Abstract:
Disclosed is a semiconductor integrated circuit for regulator including: a control transistor; a voltage divider circuit generating a feedback voltage proportional to an output voltage; a control circuit controlling the control transistor based on difference between the feedback voltage and a reference voltage; and a terminal through which an output voltage switching control signal is received, and being configured to switch the output voltage into a first voltage or into a second voltage lower than the first voltage, by varying division ratio in the voltage divider circuit in response to the signal. The semiconductor integrated circuit further includes: a discharging transistor between the output terminal and the ground; and a circuit outputting a signal for keeping the discharging transistor turned on over a period from change of the signal to fall of the output voltage from the first voltage down to the second voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present U.S. patent application claims a priority under the Paris Convention of Japanese patent application No. 2011-143659 filed on Jun. 29, 2011, which shall be a basis of correction of an incorrect translation, and is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a DC power supply and further relates to a voltage regulator which converts DC voltage, and more specifically to a technique effectively applicable to a semiconductor integrated circuit (regulator IC) which configures a series regulator (including low-dropout regulator (LDO)) having an output voltage switching function. 
         [0004]    2. Description of Related Art 
         [0005]    In DC power supply, there is a demand of switching of output voltage level, for the purpose of suppressing degradation in characteristics of a device which serves as a load to be supplied with electric power. One known control semiconductor integrated circuit composing a conventional series regulator has, as illustrated in  FIG. 4 , a control terminal through which an output voltage switching control signal CV is received, and is configured to switch the output voltage level depending on a state (high or low) of the input signal CV at the control terminal. 
         [0006]    The series regulator given the switching function illustrated in  FIG. 4  has bleeder resistors R 1 , R 2 , a resistor R 3  and a transistor M 2 . The bleeder resistors R 1 , R 2  divide output voltage V out  and supply a feedback voltage V FB  to an error amplifier AMP. The resistor R 3  and the transistor M 2  are connected in series, and in parallel with the resistor R 2 , out of the bleeder resistors R 1 , R 2 . By turning the transistor M 2  on or off using the output voltage switching control signal CV, the voltage division ratio by the bleeder resistors may be varied, and thereby the output voltage level may be switched. 
         [0007]    The regulator given the switching function illustrated in  FIG. 4 , however, allows discharge of an output capacitor Co only through the load, so that it takes a long time to bring the output voltage V out  from a high level down to a low level, proving poor switching response characteristics. One possible solution therefor is, as illustrated in  FIG. 5 , to provide a resistor Ro in parallel with the output capacitor Co, so as to improve the switching response characteristics. 
         [0008]    However, in the regulator illustrated in  FIG. 5 , the time necessary for the output voltage V out  to reach a switched level varies depending on the value of the resistor Ro or state of a device which serves as the load, as illustrated in  FIG. 2C . In addition, since current constantly flows through the resistor Ro in the general operation, so that wasteful current will increase. 
         [0009]    Japanese Laid-Open Patent Publication No. 2010-191885 discloses a series regulator aimed at improving the transient response characteristics, which has a switching transistor for bypassing current, provided in parallel with the bleeder resistors. In the series regulator disclosed in Japanese Laid-Open Patent Publication No. 2010-191885, the switching transistor is provided in parallel with the entire bleeder resistors, rather than in parallel with either one of the bleeder resistors. In addition, the invention disclosed in Japanese Laid-Open Patent Publication No. 2010-191885 is aimed at improving the transient response characteristics in case of abrupt changes in the output voltage, rather than improving the transient response characteristics when the output voltage is switched. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention was conceived in consideration of the situation described in the above, and an object of which is to provide a semiconductor integrated circuit used for regulators, capable of improving the transient response characteristics when the output voltage is switched, without increasing a wasteful current. 
         [0011]    For the purpose of attaining the above-described objects, according to the present invention, there is provided a semiconductor integrated circuit for regulator including: a control transistor connected between an input terminal and an output terminal; a voltage divider circuit which generates a feedback voltage proportional to an output voltage; a control circuit which controls the control transistor based on difference between the feedback voltage and a predetermined reference voltage; and a terminal through which an output voltage switching control signal is received from the external, and being configured to switch the output voltage into a first voltage or into a second voltage lower than the first voltage, by varying division ratio in the voltage divider circuit in response to an output voltage switching control signal. The semiconductor integrated circuit further comprising: a discharging transistor which is connected between the output terminal and the ground; and a circuit for controlling output fall during switching, which outputs a signal for keeping the discharging transistor turned on over a period from change of the control signal to fall of the output voltage from the first voltage down to the second voltage, based on difference between the feedback voltage and the reference voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
           [0013]      FIG. 1  is a circuit diagram illustrating a control IC used in a series regulator of this embodiment; 
           [0014]      FIG. 2A  is a characteristic diagram illustrating gate control signal (1/CV) of the MOS transistor M 2  of this embodiment; 
           [0015]      FIG. 2B  is a characteristic diagram illustrating output voltage of the series regulator of this embodiment; 
           [0016]      FIG. 2C  is a characteristic diagram illustrating output voltage of a conventional series regulator; 
           [0017]      FIG. 3  is a circuit diagram illustrating a modified example of the control IC used in the series regulator illustrated in  FIG. 1 ; 
           [0018]      FIG. 4  is a circuit diagram illustrating a control IC used in a conventional series regulator having an output voltage switching function; and 
           [0019]      FIG. 5  is a circuit diagram illustrating a series regulator control IC improved in the output voltage response characteristics during switching of output voltage. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Preferred embodiments of the present invention will be explained below, referring to the attached drawings. 
         [0021]      FIG. 1  illustrates one embodiment of a series regulator (including LDO) applied by the present invention. Although not specifically limited, elements composing a circuit surrounded by a one-dot chain line in  FIG. 1  are formed on a single semiconductor chip, so as to configure a semiconductor integrated circuit  10  for controlling the regulator (referred to as regulator IC, hereinafter). 
         [0022]    The regulator IC  10  of this embodiment has a voltage input terminal IN, an output terminal OUT, a voltage control transistor M 1 , bleeder resistors R 1 , R 2 , a resistor R 3 , a MOS transistor M 2 , an error amplifier  11 , a reference voltage circuit  12 , a bias circuit  13 , a starting control circuit  14 , and a logic circuit  15 . 
         [0023]    The voltage input terminal IN is applied with DC voltage VDD from an unillustrated DC voltage source. The voltage control transistor M 1  is connected between the voltage input terminal IN and the output terminal OUT. The voltage control transistor M 1  is composed of a P-channel MOSFET (metal oxide semiconductor field effect transistor, referred to as MOS transistor, hereinafter). 
         [0024]    The bleeder resistors R 1 , R 2  are connected between the output terminal OUT and a ground terminal GND. The bleeder resistors R 1 , R 2  divide the output voltage V out . Voltage V FB  produced by voltage division by the bleeder resistors R 1 , R 2  is fed back to a non-inverting input terminal of the error amplifier  11 . Output of the error amplifier  11  is fed to the gate terminal of the voltage control transistor M 1 . 
         [0025]    The error amplifier  11  controls the voltage control transistor M 1 , corresponding to potential difference between the feedback voltage V FB  and a reference voltage V ref . Resistance ratio of the bleeder resistors R 1 , R 2  is set so as to adjust the output voltage V out  to a desired value. The series regulator of this embodiment acts so as to keep the output voltage V out  constant, by the feedback control described in the above. The output terminal OUT is externally attached with an output capacitor Co which stabilizes the output voltage V out . 
         [0026]    The reference voltage circuit  12  generates the reference voltage V ref . The reference voltage circuit  12  may be configured by using a constant voltage circuit composed of a Zener diode. Alternatively, the reference voltage circuit  12  may be configured typically by using a reference voltage generation circuit which contains a depletion-mode MOS transistor as a constant current source, and an enhancement-mode MOS transistor connected thereto in series. 
         [0027]    The bias circuit  13  feeds bias current to the reference voltage circuit  12  and the error amplifier  11 . 
         [0028]    The starting control circuit  14  is configured typically by an inverter. The starting control circuit  14  brings the bias circuit  13  into an active state, in response to a chip enable signal CE. The chip enable signal CE is an externally-fed signal for turning the chip on or off. 
         [0029]    The regulator IC  10  of this embodiment has a terminal through which the chip enable signal CE is received from the external, and a terminal through which the output voltage switching control signal CV is received from the external. 
         [0030]    The resistor R 3  and the MOS transistor M 2  are connected in series, and is connected in parallel with the resistor R 2 , out of the bleeder resistors R 1 , R 2 . By turning the MOS transistor M 2  on or off, the voltage division ratio by the bleeder resistors may be varied, and thereby the level of output voltage V out  may be switched. 
         [0031]    The logic circuit  15  is configured by an inverter and so forth. The logic circuit  15  generates an internal signal of the chip, in response to the output voltage switching control signal CV. The control signal output from the logic circuit  15  is fed to the gate terminal of the MOS transistor M 2 . When the output voltage switching signal CV is at a high level, the MOS transistor M 2  turns off, the voltage division ratio of the output voltage is determined by the bleeder resistors R 1 , R 2 , and thereby the output voltage V out  is kept at the low level. On the other hand, when the output voltage switching control signal CV is at a low level, the MOS transistor M 2  turns on, the voltage division ratio of the output voltage is determined by the resistance of the resistor R 1  and a combined resistance of the resistors R 2  and R 3 , and thereby the output voltage V out  shifts from the low level to the high level. 
         [0032]    The regulator IC  10  of this embodiment is further provided with N-channel MOS transistors M 3  and M 4 , and a voltage comparator circuit  16 . 
         [0033]    The N-channel MOS transistors M 3  and M 4  are connected in parallel, between the output terminal OUT and the ground point GND. 
         [0034]    The gate terminal of the MOS transistor M 3  is fed with a control signal from the starting control circuit  14 . When the chip enable signal CE changes from the high level to the low level so as to turn the chip off, the MOS transistor M 3  turns on to discharge the output capacitor Co, and swiftly brings the output voltage V out  down to the ground potential (0 V). 
         [0035]    The gate terminal of the MOS transistor M 4  is fed with an output signal of the voltage comparator circuit  16 . 
         [0036]    The voltage comparator circuit  16  compares the feedback voltage V FB  and the reference voltage V ref . A differential amplifier circuit intentionally added with offset is used as the voltage comparator circuit  16  of this embodiment. Note that the word “intentionally” herein is used to exclude any offset which naturally occurs due to process variation. 
         [0037]    Methods of adding offset to the differential amplifier circuit typically includes a method of making difference in the ratio of gate width W and gate length L of the differential transistors; a method of making difference in the resistance value of the elements which serve as loads of the differential transistors; and a method of connecting a resistor to an input of only one of the differential transistors. 
         [0038]    When the output voltage switching signal CV changes from the low level to the high level, the MOS transistor M 2  turns off. The feedback voltage V FB  then becomes higher than the reference voltage V ref , the output signal of the voltage comparator circuit  16  changes to the high level, the MOS transistor M 4  turns on, and the output capacitor Co starts to discharge. 
         [0039]    On the other hand, when the MOS transistor M 2  turns off, the output voltage V out  falls from the high level V 1  down to the low level V 2 . When the output voltage V out  falls down to the low level V 2 , the feedback voltage V FB  falls down to the reference voltage V ref , the output signal of the voltage comparator circuit  16  falls down to the low level, and thereby the MOS transistor M 4  turns off. 
         [0040]      FIG. 2A  is a characteristic diagram illustrating gate control signal (1/CV) of the MOS transistor M 2  of this embodiment, and  FIG. 2B  is a characteristic diagram illustrating the output voltage of the series regulator of this embodiment. 
         [0041]    When the output voltage switching control signal CV changes from the low level to the high level, and thereby when the gate control voltage for the MOS transistor M 2  output from the logic circuit  15  shifts from the high level down to the low level as illustrated in  FIG. 2A , the output voltage V out  may be brought down swiftly to the target low level V 2  within a predetermined short time, irrespective of the state of load, as illustrated in  FIG. 2B . 
         [0042]    On the other hand, when the output voltage switching control signal CV changes from the high level down to the low level, the feedback voltage V FB  temporarily shifts to the low level, whereas the output signal of the voltage comparator circuit  16  remains unchanged, so that the MOS transistor M 4  will not turn on. Since the voltage comparator circuit  16  is configured by using the differential amplifier circuit intentionally added with offset, so that the MOS transistor M 4  will not turn on even if the feedback voltage V FB  varies depending on changes in load in the steady state. 
         [0043]    While the regulator IC  10  of this embodiment additionally has a thermal shut-down circuit  17  and a current limit circuit  18 , the present invention is not limited to those having these additional components. 
         [0044]    The thermal shut-down circuit  17  has a temperature detection circuit which terminates operation of the circuit when the chip temperature was detected to exceed a predetermined temperature. The thermal shut-down circuit  17  is disclosed typically in Japanese Laid-Open Patent Publication No. 2007-318028. 
         [0045]    The current limit circuit  18  protects the element from over-current, by reducing the output current while lowering the output voltage V out , when the output current increased and reached a predetermined value due to short-circuiting of the load or the like. The current limit circuit  18  is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2008-052516. The thermal shut-down circuit  17  and the current limit circuit  18  will not be detailed herein, since the both are publicly known. 
         [0046]    As described in the above, the discharging transistors M 3 , M 4  do not conduct electric current in the normal operation, but temporality turn on to swiftly bring down the output voltage V out , when the output voltage switching control signal CV varies and the output voltage V out  changes from the high level V 1  down to the low level V 2 , so that the transient response characteristics during switching of the output voltage may be improved without increasing wasteful current in the steady state. 
         [0047]    In addition, by intentionally adding offset to the voltage comparator circuit  16 , a signal which temporarily turns the discharging transistor M 4  during switching of the output voltage may be generated by a relatively simple circuit, so that the transient response characteristics during switching of the output voltage may be improved without increasing so much the circuit scale. 
         [0048]      FIG. 3  illustrates a modified example of the series regulator IC of the embodiment illustrated in  FIG. 1 . 
         [0049]    In this modified example, a P-channel MOS transistor M 5 , and a pulse generator circuit  19  are additionally provided. The P-channel MOS transistor M 5  is provided between the source voltage terminal of the voltage comparator circuit  16  and the bias circuit  13 , and functions as a power switch of the voltage comparator circuit  16 . The pulse generator circuit  19  detects change of the output voltage switching control signal CV from the low level up to the high level, and generates an one-shot pulse having a predetermined width. When the P-channel MOS transistor M 5  is turned on by the one-shot pulse generated by the pulse generator circuit  19 , operating current temporarily flows through the voltage comparator circuit  16 , and the voltage comparator circuit  16  starts to operate. 
         [0050]    By temporarily operating the voltage comparator circuit  16 , the modified example may reduce the current consumption as compared with the regulator IC illustrated in  FIG. 1 . Another possible configuration may be such as implementing on/off control of a power source of the voltage comparator circuit  16 , making use of the one-shot pulse generated by the pulse generator circuit  19 , rather than providing the MOS transistor M 5  as the power switch of the voltage comparator circuit  16 . 
         [0051]    It is still also possible to provide a CR time constant circuit for specifying the pulse width of the one-shot pulse, to the pulse generator circuit  19 . Alternatively, an external terminal allowing connection of a capacitor, which composes the CR time constant circuit and assumed as an externally attached element, may be provided to the regulator IC  10 , so as to allow the user to arbitrarily set the pulse width by appropriately selecting the capacitor, or to set the operating time of the voltage comparator circuit  16 . 
         [0052]    In the configuration provided with the pulse-width-adjustable pulse generator circuit, the voltage comparator circuit  16  is omissible, phase of the output of the pulse generator circuit may be inverted, and the MOS transistor M 4  for discharging may directly be turned on or off by the phase-inverted output. In this case, a resistor may be provided in series with the MOS transistor M 4 , so as to adjust the fall rate of the output voltage V out  based on a resistance value of the resistor. 
         [0053]    While the invention accomplished by the present inventor has been detailed referring to the embodiments, the present invention is not limited thereto. For example, while the embodiments in the above adopted a separate configuration of the MOS transistor M 3  which is directed to drop the output voltage V out  in the off time of the chip, and the MOS transistor M 4  which is directed to drop the output voltage V out  in the switching of output voltage, an alternative configuration may be such as providing these transistors as a common transistor, and also providing an OR gate which is designed to implement the OR operation of the output of the logic circuit  15  and the output of the voltage comparator circuit  16 , so as to allow on/off control of the common transistor based on the output of the OR gate. 
         [0054]    Provision of the OR gate may otherwise increase the number of elements which compose the circuit. However, in contrast to that the transistors M 3 , M 4  which are designed to allow discharge through the output terminal need a relatively large size for the configuration, the OR gate needs only small-sized elements since the load of the OR gate is only a gate capacitance of the MOS transistor. Accordingly, in the configuration having the MOS transistors M 3  and M 4  replaced by a single element, the total area occupied by the circuit may be reduced. 
         [0055]    While the embodiments described in the above used a MOS transistors as the control transistor for controlling the output voltage, the present invention is also applicable to a regulator which uses a bipolar transistor as the control transistor. 
         [0056]    While the embodiments described in the above used an offset-added differential amplifier circuit as the voltage comparator circuit  16  for controlling the MOS transistor M 4  for discharge, another possible configuration is such as using a general differential amplifier circuit having no offset, and instead feeding the feedback voltage to the differential amplifier circuit after shifted the feedback voltage by a predetermined potential corresponding to the offset. 
         [0057]    In addition, while the description in the above dealt with the case where the present invention was applied to the series regulator IC, the present invention is not limited thereto, and is also applicable to a charging control IC which configures a charger for secondary batteries.