Abstract:
Provided is a voltage regulator for limiting a rush current from an output stage transistor. The voltage regulator includes an output current limiting circuit having a low detection current value and an output current limiting circuit having a high detection current value, and is structured so as to enable operation of the output current limiting circuit having a low detection current value during a time period from a state in which an overheat protection circuit detects overheat and an output current is stopped to a state in which an overheat protection is canceled and a predetermined time passes. Accordingly, after the overheat protection is cancelled, an excessive rush current can be limited.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a voltage regulator for generating a constant voltage based on an input voltage. 
   2. Description of the Related Art 
   In order to stabilize an output voltage, an output terminal of a voltage regulator generally includes an external capacitor. Before activation of the voltage regulator, the external capacitor does not store charges. During a time period from immediately after the activation to a moment at which the output voltage generates a constant voltage, a charging current flows into the external capacitor. The charging current generated at that time is in a state in which an equivalent impedance of the external capacitor is low, and hence an excessive rush current flows. In the worst case, the rush current may be a cause of breakage such as blowout of a wire bonding. Accordingly, the voltage regulator includes a circuit for limiting an output stage transistor with respect to the generation of the rush current. 
   Hereinafter, a conventional voltage regulator is described.  FIG. 3  is a circuit diagram illustrating the conventional voltage regulator. 
   The voltage regulator includes an amplifier circuit  25 , an output stage transistor T 23  and a testing transistor T 24 , a switch circuit  30 , a current limiting circuit  20 , an on/off circuit  26 , and a counter circuit  27 . The amplifier circuit  25  compares a divided voltage obtained by dividing an output voltage of the voltage regulator by resistors R 31  and R 32  and fed back with a reference voltage generated by a reference voltage circuit to thereby control the output voltage of the voltage regulator. The output stage transistor T 23  and the testing transistor T 24  output drain currents corresponding to a voltage (gate voltage) output by the amplifier circuit  25 . The switch circuit  30  selects an output destination of the drain current of the transistor T 24 . The current limiting circuit  20  controls gate voltages of the transistor T 23  and the transistor T 24  so that the drain currents have the detection current value or smaller in a case where the drain current of the transistor T 24  has a predetermined detection current value or larger. The on/off circuit  26  controls on/off of the voltage regulator, and the counter circuit  27  counts an elapsed time from a moment at which the voltage regulator is turned on by the on/off circuit  26 . 
   The current limiting circuit  20  includes an output current limiting circuit  21  for actually controlling an excessive drain current, and an output current limiting circuit  22  for controlling, by using a detection current value smaller than a detection current value of the output current limiting circuit  21 , the excessive drain current. In the output current limiting circuit  21  and the output current limiting circuit  22 , the counter circuit  27  controls the switch circuit  30  according to an obtained elapsed time. The switch circuit  30  connects the output current limiting circuit  22  to the transistor T 24  until a predetermined elapsed time passes, and connects the output current limiting circuit  21  thereto after the predetermined elapsed time passes. 
   According to the above-mentioned voltage regulator, the on/off circuit  26  controls the voltage regulator to be turned on, the amplifier circuit  25  starts to operate, and the counter circuit  27  starts to count an elapsed time. Subsequently, the external capacitor connected to an output voltage terminal starts to be charged quickly, and hence the transistor T 23  allows an excessive drain current (rush current) to flow. Based on the rush current, the transistor T 24  allows a predetermined amount of the drain current to flow into the current limiting circuit  20 . In this case, the switch circuit  30  is in a state of selecting the output current limiting circuit  22  which is likely to control the drain current. The output current limiting circuit  22  controls the gate voltages of the transistors T 23  and T 24  so that the drain current has the detection current value or smaller in a case where the drain current has a predetermined detection current value or larger, and controls an excessive drain current to be small. After the voltage regulator is tuned on and a predetermined elapsed time passes, the switch circuit  30  selects the output current limiting circuit  21  which is unlikely to control the drain current (for example, see 2003-271251 A). 
   However, in a voltage regulator including an overheat protection circuit, not only in a case where the voltage regulator is turned on and an excessive rush current is generated, but also in a case where the overheated state is detected and the output transistor is controlled to stop an output current and thereafter the temperature decreases and the output current starts to flow again, there is generated an excessive rush current that charges the external capacitor connected to the output terminal of the voltage regulator. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned circumstances, and provides a voltage regulator in which an overheat protection circuit detects an overheated state and stops an output transistor operation, and a rush current from an output transistor is limited when the output transistor operates after a temperature decreases. 
   A voltage regulator according to the present invention includes: a first output current limiting circuit having a first detection current value; a second output current limiting circuit having a second detection current value larger than the first detection current value; an overheat protection circuit for detecting a temperature and outputting a detection signal indicating one of an overheated state and a normal state; and a detection circuit for receiving an input of the detection signal of the overheat protection circuit and an input of a rising signal of an input voltage, in which the detection circuit enables operation of the first output current limiting circuit when the detection signal indicating the overheated state is input and during a time period from when the detection signal indicating the normal state is input to when a predetermined period of time passes. 
   In the present invention, there is provided the structure in which the output current limiting circuit having a low detection current value and the output current limiting circuit having a high detection current value, and operation of the output current limiting circuit having a low detection current value is enabled during a time period from a state in which the overheat protection circuit detects overheat and an output current is stopped to a state in which an overheat protection is canceled and a predetermined time passes. Accordingly, after the overheat protection is cancelled, an excessive rush current can be limited. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a circuit diagram illustrating a voltage regulator according to the present invention; 
       FIG. 2  is a circuit diagram illustrating an example of a detection circuit of the voltage regulator according to the present invention; and 
       FIG. 3  is a circuit diagram illustrating a conventional voltage regulator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereinafter, with reference to the drawings, an embodiment of the present invention is described. 
   First, a voltage regulator is described.  FIG. 1  is a circuit diagram illustrating the voltage regulator according to the present invention. 
   The voltage regulator of  FIG. 1  includes voltage dividing resistors R 11  and R 12 , an error amplifier circuit  6 , an output transistor T 3 , testing transistors T 4  and T 5 , an output current limiting circuit  1 , an output current limiting circuit  2 , an overheat protection circuit  13 , and a detection circuit  7 . 
   The voltage dividing resistors R 11  and R 12  divide an output voltage of the voltage regulator to output a divided voltage. The error amplifier circuit  6  compares the divided voltage with a reference voltage output by a reference voltage circuit, and outputs a voltage according to the comparison result to the output transistor T 3 . The testing transistors T 4  and T 5  each have a gate connected to a gate of the output transistor T 3 . Accordingly, the testing transistors T 4  and T 5  can detect a current flowing into the output transistor T 3 . The output current limiting circuit  1  controls a gate voltage of the output transistor T 3  based on a current of the testing transistor T 5 . The output current limiting circuit  2  controls the gate voltage of the output transistor T 3  based on a current of the testing transistor T 4 . A detection current value of the output current limiting circuit  2  is larger than that of the output current limiting circuit  1 . The overheat protection circuit  13  detects an overheated state of the voltage regulator and controls the current flowing into the output transistor T 3 . The detection circuit  7  detects that the voltage regulator is turned on with use of an on/off signal of an input voltage, and detects overheat of the voltage regulator with use of a signal of the overheat protection circuit  13 , thereby outputting a signal to the output current limiting circuit  1 . 
   The voltage regulator having the above-mentioned structure prevents a rush current through the following operation. 
   In a case where an input voltage of the voltage regulator at a time of activation thereof rises, the reference voltage is input to the error amplifier circuit  6 , but an output voltage of the voltage regulator is not output. Accordingly, the dividing voltage input to the error amplifier circuit  6  becomes lower than the reference voltage. Therefore, the gate voltage input to the output transistor T 3 , which is output from the amplifier circuit  6  decreases, and hence a drain current of the output transistor T 3  becomes excessively large. Owing to the excessive drain current (rush current), the external capacitor connected to the output voltage terminal starts to be charged quickly. Based on the rush current, the testing transistors T 4  and T 5  allow a predetermined amount of the drain current to flow into the output current limiting circuit  2  and the output current limiting circuit  1 , respectively. 
   The output current limiting circuit  1  controls, in a case where a drain current of the testing transistor T 5  has a value larger than a predetermined detection current value, gate voltages of the output transistor T 3  and the testing transistors T 4  and T 5  so that the drain current becomes smaller than the detection current value, and controls so that respective drain currents thereof decrease. Note that, on this occasion, both the output current limiting circuit  1  and the output current limiting circuit  2  operate. The output current limiting circuit  1  uses a detection current value smaller than that of the output current limiting circuit  2 , and hence the output current limiting circuit  1  controls the rush current of the output transistor T 3  to be small. Further, the detection circuit  7  detects the on/off signal therein, the reference voltage, and an overheated state of the voltage regulator. In a state in which the voltage regulator is not in the overheated state and the on/off signal is in an on-state, and after a predetermined period of time has passed since the reference voltage reaches a predetermined voltage, the detection circuit  7  stops an operation of the output current limiting circuit  1  and controls so that only the output current limiting circuit  2  operates. 
   Further, in a case where the input voltage of the voltage regulator has already risen, when the overheat protection circuit  13  detects a predetermined temperature because a temperature of the voltage regulator rises due to internal heat generation in addition to an ambient temperature, the overheat protection circuit  13  increases the gate voltage of the output transistor T 3  up to a source voltage to thereby stop the output current. In this case, the overheat protection circuit  13  and the detection circuit  7  detect an overheated state of the voltage regulator and control so that the output current limiting circuit  1  operates. After that, the internal heat generation disappears because the output current is stopped, and in a case where the ambient temperature decreases to a temperature lower than a reset temperature, the gate voltage of the output transistor T 3 , which is controlled by the overheat protection circuit  13 , decreases, whereby an external capacitor connected to the output voltage terminal starts to be charged quickly. Based on this rush current, in a case where the drain current of the testing transistor T 5  has a value larger than a predetermined detection current value, the output current limiting circuit  1  controls the gate voltages of the output transistor T 3  and the testing transistors T 4  and T 5  so that the drain current has a value smaller than the detection current value, and controls so that the respective drain current thereof decrease. Note that, on this occasion, both the output current limiting circuit  1  and the output current limiting circuit  2  operate. The output current limiting circuit  1  uses a detection current value smaller than that of the output current limiting circuit  2 , and hence the output current limiting circuit  1  controls the rush current of the output transistor T 3  to be small. After the temperature of the voltage regulator decreases to the reset temperature or lower and then a predetermined period of time has passed since the output current starts to flow, the detection circuit  7  stops the operation of the output current limiting circuit  1 , and only the output current limiting circuit  2  operates. 
   Next, the detection circuit  7  is described.  FIG. 2  is a circuit diagram illustrating an example of the detection circuit  7  of the voltage regulator according to the present invention. 
   The detection circuit  7  is connected to a capacitor C 17  having one terminal grounded and another terminal connected to a reference current source  14 . Further, the detection circuit  7  is connected to a drain of an enhancement NMOS transistor  15 , a drain of an enhancement NMOS transistor  16 , and an input of a comparator  18 . The comparator  18  controls start and stop of the operation of the output current limiting circuit  1 . 
   To a gate of the enhancement NMOS transistor  15 , a control circuit  19  is connected. The control circuit  19  outputs a low signal when becoming an on-state in a state in which the voltage regulator is not in the overheated state, the on/off signal, which is an internal signal, becomes the on-state, and a reference voltage of the inside is detected and reaches a desired voltage. Further, to a gate of the enhancement NMOS transistor  16 , an output signal of an overheat protection circuit is connected. The overheat protection circuit outputs a high signal in an overheat detecting state and outputs a low signal in other states. In a case where one of the gates of the enhancement NMOS transistor  15  and the enhancement NMOS transistor  16  is in a high state, discharge of charges accumulated in the capacitor C 17  is controlled and an input signal of the comparator  18  is reduced. The comparator  18  compares a potential stored in the capacitor C 17  with a reference voltage of the inside. In a case where the reference voltage has a potential higher than a potential stored in the capacitor C 17 , the output current limiting circuit  1  is in an operating state. 
   In contrast to this, in a case where both the gates of the enhancement NMOS transistor  15  and the enhancement NMOS transistor  16  are in a low state, the reference current source  14  starts to charge the capacitor C 17  at a constant current. After a predetermined period of time has passed, a potential stored in the capacitor C 17  becomes higher than a potential of the reference voltage, whereby the output current limiting circuit  1  is in a stopped state.