Voltage regulator circuit with oscillation suppression

Abstract

 A regulator circuit comprises a phase correcting circuit portion (40). The phase correcting circuit portion (40) comprises a current correcting transistor (M11) combined with an output transistor to form a current mirror circuit. An I-V converting resistor (R4) is connected to the current correcting transistor (M11) to convert a current flowing through the phase correcting circuit portion (40) into a feedback voltage which is proportional to an output voltage and independent of an output capacitor (Co). A phase correcting capacitor (C1) and a resistor (R5) correct the error signal according to the feedback voltage.

Description

[0001] This application claims priority to prior Japanese application JP 2003-110435, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION:

[0002] This invention relates to a regulator circuit, in particular, to a series regulator integrated circuit which can prevent oscillation thereof regardless of an output capacitor connected thereto.

[0003] A regulator circuit regulates an input voltage supplied to a power supply terminal thereof from the outside and supplies a regulated voltage as an output voltage to an output terminal thereof. While the output terminal is connected to a load, an output capacitor is also connected to the output terminal. The output capacitor smoothes the output voltage supplied to the output terminal prior to supplying the output voltage to the load. Thus, the load receives the smoothed regulated voltage from a combination of the regulator circuit and the output capacitor.

[0004] Though an electrolytic capacitor or a tantalum capacitor is generally used as the output capacitor, a ceramic capacitor with smaller capacitance is coming into use to meet a demand for miniaturizing.

[0005] Now, it is known that the various capacitors have different values of equivalent series resistance (ESR). For instance, the electrolytic capacitor, the tantalum capacitor and the ceramic capacitor have the ESR between 0.1 and 100 [Ω], 0.01 to 1 [Ω] and 0.001 to 0.1 [Ω], respectively, though the ESR varies according to a frequency of an input signal supplied to each capacitor and/or an ambient temperature thereof.

[0006] When the output capacitor has the ESR inappropriate for a phase secure range of the regulator circuit connected thereto, it causes the regulator circuit oscillation. Concretely, the regulator circuit oscillates when the output capacitor has too low ESR for the regulator circuit. Accordingly, some preventive measures are necessary for preventing oscillation of the regulator circuit when an output capacitor with a smaller capacitance is used.

[0007] A related regulator circuit is manufactured by advanced micro processing to adapt to reduction of the capacitance and the ESR of the output capacitor. That is, the related regulator circuit secures a necessary oscillation margin by means of miniaturizing devices included therein to improve frequency characteristic thereof. Each transistor included in the related regulator circuit, for example, has a channel length of 0.6 [µm].

[0008] The related regulator is expensive because it is manufactured by the advanced micro processing as mentioned above.

SUMMARY OF THE INVENTION:

[0009] It is therefore an object of at least the preferred embodiments of this invention to provide a regulator circuit which inexpensively can prevent oscillation thereof regardless of an output capacitor.

[0010] Other such objects of this invention will become clear as the description proceeds.

[0011] According to an aspect of this invention, a regulator circuit is used with an output capacitor connected to an output terminal thereof. The regulator circuit comprises a phase correcting capacitor for preventing oscillation thereof. The regulator circuit is characterized in that the phase correcting capacitor is provided to be independent of the output capacitor connected to the output terminal.

[0012] The regulator circuit further comprises a power supply terminal and an output transistor connected between the power supply terminal and the output terminal. The phase correcting capacitor is included in a phase correcting circuit portion provided at a side of the power supply terminal in relation to the output transistor in the regulator circuit.

[0013] According to another aspect of this invention, a regulator circuit comprises an error amplifier for producing an error signal according to a difference between a reference voltage and a divided voltage derived from an output voltage supplied to an output terminal. An output circuit is for producing the output voltage according to the error signal. A feedback voltage producing portion is for producing a feedback voltage equivalent to the output voltage from the input voltage. A phase correcting portion is connected to both of the feedback voltage producing portion and the error amplifier and has a phase correcting capacitor to correct phase of the error signal according to the feedback output voltage.

[0014] According to still another aspect of this invention, a regulator circuit regulates an input voltage to produce an output voltage according to an error signal based on difference between a reference voltage and a divided voltage derived from the output signal. A method for preventing the regulator circuit from oscillating is characterized by the steps of producing a feedback voltage equivalent to the output voltage, and correcting a phase of the error signal according to the feedback voltage.

BRIEF DESCRIPTION OF THE DRAWING:

[0015] 

Fig. 1 is a circuit diagram of a related regulator circuit;

Fig. 2 is a graph showing open loop characteristics of the regulator circuit of Fig. 1 in a case where an output capacitor having ESR of 100 [Ω] is connected to the regulator circuit;

Fig. 3 is a graph showing open loop characteristics of the regulator circuit of Fig. 1 in a case where an output capacitor having ESR of 0.01 [Ω] is connected to the regulator circuit;

Fig. 4 is a circuit diagram of a regulator circuit according to a preferred embodiment of this invention;

Fig. 5 is a graph showing open loop characteristics of the regulator circuit of Fig. 4 in a case where an output capacitor having ESR of 100 [Ω] is connected to the regulator circuit; and

Fig. 6 is a graph showing open loop characteristics of the regulator circuit of Fig. 4 in a case where an output capacitor having ESR of 0.01 [Ω] is connected to the regulator circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

[0016] Referring to Figs. 1 to 3, description will be at first directed to a related regulator circuit for a better understanding of this invention.

[0017] Fig. 1 is a circuit diagram of a related regulator circuit. The regulator circuit comprises a power supply terminal V_DD for being connected to a power supply P_W, a grounding terminal GND for being grounded, and an output terminal V_OUT for being connected to an load RL together with an output capacitor C1. The regulator circuit is called a three terminal regulator circuit or a series regulator integrated circuit (IC).

[0018] The regulator circuit further comprises a constant current source 11, first to tenth transistors M1-M10, first to third resistors R1-R3 and a phase correcting capacitor C1.

[0019] The constant current source 11 and the first to the eighth transistors M1-M8 form an error amplifier connected between the power supply terminal V_DD and the grounding terminal GND. The error amplifier is connected to a connection point between the first and the second resistors R1 and R2 to receive a divided voltage produced by a combination of the first and the second resistors R1 and R2. The error amplifier is further connected to a reference voltage producing circuit (not shown) to receive a reference voltage supplied from the reference voltage producing circuit. The error amplifier produces an error signal according to a difference between the divided voltage and the reference voltage.

[0020] The ninth transistor M9 has a gate, a drain and a source which are connected to the error amplifier, a gate of the tenth transistor M10 and the grounding terminal GND respectively. The ninth transistor M9 receives the error signal from the error amplifier to vary a gate voltage of the tenth transistor M10 according to the error signal.

[0021] The tenth transistor M10 has a source and a drain which are connected to the power supply terminal V_DD and the output terminal V_OUT respectively. The tenth transistor M10 serves as an output transistor (or a power transistor). That is, the tenth transistor M10 regulates an input voltage supplied to the power supply terminal V_DD to supply a regulated voltage as an output voltage to the output terminal V_OUT.

[0022] The first and the second resistor R1 and R2 are connected to each other between the output terminal V_OUT and the grounding terminal GND. The first and the second resistor R1 and R2 divide the output voltage to produce the divided voltage at the connection point between them.

[0023] The third resistor R3 is connected between the gate and the source of the tenth transistor M10 to produce a control voltage (i.e. the gate voltage) at the gate of tenth transistor M10 according to an operation of the ninth transistor M9.

[0024] The phase correcting capacitor C1 is connected between the output terminal V_OUT and a gate of the first transistor M1 (or the connection point between the first and the second resistors R1 and R2). Alternatively, as shown by broken lines in Fig. 1, the phase correcting capacitor C1 is connected between the output terminal V_OUT and a gate of the eighth transistor M8.

[0025] Next, an operation of the regulator circuit will be described below.

[0026] Supplying the power supply voltage (e.g. 3-7 [V]) to the power supply terminal V_DD causes currents flowing through the first and the second transistors M1 and M2. Values of the currents flowing through the first and the second transistors M1 and M2 depend on the difference between the divided voltage and the reference voltage. The reference voltage, for example, is equal to 1.2 [V].

[0027] The third and the fourth transistors M3 and M4 form a first current mirror circuit while the fifth and the sixes transistors M5 and M6 form a second current mirror circuit. The first and the second current mirror circuits supply currents equivalent to the currents flowing through the first and the second transistors M1 and M2 to the seventh and the eighth transistors M7 and M8 respectively. A difference between the currents supplied to the seventh and the eighth transistors M7 and M8 generates a voltage (i.e. the error signal) at the gate of the ninth transistor M9. The ninth transistor M9 varies a current flowing therein according to the error signal. Consequently, the gate voltage of the tenth transistor M10 is varied according to the error signal. The tenth transistor M10 regulates the input voltage and supplies the output voltage to the output terminal V_OUT. The output voltage, for example, is equal to 2-5 [V]. The output voltage is smoothed by the output capacitor Co prior to supplying it to the load RL.

[0028] The phase correcting capacitor C1 enlarges a phase secure range of the regulator circuit to prevent the regulator circuit from oscillating. In other words, the phase correcting capacitor C1 secures phase advance of the output voltage against the input voltage. The phase correcting capacitor C1, for example, has a capacitance of 30 [pF]. The regulator circuit having the phase correcting capacitor C1 does not oscillate, even if it is connected to the output capacitor Co with relative high ESR (e.g. 100 [Ω]).

[0029] Fig. 2 shows open loop characteristics of the regulator circuit of Fig. 1 in a case where the output capacitor Co having the ESR of 100 [Ω] is connected to the regulator circuit.

[0030] As shown in Fig. 2, phase difference between input and output signals is larger than a necessary phase margin (e.g. 45°) in a frequency range from 0.01 to 10000 [kHz]. Accordingly, the regulator circuit does not oscillate in the case of Fig. 2.

[0031] Though the phase correcting capacitor C1 is effective in the case where the output capacitor Co has the relative high ESR, it is ineffective in a case where the output capacitor Co has relative low ESR (e.g. equal to or less than 0.1 [Ω]).

[0032] Fig. 3 shows open loop characteristics of the regulator circuit of Fig. 1 in a case where the output capacitor Co having the ESR of 0.01 [Ω] is connected to the regulator circuit.

[0033] As illustrated in Fig. 3, the phase difference between the input and the output signals is lower than the necessary phase margin (45°) in a frequency range over 20 [kHz]. In addition, when the phase difference is equal to 0° (or 360°), the regulator circuit has a positive gain. Therefore, the regulator oscillates in the case of Fig. 3.

[0034] The open loop characteristics of the regulator circuit can be improved by miniaturizing components (or devices) of the regulator circuit. When the frequency characteristics of the regulator circuit are improved, the necessary oscillation margin can be secured. Accordingly, the regulator circuit having miniaturized components does not oscillate even if the output capacitor connected thereto has the relative low ESR.

[0035] However, an advanced micro processing is necessary to manufacture the regulator circuit having the miniaturized components. Therefore, the regulator circuit having the miniaturized components is expensive.

[0036] Referring to Figs. 4 to 6, the description will be proceed to a regulator circuit according to a preferred embodiment of this invention.

[0037] Fig. 4 is a circuit diagram of the regulator circuit (or a series regulator IC). The regulator circuit is basically similar to the related regulator circuit of Fig. 1 and different in having a phase correcting circuit portion 40 including a phase correcting capacitor C1'.

[0038] In detail, the regulator circuit comprises a constant current source 11, first to tenth transistors M1-M10, first to third resistors R1-R3 and the phase correcting circuit portion 40.

[0039] The phase correcting circuit portion 40 comprises an eleventh transistor M11, forth and fifth resistors R4 and R5, and the phase correcting capacitor C1'. For example, the forth and the fifth resistors R4 and R5 has resistance of 60 [kΩ] and 100 [kΩ] respectively, while the phase correcting capacitor C1' has capacitance of 5 [pF].

[0040] The constant current source 11 and the first to the eighth transistors M1-M8 form an error amplifier (as an error amplifying means) to produce an error signal according to two input signals (i.e. a divided voltage and a reference voltage as mentioned below).

[0041] The first and the second transistors M1 and M2 comprise N channel FETs (e.g. MOS FETs) which are identical to each other. The first and the second transistors M1 and M2 have sources connected to the constant current source 11 in common, and drains connected to those of the third and the fifth transistors M3 and M5. A gate of the first transistor M1 is connected to a connection point between the first and the second resistors R1 and R2. A gate of the second transistor M2 is connected to a reference voltage producing circuit (not shown) to receive the reference voltage produced by the reference voltage producing circuit. The first and the second transistors M1 and M2 divides a constant current produced by the constant current source 11 according to the difference between voltages supplied to the gates of the first and the second transistors M1 and M2.

[0042] The third and the fourth transistors M3 and M4 comprise P channel FETs (e.g. MOS FETs) which are identical to each other. The third and the fourth transistors M3 and M4 have sources connected to a power supply terminal V_DD, and gates connected to a drain of the third transistor M3. The drain of the third transistor M3 is connected to the drain of the first transistor M1. A drain of the fourth transistor M4 is connected to a drain of the seventh transistor M7. The third and the fourth transistors M3 and M4 form a first current mirror circuit to supply a current equivalent to that flowing through the first transistor M1 to the seventh transistor M7.

[0043] The fifth and the sixth transistors M5 and M6 are similar to the third and the fourth transistor M3 and M4. That is, the fifth and the sixth transistors M5 and M6 comprise P channel FETs (e.g. MOS FETs) which are identical to each other. The fifth and the sixth transistors M5 and M6 have sources connected to a power supply terminal V_DD, and gates connected to a drain of the fifth transistor M5. The drain of the fifth transistor M5 is connected to the drain of the first transistor M1. A drain of the sixth transistor M6 is connected to a drain of the eighth transistor M8. The fifth and the sixth transistors M5 and M6 form a second current mirror circuit to supply a current equivalent to that flowing through the second transistor M2 to the eighth transistor M8.

[0044] The seventh and the eighth transistors M7 and M8 comprise N channel FETs (e.g. MOS FETs) which are identical to each other. The seventh transistor M7 has a source connected to the grounding terminal GND, and a gate connected to both of the drain thereof and one of the terminals of the fifth resistor R5 of the phase correcting circuit portion 40. On the other hand, the eighth transistor M8 has a source connected to the grounding terminal GND, and a gate connected to the other terminal of the fifth resistor R5 of the phase correcting circuit portion 40. The eighth transistor M8 is referred to as an error signal producing transistor. The seventh and the eighth transistors M7 and M8 form a third current mirror circuit to generate a voltage (i.e. the error signal) at the drain of the eighth transistor M8 according to a difference between currents supplied from the fourth and the sixth transistors M4 and M6.

[0045] The ninth transistor M9 comprises an N channel FET. The ninth transistor M9 has a source connected to the grounding terminal GND, a drain connected to a gate of the tenth transistor M10, and a gate connected to the drain of the eighth transistor M8 (and to the drain of the sixth transistor M6). The ninth transistor M9 cooperates with a third resistor R3 to generate a voltage (i.e. a control voltage) at the gate of the tenth transistor M10 according to a gate voltage (i.e. the error signal supplied from the error amplifier) of the ninth transistor M9.

[0046] The tenth transistor M10 comprises a P channel FET. The tenth transistor M10 has a source connected to the power supply terminal V_DD and to the gate thereof through the third resistor R3. A drain of the tenth transistor M10 connected to the output terminal V_OUT. The tenth transistor M10 regulates an input voltage (or a power supply voltage) supplied to the power supply terminal V_DD from the power supply P_W to produce an output voltage (or a regulated voltage) supplied to the output terminal V_OUT. The tenth transistor M10 is referred to as an output transistor.

[0047] The ninth and the tenth transistors M9 and M10 and the third resistor R3 serve as an output circuit means.

[0048] The first and the second resistors R1 and R2 are connected to each other between the output terminal V_OUT and the grounding terminal GND. As mentioned above, the connection point between the first and the second resistors R1 and R2 is connected to the gate of the first transistor M1. The first and the second resistor R1 and R2 divide the output voltage supplied to the output terminal V_OUT to produce the divided voltage supplied to the gate of the first transistor M1.

[0049] The eleventh transistor M11 comprises a P channel transistor. The eleventh transistor M11, together with the tenth transistor M10, forms a current mirror circuit. The eleventh transistor M11 is referred to as a correcting circuit transistor or a feedback transistor. The eleventh transistor M11 has a source connected to the power supply terminal V_DD, a drain connected to the grounding terminal GND through the fourth resistor R4, and a gate connected to the gate of the tenth transistor M10.

[0050] The fourth resistor R4 is referred to as a current-voltage-converting resistor. The fourth resistor R4 cooperates with the eleventh transistor M11 produces a feedback voltage proportional to (or equivalent to) the output voltage at a connection point between the fourth resistor R4 and the eleventh transistor M11. That is, the fourth resistor R4 and the eleventh transistor M11 serves as a feedback voltage producing means. The feedback voltage is not influenced by an output capacitor Co (especially, its ESR) connected to the output terminal V_OUT. This is because the source of the eleventh transistor M11 is not connected to the output terminal V_OUT but to the power supplying terminal V_DD.

[0051] The phase correcting capacitor C1' has a pair of terminals, one of which is connected to the connection point between the eleventh transistor M11 and the fourth resistor R4, and the other of which is connected to the gate of the eighth transistor M8. IN other words, the other of the terminals of the phase correcting capacitor is connected to both of the resistor R5 and the eighth transistor M8. The phase correcting capacitor C1' varies the gate voltage of the eighth transistor M8 according to the feedback voltage to correct the error signal output from the error amplifier. Thus, the phase correcting capacitor C1' and the resistor R5 serve as a phase correcting means.

[0052] With the above mentioned structure, the regulator circuit of Fig. 4 regulates the input voltage supplied to the power supply terminal V_DD to produce the regulated voltage. The regulator circuit supplies the regulated voltage to the output terminal V_OUT as the output voltage.

[0053] The phase correcting circuit 40 is connected between the input terminal V_DD and the output transistor M10 as mentioned above. That is, the phase correcting circuit 40 is provided at a power supplying terminal side of the output terminal 10. Accordingly, the phase correcting circuit 40 can execute phase correction without influence of the output capacitor Co (or its ESR) when the regulator circuit regulates the power supply voltage. That is, the phase correcting circuit 40 feeds the feedback voltage proportional to the output voltage to the error amplifier regardless of the output capacitor Co (or the ESR) and corrects the error signal output from the error amplifier to suppress variation of the output voltage. Thus, the regulator circuit of this embodiment can execute the phase correction between the input and the output voltages regardless of the ESR of the output capacitor Co. Therefore, the regulator circuit secures the necessary phase margin regardless of a frequency of variation of the power supply voltage. At least, the necessary phase margin is secured for the ESR from 10 [mΩ] to 100 [Ω].

[0054] Fig. 5 shows open loop characteristics of the regulator circuit of Fig. 4 in a case where the output capacitor Co having the ESR of 100 [Ω] is connected to the regulator circuit. The regulator circuit is manufactured by means of normal fine processing while each transistor included in the regulator circuit has a channel length of 1.2 [µm].

[0055] As shown in Fig. 5, phase difference between input and output signals is larger than the necessary phase margin (e.g. 45°) over a frequency range from 0.01 to 10000 [kHz]. Accordingly, the regulator circuit does not oscillate in the case of Fig. 5.

[0056] Fig. 6 shows open loop characteristics of the regulator circuit of Fig. 4 in a case where the output capacitor Co has the ESR of 0.01 [Ω].

[0057] As illustrated in Fig. 6, the necessary phase margin (45°) is secured in a frequency range under 1000 [Hz]. In addition, the regulator circuit has negative gain in a frequency range over 1000 [Hz]. Accordingly, the regulator circuit does not oscillate over a frequency range from 0.01 to 10000 [kHz].

[0058] As mentioned above, the regulator circuit of this embodiment can be used with the output capacitor having a small capacitance and low ESR, even if it is manufactured by the normal fine processing. Therefore, the regulator circuit is inexpensive.

[0059] Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

[0060] Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

[0061] The description of the invention with reference to the drawings is by way of example only.

[0062] The text of the abstract filed herewith is repeated here as part of the specification

[0063] A regulator circuit comprises a phase correcting circuit portion. The phase correcting circuit portion comprises a current correcting transistor M11 combined with an output transistor to form a current mirror circuit. An I-V converting resistor R4 is connected to the current correcting transistor M11 to convert a current flowing through the phase correcting circuit portion into a feedback voltage which is proportional to an output voltage and independent of an output capacitor Co. A phase correcting capacitor C1 and a resistor R5 corrects the error signal according to the feedback voltage.

Claims

1. A regulator circuit for use with an output capacitor connected to an output terminal thereof, comprising a phase correcting capacitor for preventing oscillation of the circuit, characterized in that:

said phase correcting capacitor is isolated from said output capacitor.

2. A regulator circuit as claimed in Claim 1, comprising a power supply terminal and an output transistor connected between said power supply terminal and said output terminal, wherein
   said phase correcting capacitor is included in a phase correcting circuit portion provided on the power supply terminal side of said output transistor.

3. A regulator circuit as claimed in Claim 2, wherein said phase correcting circuit portion comprises:

a feedback transistor combined with said output transistor to form a current mirror circuit; and

an I-V converting resistor connected to said feedback transistor at a connection point for converting a current flowing through said correcting transistor into a feedback voltage;

said phase correcting capacitor having a terminal connected to said connection point.

4. A regulator circuit as claimed in Claim 3, comprising an error amplifier for producing an error signal according to difference between a divided voltage derived from an output voltage and a reference voltage by the use of an error signal producing transistor, wherein
   another terminal of said phase correcting capacitor is connected to said error signal producing transistor.

5. A regulator circuit as claimed in Claim 4, comprising another transistor combined with said error signal producing transistor to form another current mirror circuit, wherein
   said phase correcting circuit portion further comprises another resistor connected between said error signal producing transistor and the other transistor,
   the said other terminal of said phase correcting capacitor is connected to both of said resistor and said error signal producing transistor.

6. A regulator circuit claimed in Claim 1, comprising:

an error amplifying means for producing an error signal according to a difference between a reference voltage and a divided voltage derived from an output voltage supplied to said output terminal;

an output circuit means for producing the output voltage according to the error signal;

a feedback voltage producing means for producing a feedback voltage equivalent to the output voltage from the input voltage; and

a phase correcting means connected to both of said feedback voltage producing means and said error amplifying means and including said phase correcting capacitor for correcting phase of the error signal according to the feedback output voltage.

7. A regulator circuit as claimed in Claim 6, wherein
   said output circuit means comprises an output transistor, and wherein
   said feedback voltage producing means comprises a feedback transistor combined with said output transistor for forming a current mirror circuit; and
   a resistor connected to said feedback transistor at a connection point for converting a current flowing through said feedback transistor into the feedback voltage, and wherein
   said phase correcting capacitor is connected between said connection point and said error amplifying means.

8. A method for preventing a regulator circuit from oscillating, said regulator circuit regulating an input voltage to produce an output voltage according to an error signal based on difference between a reference voltage and a divided voltage derived from the output signal, characterized by the steps of:

producing a feedback voltage equivalent to said output voltage, and

correcting the phase of said error signal according to said feedback voltage.

9. A regulator circuit configured to regulate an input voltage to produce an output voltage according to an error signal based on a difference between a reference voltage and a divided voltage derived from the output signal, characterized by means for producing a feedback voltage equivalent to the output voltage and means for correcting the phase of the error signal according to the feedback voltage.

Drawing