Patent Abstract:
Provided is a switching regulator having an improved power supply voltage variation response characteristic while maintaining the stability of an output voltage to oscillation. An output resistance of an error amplifier is adjusted by a power supply voltage variation response improving circuit to allow a gain of the error amplifier to change.

Full Description:
[0001]     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. JP2005-354635 filed Dec. 8, 2005, the entire content of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a switching regulator, and more particularly, to an improvement of a power supply voltage variation response characteristic of a switching regulator.  
         [0004]     2. Description of the Related Art  
         [0005]     A conventional switching regulator includes a transconductance amplifier error amplifier. The transconductance amplifier error amplifier compares a voltage generated by dividing a voltage at an output voltage terminal by dividing resistors with a reference voltage produced from a reference voltage circuit and amplifies a potential difference therebetween. A gain of the transconductance amplifier error amplifier is determined based on an output current and a transconductance amplifier error amplifier output resistance. The switching regulator includes an LC filter which is composed of a coil and an output capacitor, so a significant phase delay occurs near a cutoff frequency of the LC filter. When the gain is equal to or larger than 0 dB in a frequency region in which the phase is delayed by 180 degrees or more, abnormal oscillation occurs. Therefore, it is necessary to reduce the gain in a high-frequency region near the cutoff frequency of the LC filter (see “SII CMOS IC DATA BOOK 2004, Power Supply IC·MOS FET Part”, pp. 4-314,  FIG. 12 ).  
         [0006]      FIG. 3  is a block diagram showing a switching regulator. The switching regulator includes a switching regulator control IC  34 , a power supply  29 , an output driver transistor  30 , a diode  31 , a coil  32 , and an output capacitor  33 .  
         [0007]     The switching regulator control IC  34  has the following structure and operates as follows. A voltage at an output voltage terminal  35  is divided by a dividing resistor  37  to sense an output voltage. The output voltage generated by the voltage division using the dividing resistor  37  is compared with a reference voltage  39  by a transconductance amplifier error amplifier  38  and a result obtained by the comparison is amplified by the transconductance amplifier error amplifier  38 . An output terminal of the transconductance amplifier error amplifier  38  is connected with a transconductance amplifier output resistor portion  40 . A voltage outputted from the transconductance amplifier error amplifier  38  is compared with a voltage outputted from a triangular wave generating circuit  42  by a PWM comparator  41  to generate a PWM waveform corresponding to the voltage outputted from the transconductance amplifier error amplifier  38 . The PWM waveform outputted from the PWM comparator  41  passes through a buffer  43  and then is inputted to a gate of the output driver transistor  30 . A gain of the transconductance amplifier error amplifier  38  is determined by “(output current from transconductance amplifier error amplifier  38 )×(resistance of transconductance amplifier output resistor portion  40 )”.  
         [0008]      FIG. 4  is a circuit diagram showing an example of the transconductance amplifier output resistor portion  40  of the conventional switching regulator. Resistors  3  and  4  are connected in parallel. One end of each of the resistors  3  and  4  is connected with an output-resistance input terminal  1 . The other end of the resistor  3  is grounded and the other end of the resistor  4  is grounded through a capacitor  5 . The output-resistance input terminal  1  is connected with the output terminal of the transconductance amplifier error amplifier  38 . An output resistance in a low-frequency region corresponds to a resistance of the resistor  3 . On the other hand, the output resistance in a high-frequency region in which the capacitor  5  appears to be a short-circuit is a parallel resistance of the resistors  3  and  4 , thereby determining the gain. That is, (resistance of resistor  3 )&gt;(parallel resistance of resistors  3  and  4 ) is satisfied, so (gain in low-frequency region)&gt;(gain in high-frequency region) is satisfied.  
         [0009]     In the switching regulator as describe above, in order to maintain the stability of the output voltage to the oscillation, the resistance of the transconductance amplifier output resistor portion is reduced in the high-frequency region near the cutoff frequency of the LC filter, thereby reducing the gain.  
         [0010]     However, the conventional switching regulator has the following problem. In order to prevent the abnormal oscillation, the gain of the transconductance amplifier error amplifier is reduced in the high-frequency region near the cutoff frequency of the LC filter. Then, even when a variation in output voltage occurs in the high-frequency region, a response of the transconductance amplifier error amplifier is slow, so a power supply voltage variation response characteristic is low.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention has been made to solve the above-mentioned problem. An object of the present invention is to provide a switching regulator having an improved power supply voltage variation response characteristic while maintaining the stability of an output voltage to oscillation.  
         [0012]     In the switching regulator according to the present invention, a power supply voltage variation response improving circuit is added into a switching regulator control IC to temporarily change an output resistance of an error amplifier. Therefore, the above-mentioned problem is solved, so the power supply voltage variation response characteristic is improved.  
         [0013]     According to the switching regulator control IC in the present invention, it is possible to provide the switching regulator having the improved power supply voltage variation response characteristic while maintaining the stability of the output voltage to oscillation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     In the accompanying drawings:  
         [0015]      FIG. 1  is a circuit diagram showing a transconductance amplifier output resistor portion of a switching regulator according to the present invention;  
         [0016]      FIG. 2  is a circuit diagram showing a power supply voltage variation response improving circuit of the switching regulator according to the present invention;  
         [0017]      FIG. 3  is a block diagram showing a conventional switching regulator; and  
         [0018]      FIG. 4  is a circuit diagram showing a transconductance amplifier output resistor portion of the conventional switching regulator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]      FIG. 1  is a circuit diagram showing a transconductance amplifier output resistor portion of a switching regulator according to the present invention. In the transconductance amplifier output resistor portion, a resistor  4  and a capacitor  5  which are connected in series are connected in parallel with a resistor  3 . One end of the resistor  4  is connected with an output-resistance input terminal  1 . One end of the capacitor  5  is grounded through an N-type MOS transistor  6 . A gate of the N-type MOS transistor  6  is connected with an output terminal of a power supply voltage variation response improving circuit  7 . An input terminal  2  of the power supply voltage variation response improving circuit  7  is connected with the power supply  29  shown in  FIG. 3 . The output-resistance input terminal  1  is connected with the output terminal of the transconductance amplifier error amplifier  38  of the switching regulator control IC shown in  FIG. 3 .  
         [0020]     It is assumed that a normal operation state is a state in which a voltage of the power supply  29  does not vary, and that a power supply voltage variation state is a state in which the voltage of the power supply  29  varies. When a voltage at the input terminal  2  does not vary, the power supply voltage variation response improving circuit  7  generates a High-level. When the voltage at the input terminal  2  varies, the power supply voltage variation response improving circuit  7  generates a Low-level for a predetermined period.  
         [0021]     That is, the transconductance amplifier output resistor portion in the present invention operates based on the voltage of the power supply  29  as follows. In the normal operation state, the voltage at the input terminal  2  does not vary, so the power supply voltage variation response improving circuit  7  generates the High-level. Then, the N-type MOS transistor  6  is turned on. Therefore, an output resistance value in a low-frequency region is a resistance of the resistor  3 . An output resistance value in a high-frequency region is a parallel resistance of the resistors  3  and  4 . On the other hand, in the power supply voltage variation state, the voltage at the input terminal  2  varies, so the power supply voltage variation response improving circuit  7  generates the Low-level for the predetermined period. Then, the N-type MOS transistor  6  is off for the predetermined period. Therefore, even in the high-frequency region, the output resistance value is the resistance of the resistor  3 , so the gain does not reduce. After the lapse of the predetermined period, the output voltage of the power supply voltage variation response improving circuit  7  becomes the High-level. Then, the N-type MOS transistor  6  is turned on, so the output resistance value becomes equal to that in the normal operation state.  
         [0022]      FIG. 2  is a circuit diagram showing a power supply voltage variation response improving circuit  7  of the switching regulator according to the present invention. The power supply voltage variation response improving circuit  7  includes a circuit  10  (for case of increase in power supply voltage) which operates in the case where a power supply voltage increases and a circuit  11  (for case of decrease in power supply voltage) which operates in the case where the power supply voltage decreases. Output signals of the circuits  10  and  11  are inputted to a NOR circuit  28 . An output of the NOR circuit  28  is outputted to an output terminal  9 .  
         [0023]     First, an operation of the circuit  10  (for case of increase in power supply voltage) will be described. A current value of a constant current source  12  is equal to that of a constant current source  14 . A threshold voltage of an N-type MOS transistor  13  is equal to that of an N-type MOS transistor  15 . An N-type MOS transistor  18  is a transistor having a threshold voltage higher than that of the N-type MOS transistor  15 . In the normal operation state, a drain voltage of the N-type MOS transistor  15  is lower than the threshold voltage of the N-type MOS transistor  18 , so the N-type MOS transistor  18  is off. Therefore, a drain voltage of the N-type MOS transistor  18  becomes a High-level, with the result that an output of an inverter  19  becomes a Low-level.  
         [0024]     In a power supply voltage variation state in which the power supply voltage increases, the drain voltage of the N-type MOS transistor  15  is increased by a capacitor  16  for a predetermined time, so the N-type MOS transistor  18  is turned on. Therefore, the drain voltage of the N-type MOS transistor  18  becomes the Low-level, with the result that the output of the inverter  19  becomes the High-level. The time for which the N-type MOS transistor  18  is on is substantially determined by “(current value of constant current source  14 )×(variation value of power supply voltage)/(capacitance value of capacitor  16 )”. In a power supply voltage variation state in which the power supply voltage decreases, the drain voltage of the N-type MOS transistor  15  decreases. However, the N-type MOS transistor  18  is off, so the output of the inverter  19  becomes the Low-level as in the normal operation state.  
         [0025]     Next, an operation of the circuit  11  (for case of decrease in power supply voltage) will be described. A current value of a constant current source  20  is equal to that of a constant current source  23 . A threshold voltage of an N-type MOS transistor  21  is equal to that of an N-type MOS transistor  24 . An N-type MOS transistor  26  is a transistor having a threshold voltage higher than that of the N-type MOS transistor  24 . In the normal operation state, a drain voltage of the N-type MOS transistor  24  is lower than the threshold voltage of the N-type MOS transistor  26 , so the N-type MOS transistor  26  is off. Therefore, a drain voltage of the N-type MOS transistor  26  becomes a High-level, with the result that an output of an inverter  27  becomes a Low-level.  
         [0026]     In a power supply voltage variation state in which the power supply voltage decreases, a gate voltage of the N-type MOS transistor  24  is decreased by a capacitor  22  for a predetermined time and the drain voltage of the N-type MOS transistor  24  increases, so the N-type MOS transistor  26  is turned on. Therefore, the drain voltage of the N-type MOS transistor  26  becomes the Low-level, with the result that the output of the inverter  27  becomes the High-level. The time for which the N-type MOS transistor  26  is on is substantially determined by “(current value of constant current source  22 )×(variation value of power supply voltage)/(capacitance value of capacitor  23 )”. In a power supply voltage variation state in which the power supply voltage increases, the gate voltage of the N-type MOS transistor  24  increases and the drain voltage of the N-type MOS transistor  24  decreases. However, the N-type MOS transistor  26  is off, so the output of the inverter  27  becomes the Low-level as in the normal operation state.  
         [0027]     Thus, in the normal operation state, each of the output voltage of the circuit  10  (for case of increase in power supply voltage) and the output voltage of the circuit  11  (for case of decrease in power supply voltage) is a Low-level, so the output of the NOR circuit  28  becomes a High-level. In the power supply voltage variation state in which the power supply voltage increases, the output of the circuit  10  (for case of increase in power supply voltage) is the High-level and the output of the circuit  11  (for case of decrease in power supply voltage) is the Low-level, with the result that the output of the NOR circuit  28  becomes a Low-level. In the power supply voltage variation state in which the power supply voltage decreases, the output of the circuit  10  (for case of increase in power supply voltage) is the Low-level and the output of the circuit  11  (for case of decrease in power supply voltage) is the high-level, with the result that the output of the NOR circuit  28  becomes the Low-level.  
         [0028]     According to the above-mentioned structure, when the power supply voltage varies, the output resistance of the transconductance amplifier error amplifier can be changed for the predetermined period. Therefore, the power supply voltage variation response characteristic can be improved while the stability of the output voltage to oscillation is maintained.

Technology Classification (CPC): 7