]     The differential amplifier  60  controls the bipolar transistors  12   a  and  12   b  by applying a voltage to the bases of the bipolar transistors  12   a  and  12   b  so that the voltage which the current detector circuit  11  generates becomes equal to the voltage which the reference voltage circuit  5  generates. By this operation, the bipolar transistors  12   a  and  12   b  maintain their output voltages constant, and supply the collector currents to the load  10 , respectively, so that the voltage which the current detector circuit  11  generates and the voltage which the reference voltage circuit  5  generates become equal.  
         [0051]     For example, when the differential amplifier circuit  2  and the resistor  30  of the first voltage-current conversion circuit  3  are set in such a way that a current of up to 200 μA flows in the bipolar transistor  31   a , the bipolar transistor  40  is enabled to feed a current of up to 2 mA to the reference supply circuit  5 .  
         [0052]     At this time, when the resistor  50  of the reference supply circuit  5  is 200 Ω and the resistors  110   a  and  110   b  of the current detector circuit  11  are both 1 Ω, since the voltage which the reference supply circuit  5  generates is 0.4 V at the maximum, and then each of the bipolar transistors  12   a  and  12   b  can supply an output current of up to 400 mA to the load  10 , respectively. As a result, the power supply circuit  1  can supply its output current of up to 900 mA, which is a sum of the drain current of the field effect transistor  33   b  and the collector currents of the bipolar transistors  12   a  and  12   b , to the load  10 .  
         [0053]     According to the third embodiment, by attaching the resistors  110   a  and  110   b  forming the current detector circuit  11  and the bipolar transistors  12   a  and  12   b  externally to the IC  8 , the power supply circuit  1  can ensure the output voltage and can further increase its output current of up to, for example, 900 mA.  
         [0054]     In the third embodiment, more than two pairs of a transistor and a resistor which are connected in series may be connected in parallel between the VCC terminal and the terminal VK of the IC  8  to form the current detector circuit.  
         [0055]     The present invention should not be limited to the disclosed embodiments, but may be modified in other ways without departing from the spirit of the invention.