Abstract:
Provided is a switching regulator configured to suppress noise coupling, which may occur when a comparator is switched between a normal current operation and a low current consumption operation, to thereby operate stably. The switching regulator has a configuration in which switches are connected to input terminals of the comparator, and a feedback resistor having a large resistance value is disconnected from the input terminal of the comparator when switching is performed between the normal current operation and the low current consumption operation.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2016-035932 filed on Feb. 26, 2016, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switching regulator, and more particularly, to a technology of reducing current consumption in a light load state. 
     2. Description of the Related Art 
     Electronic devices are demanded to have low power consumption. In particular, smartphones, mobile devices, wearable devices, and the like are battery driven, and hence demands for those electronic devices to have low power consumption are especially strong. A switching regulator is used as a voltage supply source in various electronic devices. The switching regulator is demanded to maintain a high efficiency even when currents that are supplied from an output terminal to a load vary over a wide range of from a low current to a high current. 
       FIG. 3  is a circuit diagram of a synchronous rectification type switching regulator  300  of the related art. The switching regulator  300  is formed of a feedback resistor  7 , a comparator  10 , a reference voltage circuit  12 , an R-S flip-flop  13 , an ON-time control circuit  14 , an output control circuit  15 , driver circuits  16  and  17 , a comparator  18 , power FETs  2  and  4 , an inductor  3 , and a capacitor  5  (see, for example, U.S. Pat. No. 8,970,199). 
     The feedback resistor  7  divides an output voltage VOUT so that a feedback voltage VFB is output. The reference voltage circuit  12  outputs a reference voltage VREF. The comparator  10  compares the feedback voltage VFB and the reference voltage VREF to each other, to thereby output a set signal. The R-S flip-flop  13  receives the set signal at a set terminal S thereof, and then outputs a high level signal from an output terminal Q thereof. The ON-time control circuit  14  receives the high level signal from the output terminal Q, and then outputs a reset signal to a reset terminal R after a predetermined amount of time. The R-S flip-flop  13  receives the reset signal at the reset terminal R thereof, and then outputs a low level signal from the output terminal Q. 
     The output control circuit  15  generates drive signals for the power FETs  2  and  4  based on the signal output from the output terminal Q of the R-S flip-flop  13 . 
     The switching regulator  300  as described above achieves low power consumption by operating as follows. 
     In a heavy load mode, the power FETs  2  and  4  are the parts in the switching regulator  300  that mainly consume power. Therefore, in the switching regulator  300 , the low power consumption and the high efficiency may be achieved by setting an ON resistance of the power FETs  2  and  4  to be small. 
     In a light load mode, the power consumption by the power FETs  2  and  4  is small, and hence the power loss is mainly due to the power consumption by circuits, for example, the comparator  10 . Thus, an effective measure to achieve the low power consumption and high efficiency is to reduce the power loss of the comparator  10 , for example. 
     For example, the comparator  18  compares a voltage at one end of the inductor  3  and a voltage at a GND terminal to each other, and outputs, when a relationship between the voltages is reversed, a detection signal to the comparator  10 . The comparator  10  receives the detection signal from the comparator  18 , and then shifts to a low current consumption operation in which an operation current is reduced. 
     For example, in general, the comparator  10  consumes a current in the order of from several microamperes to several tens of microamperes in normal operation. In order to maintain the high efficiency in the light load mode in which a load current is 1 μA or less or several microamperes, it is required that current consumption of the comparator  10  be suppressed to be 1 μA or less. Further, the feedback resistor  7  is required to have its resistance value set to from several megohms to several hundreds of megohms, to reduce current consumption thereof. 
     In the case in which the resistance value of the feedback resistor  7  is set to from several megohms to several hundreds of megohms, when the operation current of the comparator  10  is switched to a small current for the low current consumption operation, the feedback voltage VFB at an input terminal of the comparator  10  that is connected to the feedback resistor  7  tends fluctuate due to noise coupling. Further, because the resistance value of the feedback resistor  7  is large, a large amount of time is needed for the fluctuated feedback voltage VFB to return to a normal voltage. Therefore, the switching regulator has a problem in that an operation margin is decreased or a malfunction occurs. 
     SUMMARY OF THE INVENTION 
     In order to solve the problem of the related art, a switching regulator according to one embodiment of the present invention has the following configuration. 
     The switching regulator includes: 
     a feedback resistor for outputting a feedback voltage; 
     a reference voltage circuit configured to generate a reference voltage; 
     a comparator, which includes a first input terminal connected to an output terminal of the feedback resistor, and a second input terminal connected to an output terminal of the reference voltage circuit, and which is configured to compare the feedback voltage and the reference voltage to each other; 
     a light load mode detecting circuit configured to detect a light load mode; 
     a first switch arranged between the output terminal of the feedback resistor and the first input terminal; and 
     a second switch arranged between the first input terminal and the second input terminal. 
     The switching regulator is configured to control, based on a signal output from the comparator, a power FET connected between a voltage input terminal and an output terminal, to thereby output a desired voltage. 
     The first switch is turned off and the second switch is turned on when the comparator switches an operation current based on a detection signal from the light load mode detecting circuit. 
     According to the switching regulator of the present invention, because the switches are connected to the input terminals of the comparator, and the feedback resistor having a large resistance value is disconnected from the input terminal of the comparator when switching is performed between a normal current operation and a low current consumption operation, noise coupling can be suppressed and a malfunction of the switching regulator can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram for illustrating a switching regulator of an embodiment of the present invention. 
         FIG. 2  is a circuit diagram for illustrating another example of the switching regulator of this embodiment. 
         FIG. 3  is a circuit diagram for illustrating a synchronous rectification type switching regulator of the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a circuit diagram for illustrating a switching regulator  100  of an embodiment of the present invention. 
     The switching regulator  100  of this embodiment includes a feedback resistor  7 , switches  8  and  9 , a comparator  10 , a capacitor  11 , a reference voltage circuit  12 , an R-S flip-flop  13 , an ON-time control circuit  14 , an output control circuit  15 , driver circuits  16  and  17 , a power FET  2 , which serves as a high-side switching element, a power FET  4 , which serves as a low-side switching element, an inductor  3 , and a capacitor  5 . 
     The feedback resistor  7  divides an output voltage VOUT, which is from an output terminal OUT, so that a feedback voltage VFB is output. The reference voltage circuit  12  outputs a reference voltage VREF. The comparator  10  compares the feedback voltage VFB and the reference voltage VREF to each other, to thereby output a detection signal. The R-S flip-flop  13  receives a set signal at a set terminal S thereof, and then outputs a high level signal from an output terminal Q thereof. The ON-time control circuit  14  receives the high level signal from the output terminal Q, and then outputs a reset signal to a reset terminal R after a predetermined amount of time. The R-S flip-flop  13  receives the reset signal at the reset terminal R thereof, and then outputs a low level signal from the output terminal Q. The output control circuit  15  generates drive signals for the power FETs  2  and  4  based on an output signal from the output terminal Q of the R-S flip-flop  13 . The comparator  18  compares a voltage at one end of the inductor  3  and a voltage at a GND terminal to each other, and outputs, when a relationship between the voltages is reversed, a detection signal to the comparator  10 . The comparator  10  receives the detection signal from the comparator  18 , and then shifts to a low current consumption operation in which an operation current is reduced. 
     Next, the switches  8  and  9  are described. 
     The switch  8  is connected between an output terminal of the feedback resistor  7  and an inverting input terminal of the comparator  10 . The switch  9  is connected between the inverting input terminal and a non-inverting input terminal of the comparator  10 . 
     When the switching regulator  100  is operating in a heavy load mode, the switch  8  is turned on and the switch  9  is turned off. When the switches  8  and  9  are in this state, the switching regulator  100  has a circuit configuration that is the same as a general switching regulator. Further, the comparator  10  operates at a normal operation current. 
     When the switching regulator  100  shifts from the heavy load mode to a light load mode, the comparator  10  receives the detection signal from the comparator  18 , and shifts to the low current consumption operation in which the operation current is reduced. For example, switching is performed between a first constant current source configured to cause a normal operation current to flow to the comparator  10 , and a second constant current source configured to cause less current to flow to the comparator  10  as compared to the first constant current source. 
     When the comparator  18  outputs the detection signal, first, the switch  8  is turned off and the switch  9  is turned on. As a result, the inverting input terminal of the comparator  10  and the feedback resistor  7  are disconnected from each other, and the inverting input terminal and the non-inverting input terminal of the comparator  10  are connected to each other. Next, the comparator  10  shifts to the low current consumption operation in which the operation current is reduced. Then, the switch  8  is turned on and the switch  9  is turned off, and hence the switching regulator returns to a normal circuit configuration. 
     In this case, the switches  8  and  9  may be switched after a target node of the comparator  10  reaches a desired operating point, or comes close to a voltage thereof. Alternatively, each operation described above may be conducted in sequence at a desired timing using a timer circuit, for example. 
     Further, also when the switching regulator  100  shifts from the light load mode to the heavy load mode, the operation current of the comparator  10  is switched after the connections of the switches  8  and  9  are switched in the same manner as above. 
     As described above, when the comparator  10  switches from a normal current operation to the low current consumption operation, the inverting input terminal of the comparator  10  and the feedback resistor  7  are disconnected from each other. Therefore, the inverting input terminal is not affected by noise coupling. As a result, a malfunction of the switching regulator may be suppressed. 
     Further, the inverting input terminal and the non-inverting input terminal of the comparator  10  are connected to each other through the switch  9 , and thus a voltage of the comparator  10  is set to the reference voltage VREF. In general, the reference voltage circuit  12  has a low output impedance, and thus has a high resistance to noise coupling. Even when the output impedance of the reference voltage circuit  12  is high, the impedance may be maintained to be low by adding the capacitor  11  so that the reference voltage circuit  12  may have a high resistance to noise coupling. 
     In the description of this embodiment, the synchronous rectification type switching regulator  100  is used, but the configuration of the switching regulator is not limited thereto and only needs to be adapted to the comparator, which is configured to detect the light load mode to switch the operation current. For example, the switching regulator may be a switching regulator  200  including a triangular wave generating circuit  21 , an error amplifier  22 , a reference voltage circuit  23 , a comparator  24 , and a Schottky diode  25 , as illustrated in  FIG. 2 .