Abstract:
There is provided a DC-DC converter which is safe and secure, but yet with low power consumption. The DC-DC converter is configured to monitor the output voltage of an error amplifier, and detect that the output voltage of the error amplifier becomes a fixed value or smaller to drive an overheat protection circuit and a supply voltage monitoring circuit intermittently.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2016-091200 filed on Apr. 28, 2016, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a DC-DC converter, and particularly to a technique for reducing current consumption under light load. 
       Background Art 
       [0003]    Recently, the power consumption of an electronic device equipped with a battery has been increasingly reduced. Particularly, an electronic device such as a smartphone, a mobile device, or a wearable device has been required to reduce power consumption more and more in order to make the drive time longer. Therefore, a semiconductor integrated circuit incorporated in the electronic device is also required to reduce power consumption significantly. 
         [0004]    In the meantime, the safety of an electronic device, such as a mobile device directly handled by a person, not to affect the human body adversely like an explosion or electric shock is particularly required. 
         [0005]    For example, a DC-DC converter incorporated in a battery-driven electronic device to operate at the battery voltage includes an overheat protection circuit that stops the operation when the chip temperature in a semiconductor integrated circuit rises and reaches a temperature equal to or higher than a predefined temperature, a supply voltage monitoring circuit that prevents the malfunction of an internal circuit when the battery voltage drops significantly, and the like (for example, see Patent Document 1). 
         [0006]    [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-328589 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide a DC-DC converter which is safe and secure, but yet with low power consumption. 
         [0008]    According to one embodiment of the present invention, there is provided a DC-DC converter including: a switching element connected between one end of an inductor, which includes another end at which an output voltage is generated, and a input terminal of the DC-DC converter; an error amplifier that monitors the output voltage; an output control circuit that outputs a control signal to the gate of the switching element based on an output signal of the error amplifier; and a protection circuit that outputs a signal to the output control circuit when detecting an abnormal state to turn off the switching element, wherein the protection circuit performs intermittent operation to operate only for a predetermined period of time in response to a signal based on the output signal of the error amplifier. 
         [0009]    Since the DC-DC converter of the present invention is configured to operate the protection circuit intermittently only for the predetermined period, the current consumption under light load can be particularly reduced, and hence power efficiency can be improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a circuit diagram illustrating an example of a DC-DC converter of an embodiment of the present invention. 
           [0011]      FIG. 2A  is a timing chart illustrating the operation of a comparator  20  under heavy load in the DC-DC converter of the embodiment. 
           [0012]      FIG. 2B  is a timing chart illustrating the operation of the comparator  20  under light load in the DC-DC converter of the embodiment. 
           [0013]      FIG. 3  is a circuit diagram illustrating an example of an overheat protection circuit in the DC-DC converter of the embodiment. 
           [0014]      FIG. 4  is a circuit diagram illustrating an example of a supply voltage monitoring circuit in the DC-DC converter of the embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]      FIG. 1  is a circuit diagram illustrating an example of a DC-DC converter of an embodiment. A DC-DC converter  100  is a synchronous rectification type DC-DC converter that converts, to constant voltage, supply voltage Vin input to an input terminal  1 , and outputs the voltage to an output terminal  7  as output voltage Vout. 
         [0016]    The DC-DC converter  100  of the embodiment includes a PMOS transistor  3  as a first switching element, an NMOS transistor  4  as a second switching element, an inductor  5 , an output capacitor  6 , an error amplifier  10 , an oscillation circuit  11 , reference voltage circuits  12  and  21 , comparators  13  and  20 , an output control circuit  14 , buffer circuits  15  and  16 , voltage-dividing resistors  17  and  18 , an overheat protection circuit  22 , and a supply voltage monitoring circuit  23 . 
         [0017]    The voltage-dividing resistors  17  and  18  output feedback voltage Vfb corresponding to the output voltage Vout. The reference voltage circuit  12  outputs reference voltage Vref 1 . The error amplifier  10  compares the feedback voltage Vfb with the reference voltage Vref 1 , and outputs, to the comparators  13  and  20 , voltage Verr obtained by amplifying a difference between the feedback voltage Vfb and the reference voltage Vref 1 . The oscillation circuit  11  outputs a constant-period triangular wave signal. The comparator  13  compares the triangular wave signal of the oscillation circuit  11  with the voltage Verr of the error amplifier  10 , and outputs a signal as the comparison result. The output control circuit  14  receives the output signal of the comparator  13 , and outputs a control signal to the buffer circuits  15  and  16 . The buffer circuit  15  controls the PMOS transistor  3 , and the buffer circuit  16  controls the NMOS transistor  4 . 
         [0018]    The reference voltage circuit  21  outputs reference voltage Vref 2 . The comparator  20  compares the voltage Verr of the error amplifier  10  with the reference voltage Vref 2 , and outputs a control signal Vcont. When the voltage Verr of the error amplifier  10  exceeds the reference voltage Vref 2 , the comparator  20  outputs the control signal Vcont to the overheat protection circuit  22  and the supply voltage monitoring circuit  23  to stop the operation. 
         [0019]    The overheat protection circuit  22  monitors the temperature of the DC-DC converter, and outputs a signal to the output control circuit  14  when determining an overheated state as a result of heating the DC-DC converter. The output control circuit  14  that received the signal from the overheat protection circuit  22  turns off the PMOS transistor  3  through the buffer circuit  15  to protect a breakdown of the DC-DC converter by heating. 
         [0020]    The supply voltage monitoring circuit  23  monitors supply voltage, and outputs a signal to the output control circuit  14  when determining that the supply voltage drops lower than a predetermined voltage. When receiving the signal from the supply voltage monitoring circuit  23 , the output control circuit  14  turns off the PMOS transistor  3  through the buffer circuit  15 . Thus, the supply voltage monitoring circuit  23  prevents a breakdown of the DC-DC converter by unintended switching operation. 
         [0021]    Referring next to timing charts of  FIG. 2 , the operation of the comparator  20  will be described.  FIG. 2A  illustrates a case of heavy load, where the load connected to the output terminal  7  is heavy, and  FIG. 2B  illustrates a case of light load, where the load connected to the output terminal  7  is light. 
         [0022]    In the case of heavy load, since current output from the output terminal  7  increases, the output voltage Vout drops, i.e., the feedback voltage Vfb drops. Therefore, the voltage Verr of the error amplifier  10  rises so that the output of the comparator  13  will be an oscillation signal long in H period. Thus, the output control circuit  14  outputs a signal long in L period to the gate of the PMOS transistor  3 , and a signal long in H period to the gate of the NMOS transistor  4 . 
         [0023]    At this time, since the voltage Verr of the error amplifier  10  always exceeds the reference voltage Vref 2  of the reference voltage circuit  21  as illustrated in  FIG. 2A , the comparator  20  always outputs an H control signal Vcont. In response to this H signal, the overheat protection circuit  22  and the supply voltage monitoring circuit  23  always monitor the temperature and the supply voltage, respectively. 
         [0024]    When the load is light, since the output voltage Vout rises, i.e., the voltage Verr of the error amplifier  10  drops, the output of the comparator  13  becomes an oscillation signal long in L period. Thus, the output control circuit  14  outputs the signal long in H period to the gate of the PMOS transistor  3 , and the signal long in L period to the gate of the NMOS transistor  4 . 
         [0025]    At this time, since the voltage Verr of the error amplifier  10  drops to that equivalent to the reference voltage Vref 2  of reference voltage circuit  21  as illustrated in  FIG. 2B , the control signal Vcont of the comparator  20  varies between the H level and the L level depending on the magnitude of the voltage difference. In response to this control signal Vcont, the overheat protection circuit  22  and the supply voltage monitoring circuit  23  intermittently monitor the temperature and the supply voltage, respectively. 
         [0026]    As described above, since the comparator  20  intermittently controls the operation of the overheat protection circuit  22  and the supply voltage monitoring circuit  23 , the power consumption of the overheat protection circuit  22  and the supply voltage monitoring circuit  23  can be reduced. 
         [0027]    Particularly, in the overheat protection circuit  22  under light load, since there is no danger of braking down any element due to a temperature rise by current flowing through the NMOS transistor  4 , there is no problem that if the overheat protection circuit  22  is in a stopped state. 
         [0028]    When an overheated state or a power drop state is determined, the overheat protection circuit  22  and the supply voltage monitoring circuit  23  continue detection operation until it is determined not to the overheated state or the power drop state. This operation can protect the DC-DC converter without fail even when the overheat protection circuit  22  and the supply voltage monitoring circuit  23  perform intermittent operation under light load. 
         [0029]      FIG. 3  is a circuit diagram illustrating an example of the overheat protection circuit of the present invention. The overheat protection circuit  22  includes a thermosensor  31 , a reference voltage circuit  32 , a comparator  33  that compares the voltage of the thermosensor  31  with the output voltage of the reference voltage circuit  32  to detect temperature, a bias circuit  34  that supplies current to the thermosensor  31 , a bias circuit  35  that supplies current to the comparator  33 , a switch  36  that controls the supply of current from the bias circuit  34  to the thermosensor  31 , and a switch  37  that controls the supply of current from the bias circuit  35  to the comparator  33 . The switch  36  is provided between the thermosensor  31  and the bias circuit  34 . The switch  37  is provided between the comparator  33  and the bias circuit  35 . 
         [0030]    When the H signal of the comparator  20  is input to an IN terminal, the switch  36  and the switch  37  are turned on to supply current to the thermosensor  31  and the comparator  33 . After the current is supplied and the voltage of the thermosensor  31  and the comparator  33  are stabilized in a state where a comparison can be made, the comparator  33  compares the output voltage of the reference voltage circuit  32  with the voltage of the thermosensor  31  to determine the temperature. When an overheated state is determined, the bias circuits  34 ,  35  continue to supply current to the thermosensor  31  and the comparator  33  in order to continue the detection of temperature. When no overheated state is determined, an L signal is input according to a signal input to the IN terminal to turn off the switch  36  and the switch  37  so as to stop the supply of current to the thermosensor  31  and the comparator  33 . 
         [0031]      FIG. 4  is a circuit diagram illustrating an example of the supply voltage monitoring circuit of the present invention. The supply voltage monitoring circuit  23  includes voltage-dividing resistors  41 ,  42 , a reference voltage circuit  43 , a comparator  44  that latches an output signal, and a bias circuit  45 . The supply voltage monitoring circuit  23  also includes a switch  46  that controls the supply of current from the bias circuit  45  to the comparator  44 , and a switch  47  that shuts off the current flowing through the voltage-dividing resistors  41 ,  42 . 
         [0032]    In a state where the switches  46 ,  47  are off, the divided voltage of a voltage-dividing resistor circuit composed of the voltage-dividing resistors  41 ,  42  is pulled up to the supply voltage Vin. 
         [0033]    When the H signal output from the comparator  20  is input to the IN terminal, the switch  46  and the switch  47  are turned on to supply current to the voltage-dividing resistors  41 ,  42 , and the comparator  44 . The comparator  44  compares the divided voltage with reference voltage of the reference voltage circuit  43  to monitor the supply voltage Vin. When the divided voltage is determined to be low voltage lower than the reference voltage value, the comparator  44  outputs the L signal from an OUT terminal. Then, power monitoring is continued until the divided voltage is determined not to be the low voltage. When the divided voltage becomes voltage higher than the reference voltage value, the comparator  44  outputs the H signal from the OUT terminal. On this occasion, the switches  46 ,  47  perform on/off operation based on a signal input from the comparator  20  to the IN terminal. When the switch  46  is off, the comparator  44  latches a signal when the switch is on to output an intermittent signal from the OUT terminal. The voltage in the supply voltage monitoring circuit  23  to determine whether the supply voltage Vin is the low voltage or not is decided based on the reference voltage value of the reference voltage circuit  43 , and a voltage dividing ratio of the voltage-dividing resistor circuit. 
         [0034]    In the above description, the supply voltage monitoring circuit monitors the supply voltage Vin to stop the operation of the DC-DC converter in order to protect the DC-DC converter, but it can also be applied to a monitoring circuit that monitors internal power supply to drive a control circuit in the DC-DC converter circuit. 
         [0035]    The supply voltage monitoring circuit can further be applied to an OVP (Over Voltage Protection) circuit that monitors the output voltage Vout and detects an overvoltage state to stop the operation of the DC-DC converter. 
         [0036]    Further, the supply voltage monitoring circuit can be applied to a UVP (Under Voltage Protection) circuit that detects a decrease in output voltage Vout to stop the operation of the DC-DC converter.