Abstract:
The present invention provides a switching regulator for controlling the overshoot voltage which is generated during transition from power ON, when soft start activates, to normal operation, when the load side output is maintained at the output setting voltage by the feedback circuit. A clamp circuit is disposed between a soft start circuit and an error amplifier, an upper limit value from the output of the soft start circuit (SS) to the output of the error amplifier (FB) is set, so that the rise of voltage of the output of the error amplifier (FB) is controlled when power is turned ON, and time until the output of the error amplifier (FB) is stabilized is decreased, by which the overshoot voltage generated in the load side output (VOUT) is controlled.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switching regulator for supplying power from the power supply to the load side via a switching element, with which a predetermined voltage at the load side (output setting voltage) can be acquired by switching this switching element under a predetermined control, and more particularly to a switching regulator that has a soft start function. 
     2. Description of the Related Art 
     As one system of acquiring output setting voltage, a switching regulator system, which switches a switching element under a predetermined control, has been conventionally used since it is compact and can implement high efficiency. In order to control rush current which flows into the switching element when power is turned ON, a soft start operation is executed (e.g. Japanese Patent Application Laid-On No. 7-336999). 
       FIG. 6  is a circuit diagram of a conventional switching regulator. In the switching regulator  101 , the switching element  114 , which is a PMOS transistor, supplies power from the power supply side (VCC) to the load side, and holds the load side output (VOUT) at the output setting voltage by switching under a predetermined control, that is, according to the output of the comparator  110 , which is described later. To the load side of the switching element  114 , a smoothing circuit  115 , which comprises a coil, capacitor and diode, is connected for smoothing the voltage from the switching element  114 . The load side voltage (VOUT) is divided by the voltage divider  116 , which is composed of a serial resistor, and is input to the inversion input side of the error amplifier  111 . Between the output (FB) and the inversion input side of the error amplifier  111 , an oscillation stop circuit  117 , which is composed of a resistor and capacitor, is connected. The error amplifier  111  amplifies the error between the error comparison reference voltage  118  and the voltage which is input to the inversion input side, and outputs the amplified error. The output of the error amplifier  111  (FB) and the output of the later mentioned soft start circuit  113  (SS) are input to the comparator  110 , and the lower voltage thereof and the triangular wave voltage from the triangular wave generator (TRI)  112  are compared. The output of the comparator  110  is inverted by the inversion buffer  119 , and is input to the gate of the above mentioned switching element  114 . In this way, the switching element  114  is controlled by the feedback circuit from the load side output (VOUT). 
     The above mentioned soft start circuit  113  comprises a constant current source  122  and a capacitor  123  for generating voltage which gradually rises when power is turned ON, and further comprises a low power supply voltage malfunction prevention circuit (UVLO)  120  and a thermal shutdown circuit (TSD)  121 . 
     The reference voltage generator (VREF)  124  is also installed, and the output thereof (Vref) is the power supply for the error amplifier  111 , triangular wave generator  112  and soft start circuit  113 . 
       FIG. 7  is a characteristics diagram showing the voltage values in transition from the time of power ON to normal operation. When power is turned ON, the voltage of the output of the soft start circuit  113  (SS) is lower than that of the output of the error amplifier  111  (FB). Therefore in the comparator  110 , the output of the soft start circuit  113  (SS) and the triangular wave are compared. If the output of the soft start circuit  113  (SS) is higher than the triangular wave, high level is output from the comparator  110 , which is inverted by the inversion buffer  119 , and low level is input to the gate of the switching element  114 . As a result, the switching element  114  closes (turns ON). If the output of the soft start circuit  113  (SS) is lower than the triangular wave, low level is output from the comparator  110 , which is inverted by the inversion buffer  119 , and high level is input to the gate of the switching element  114 . As a result, the switching element  114  opens (turns OFF). 
     And in the beginning of power ON, the period when low level is input to the gate of the switching element  114  is short and the high level period is long. And by gradually increasing the low level period and decreasing the high level period, the rush current to flow into the switching element  114  is controlled. 
     When normal operation starts, the voltage of the output of the error amplifier  111  (FB) becomes lower than that of the output of the soft start circuit  113  (SS). Therefore in the comparator  110 , the output of the error amplifier  111  (FB) and the triangular wave are compared. 
     If the output of the error amplifier  111  (FB) is higher than the triangular wave, the high level is output from the comparator  110 , which is inverted by the inversion buffer  119 , and low level is input to the gate of the switching element  114 . If the output of the error amplifier  111  (FB) is lower than the triangular wave, low level is output from the comparator  110 , which is inverted by the inversion buffer  119 , and high level is input to the gate of the switching element  114 . 
     SUMMARY OF THE INVENTION 
     As described above, in the comparator  110 , the output of the soft start circuit  113  (SS) and the triangular wave are compared at power ON, and when normal operation starts, the output of the error amplifier  111  (FB) and the triangular wave are compared. The output of this soft start circuit  113  (SS) gradually rises after power ON, and asymptotically approaches the reference voltage (Vref). The output of the error amplifier  111  (FB), on the other hand, rises close to the reference voltage (Vref) immediately after power ON, and starts to drop by the function of the feedback circuit when the load side output (VOUT) exceeds the output setting voltage. By this drop, the voltage becomes lower than the output of the soft start circuit  113  (SS), and becomes the stable voltage for the normal operation. 
     Along with this, the load side output (VOUT) stabilizes at the output setting voltage, but a high overshoot voltage is generated during this time. In order to stabilize the load side voltage (VOUT) quickly and enable the operation of the devices related to the load side output (VOUT) quickly, this overshoot period must be decreased. Also this high overshoot voltage has the danger of damaging the devices related to the load side output (VOUT). 
     With the foregoing in view, it is an object of the present invention to provide a switching regulator that can control the overshoot voltage. 
     To solve the above problem, a switching regulator according to the present invention comprises a switching element for supplying power from a power supply side to a load side and holding the load side at an output setting voltage by switching, a voltage divider for dividing the voltage at the load side, an error amplifier for receiving voltage from the voltage divider, a triangular wave generator for generating triangular wave, a soft start circuit for generating voltage that gradually rises when the voltage at the power supply side starts up, a clamp circuit for setting an upper limit value from the output voltage of the soft start circuit to the output voltage of the error amplifier, and a comparator for comparing the lower voltage, between the output voltage of the soft start circuit and the output voltage of the error amplifier, with the triangular wave voltage, wherein the switching element is switched according to the output of the comparator. 
     Since the upper limit value from the output of the soft start circuit to the output of the error amplifier is set by this clamp circuit, the rise of the output voltage of the error amplifier is controlled when power is turned ON, and time until the output of the error amplifier stabilizes can be decreased, and therefore the overshoot voltage to be generated in the load side output is controlled. 
     In the switching regulator according to the present invention, the clamp circuit can concretely comprise a first transistor of which emitter is connected to the output of the error amplifier, and a second transistor of which base is connected to the output of the soft start circuit, and of which emitter is connected to the base of the first transistor, and of which the emitter-base voltage is smaller than that of the first transistor, and the upper limit value from the output voltage of the soft start circuit to the output voltage of the error amplifier is set depending on the difference of the emitter-base voltages. 
     This clamp circuit has a small number of composing elements, so an area occupied by the clamp circuit in a semiconductor integrated circuit can be decreased. Also since the number of composing element is small, the difference of the emitter-base voltages generated by the difference of the emitter-base junction areas can be adjusted relatively simply. 
     In another switching regulator according to the present invention, the clamp circuit can concretely comprises a first transistor of which emitter is connected to the output of the error amplifier, a second transistor of which the emitter is connected to the base of the first transistor, a third transistor of which the emitter is connected to the base of the second transistor, a resistor to which the base of the third transistor and a constant current source are connected, and a fourth transistor of which the emitter is connected to the other end of the resistor and of which base is connected to the output of the soft start circuit, and the upper limit value from the output voltage of the soft start circuit to the output voltage of the error amplifier is set depending on the difference of the emitter-base voltages between the first and second transistors, and on the voltage that is generated by the current of the constant current source flowing through the resistor. 
     In this clamp circuit, the voltage generated by the current flowing through the resistor is also included in the factors of setting the clamp voltage, so it is possible to increase the emitter-base junction area of the first transistor, which is related to the output of the error amplifier, and voltage can be sufficiently clamped even if the drive capability of the error amplifier is large. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram depicting the configuration of the switching regulator according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram depicting the configuration of the clamp circuit thereof; 
         FIG. 3  is a circuit diagram depicting the configuration of the clamp circuit of the switching regulator according to another embodiment of the present invention; 
         FIG. 4  is a waveform diagram depicting the transition from power ON to normal operation according to an embodiment; 
         FIG. 5  is a waveform diagram depicting a moment of transition from power ON to normal operation in detail according to the embodiment; 
         FIG. 6  is a circuit diagram depicting the configuration of a switching regulator according to a prior art; and 
         FIG. 7  is a waveform diagram depicting the transition from power ON to normal operation according to prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the drawings.  FIG. 1  is a circuit diagram depicting the configuration of a switching regulator according to an embodiment of the present invention. This switching regulator  1  is composed of basically the same composing elements as those described in the “Description of the Related Art”, to which a later described clamp circuit is added. 
     In the switching regulator  1 , the switching element  14  supplies power from the power supply side (VCC) to the load side, and holds the load side output (VOUT) at the output setting voltage under a predetermined control by switching, that is according to the output of the later mentioned comparator  10 . At the load side of the switching element  14 , a smoothing circuit  15 , which comprises a coil, a capacitor and a diode, is connected, so as to smooth the voltage from the switching element  14 . The load side output (VOUT) is divided by the voltage divider  16  which is composed of a serial resistor, and is input to the inversion input side of an error amplifier  11 . An oscillation stop circuit  17 , which is composed of a resistor and a capacitor, is connected between the output of the error amplifier  11  (FB) and the inversion input side. The error amplifier  11  amplifies the error between the error comparison reference voltage  18  and the voltage that is input at the inversion input side, and outputs it. The output of the error amplifier  11  (FB) and the output of the soft start circuit  13  (SS) are input to the comparator  10 , where the lower voltage thereof and the triangular wave voltage from a triangular wave generator (TRI)  12  are compared. The output of the comparator  10  is inverted by the inversion buffer  19 , and is input to the gate of the above mentioned switching element  14 . In this way, the switching element  14  is controlled by the feedback circuit from the load side output (VOUT). 
     The above mentioned soft start circuit  13  comprises a constant current source  22  and a capacitor  23  for generating voltage which gradually rises when power is turned ON, and further comprises a low power supply voltage malfunction prevention circuit (UVLO)  20  and a thermal shutdown circuit (TSD)  21 . The low power supply voltage malfunction prevention circuit (UVLO)  20  outputs high level when the power supply side voltage (VCC) is lower than a predetermined voltage, so as to forcibly fix the output of the soft start circuit  13  (SS) to the ground potential and prevent a malfunction of the switching regulator  1 . In the same way, the thermal shutdown circuit (TSD)  21  prevents a malfunction of the switching regulator  1  due to abnormal high temperatures. 
     And in order to control the overshoot voltage of the load side output (VOUT) during the transition from power ON to normal operation, a clamp circuit (clamper)  9  is disposed between the output of the soft start circuit  13  (SS) and the output of the error amplifier  11  (FB). 
     The reference voltage generator (VREF)  24  supplies the output thereof, that is, the reference voltage (Vref) for the error amplifier  11 , triangular wave generator  12 , soft start circuit  13  and clamp circuit  9 . 
     In the present embodiment, the switching element  14 , smoothing circuit  15 , voltage divider  16 , oscillation stop circuit  17  and capacitor  23  are external, and the other elements are enclosed in the semiconductor integrated circuit  2 . 
     Now the clamp circuit  9  will be described with reference to  FIG. 2 . The clamp circuit  9  is composed of a PNP transistor  31 , of which the emitter-base junction area is M times (xM) the basic transistor, an NPN transistor, of which the emitter-base junction area is N times (xN) thereof, and a constant current source  33 . And the emitter of the PNP transistor  31  is connected to the output of the error amplifier  11  (FB), the collector is grounded, and the base is connected to the emitter of the NPN transistor  32  and to the constant current source  33 , of which the other end is grounded. The collector of the NPN transistor  32  is connected to the reference voltage (Vref), and the base is connected to the output of the soft start circuit  13  (SS). 
     The mask patterns of the respective basic transistor of the PNP transistor and the NPN transistor are naturally different, but the transistor characteristics are assumed to be the same. And, hereinafter, x means the ratio of the emitter-base junction area to the basic transistor. Also according to this embodiment, the value M of the PNP transistor  31  is set to 1, the value N of the NPN transistor  32  is set to 8, and the constant current source  33  is set to 10–100 μA respectively. 
     In the NPN transistor  32 , of which base the output of the soft start circuit  13  (SS) is connected to, the emitter voltage is lower than the base for the amount of the emitter-base voltage. In the PNP transistor  31 , of which emitter the output of the error amplifier  11  (FB) is connected to, the base voltage is lower than the emitter for the amount of the emitter-base voltage. Here, according to the difference between the emitter-base junction areas of the PNP transistor  31  and the NPN transistor  32 , the emitter-base voltage of the PNP transistor  31  is restricted to be a voltage of about 0.1V higher than the emitter-base voltage of the NPN transistor  32 . 
     Therefore the output of the error amplifier  11  (FB) is restricted to a voltage amount about 0.1V higher than the output of the soft start circuit  13  (SS). If the output of the error amplifier  11  (FB) is less than this, current does not flow into the PNP transistor  31 , so the output of the error amplifier  11  (FB) is not influenced by the clamp circuit  9 . 
     In the first embodiment of the clamp circuit, even if the current drive capability of the PNP transistor  31  connected to the output of the error amplifier  11  (FB) is not very high, the number of composing elements can be decreased and the cost of the semiconductor integrated circuit  2  can be decreased. 
     Now a switching regulator, which is another embodiment of the present invention, will be described. The only difference of this from the previous embodiment is the clamp circuit, and  FIG. 3  shows the circuit diagram thereof. The clamp circuit  9 ′ is composed of a PNP transistor  51 , which is xM′, NPN transistor  52 , which is xN′, NPN transistor  54 , which is x 1 , PNP transistor  58 , which is x 1 , and a plurality of constant current sources  53 ,  55  and  56 , and resistor  57 . 
     And the emitter of the PNP transistor  51  is connected to the output of the error amplifier  11  (FB), the collector is grounded and the base is connected to the emitter of the NPN transistor  52  and to the constant current source  53 , of which the other end is grounded. The collector of the NPN transistor  52  is connected to the reference voltage (Vref), and the base is connected to the emitter of the NPN transistor  54 , which is x 1 , and to the constant current source  55 , of which the other end is grounded. The collector of the NPN transistor  54  is connected to the reference voltage (Vref), the base is connected to one end of the resistor  57 , and to the constant current source  56 , of which the other end is connected to the reference voltage (Vref). Also the emitter of the PNP transistor  58  is connected to the other end of the resistor  57 , the collector is grounded, and the base is connected to the output of the soft start circuit  13  (SS). 
     In this clamp circuit  9 ′, the value M′ of the PNP transistor  51  is set to 12, the value N′ of the NPN transistor  52  is set to 8, the constant current sources  53 ,  55  and  56  are set to 10–100 μA, and the resistor  57  is set to 1 k–50 kΩ respectively. The clamp voltage of the output of the error amplifier  11  (FB), with respect to the output of the soft start circuit  13  (SS), is influenced by the ratio of the emitter-base junction areas of the transistors, but is more strongly influenced by the resistance value of the resistor  57 . For example, when the constant current source  56  is 10 μA and the resistor  57  is 10 kΩ, about a 0.1V voltage is generated to the resistor  57 . 
     In the PNP transistor  58  of which base the output of the soft start circuit  13  (SS) is connected to, the emitter voltage is higher than the base for the amount of the emitter-base voltage. The base voltage of the NPN transistor  54  is higher than the emitter of the PNP transistor  58  for the amount of 0.1V by the resistor  57 . The emitter voltage of the NPN transistor  54  is lower than the base for the amount of the emitter-base voltage. If the emitter-base voltage of the PNP transistor  58  and that of the NPN transistor  54  are the same, then the emitter voltage of the NPN transistor  54  is 0.1V higher than the voltage of the output of the soft start circuit  13  (SS). 
     In the NPN transistor  52 , the emitter voltage is lower than the base for the amount of the emitter-base voltage, and the emitter voltage of the PNP transistor  51  is higher than the base for the amount of the emitter-base voltage. Here the difference of the emitter-base voltages between the NPN transistor  52  and the PNP transistor  51  changes depending on the values N′ and M′, and can be much smaller than 0.1V. In this way, the output of the error amplifier  11  (FB) connected to the emitter of the PNP transistor  51  can be 0.1V higher than the output of the soft start circuit  13  (SS). 
     In the present embodiment, the PNP transistor  51  connected to the output of the error amplifier  11  (FB) is x 12 , and has a high current drive capability, so voltage can be sufficiently clamped even if the drive capability of the error amplifier  11  is large. 
     In the clamp circuits according to the above two embodiments, the emitter-base junction area, resistance value and current value of the constant current source are examples, and needless to say they can be arbitrarily adjusted according to the desired characteristics. 
       FIG. 4  is a diagram depicting the transition from power ON to normal operation at each node.  FIG. 5  is a diagram depicting the amount of the transition from power ON to normal operation in detail. When power is turned ON, the voltage of the output of the soft start circuit  13  (SS) is lower than that of the output of the error amplifier  11  (FB). Therefore in the comparator  10 , the output of the soft start circuit  13  (SS) and the triangular wave are compared. If the voltage of the output of the soft start circuit  13  (SS) is higher than the triangular wave voltage, high level is output from the comparator  10 , which is inverted by the inversion buffer  19 , and low level is output from the output of the inversion buffer  19  (OUTH) to the gate of the switching element  14 . If the voltage of the output of the soft start circuit  13  (SS) is lower than the triangular wave voltage, low level is output from the comparator  10 , which is inverted by the inversion buffer  19 , and high level is output from the output of the inversion buffer  19  (OUTH) to the gate of the switching element  14 . 
     When normal operation starts, the voltage of the output of the error amplifier  11  (FB) is lower than that of the output of the soft start circuit  13  (SS). Therefore in the comparator  10 , the output of the error amplifier  11  (FB) and the triangular wave are compared. If the voltage of the output of the error amplifier  11  (FB) is higher than the triangular wave voltage, high level is output from the comparator  10 , which is inverted by the inversion buffer  19 , and low level is output from the output of the inversion buffer  19  (OUTH) to the gate of the switching element  14 . If the voltage of the output of the error amplifier  11  (FB) is lower than the triangular wave voltage, low level is output from the comparator  10 , which is inverted by the inversion buffer  19 , and high level is output from the output of the inversion buffer  19  (OUTH) to the gate of the switching element  14 . 
     The output of the soft start circuit  13  (SS) gradually rises after the power ON, and asymptotically approaches the reference voltage (Vref). The output of the error amplifier  11  (FB) rises after power ON to the voltage with which the clamp circuit  9  (or  9 ′) operates normally, then is clamped to the voltage which is 0.1V (clamp voltage) higher than the output of the soft start circuit  13  (SS). When the load side output (VOUT) exceeds the output setting voltage, the output of the error amplifier  11  starts to drop by the function of the feedback circuit. Then the output of the error amplifier  11  starts lower than the output of the soft start circuit  13  (SS) and becomes a stable voltage at normal operation. The output of the error amplifier  11  (FB) has been clamped by the clamp circuit  9  (or  9 ′), and the voltage difference to be dropped to the stable voltage is small. 
     Along with this, the load side output (VOUT) is also stabilized at the output setting voltage. This period when the overshoot voltage is generated is shorter since the output of the error amplifier  11  (FB) has been clamped by the clamp circuit  9  (or  9 ′). The value of the overshoot voltage is also controlled. 
     The above embodiments are examples for describing the present invention, where a step-down type switching regulator is used for the description, but the present invention may be applied to a step-up type switching regulator by changing the configuration and the wiring of the switching element  14  and the smoothing circuit. 
     The present embodiment is not limited to the above mentioned embodiment, but the design can be changed in various ways within the scope of the particulars stated in the claims.