JP2008136093A - Oscillation circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce rapid deterioration of a period of a clock signal even when a supply voltage is low. <P>SOLUTION: A supply voltage detection circuit 50 is arranged for detecting changes in a supply voltage VCC. Then, a first current generation circuit 20 for generating a first bias current in which a bias current changes depending on the supply voltage VCC is arranged. Also a second current generation circuit 30 is arranged. This circuit has a characteristic that a ratio of changes in a current value of a bias current output depending on changes in the supply voltage VCC is smaller than that of the first current generation circuit 20. Using a selection circuit 40, the first bias current is supplied to a ring oscillator 101 in a state that the supply voltage VCC is higher than a reference voltage Vx (a high voltage state), and a second bias current is supplied to the ring oscillator 101 in a state that the supply voltage VCC is lower than the reference voltage Vx (a low voltage state). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a ring oscillator type oscillation circuit.

  Conventionally, a ring oscillator type oscillation circuit has been widely used as an oscillation circuit constituting a semiconductor integrated circuit.

  FIG. 6 is a circuit diagram of a conventional ring oscillator type oscillation circuit. The oscillation circuit 100 includes a ring oscillator 101, a current generation circuit 102, a P-channel MOS transistor (hereinafter referred to as PMOS) 103, and an N-channel MOS transistor (hereinafter referred to as NMOS) 104.

The ring oscillator 101 has an odd number of stages, and three or more stages of inverters 105a to 105x are connected in series in a ring shape. The output terminal Out of the inverter 105x is the output terminal of the oscillation circuit 100, and the clock signal CLK is output from the output terminal Out.
The inverters 105a to 105x are so-called CMOS inverters having a configuration in which a PMOS 107 and an NMOS 108 are connected in series between a power supply terminal 106 and a ground terminal GND. One end of the capacitive element C is connected to the connection point of the inverters 105a to 105x. The other terminal of each capacitive element C is connected to the ground terminal GND.

  Further, between the drain of the PMOS 107 and the power supply terminal 106, the PMOS 103 that supplies the current I flowing through each of the inverters 105a to 105x via the current generation circuit 102 is connected. Further, between the drain of the NMOS 108 and the ground terminal GND, an NMOS 104 that supplies a current flowing from each of the inverters 105a to 105x to the ground terminal GND is connected.

  The current generation circuit 102 supplies a bias current to the gates of the PMOS 103 and the NMOS 104, controls the current I flowing through the inverters 105a to 105x, and as a result, controls the cycle of the clock signal CLK.

  A configuration of the current generation circuit 102 will be described. The current generation circuit 102 includes NMOSs 110 and 111, a resistor 112, and a PMOS 113.

  The sources of the NMOSs 110 and 111 are connected to the ground terminal GND. The gate and drain of the NMOS 110 are short-circuited, and a resistor 112 is connected between the connection point and the power supply terminal 106.

  The gate of the NMOS 110 is connected to the gate of the NMOS 111 to form a current mirror circuit 114. A connection point between the NMOS 110 and the NMOS 111 is connected to each gate of the NMOS 104. That is, the NMOS 110 and each NMOS 104 constitute a current mirror circuit.

  In addition, a PMOS 113 whose gate and drain are short-circuited is connected in series between the drain of the NMOS 111 and the power supply terminal 106. A connection point between the NMOS 111 and the PMOS 113 is connected to each gate of the PMOS 103. That is, the PMOS 113 and each PMOS 103 constitute a current mirror circuit.

A current I ₁ flowing through the NMOS 110 is expressed by I ₁ = (VCC−Vt) ÷ R (1). Here, VCC is a power supply voltage supplied from the power supply terminal 106, Vt is a threshold value of the NMOS 110, and R is a resistance value of the resistor 112.

The characteristic of the current mirror circuit, a current _{I 1} flowing through the NMOS 110, the current I flowing through each inverter 105a~105x are equal. A period T of charging or discharging time per stage of the inverters 105a to 105x is expressed by T = C × VCC / I (2). Therefore, when I ₁ in the formula (1) is substituted for I in the formula (2), the cycle T is expressed by T = C / R (1−Vt / VCC) (3).

  As can be seen from the equation (3), when the power supply voltage VCC is sufficiently larger than the threshold value Vt of the NMOS 110 and Vt / VCC is close to zero, the equation (3) can be approximated to T≈C / R. .

Techniques related to the present invention are described in, for example, the following patent documents.
JP 2005-184793 A

  As described above, when the power supply voltage VCC is sufficiently larger than Vt, the period T is constant without depending on the power supply voltage VCC.

  However, the operating voltage (VCC) of the oscillation circuit needs to be lowered in the same manner as the power supply voltage used in accordance with the miniaturization of semiconductor devices has been reduced in recent years. Therefore, “Vt / VCC” in equation (3) tends to be a value that cannot be ignored as an error. Therefore, in the case of the conventional circuit configuration, as shown in FIG. 7, the period T increases exponentially in a range where the power supply voltage VCC is equal to or lower than a certain voltage Vy, and the clock signal CLK having a stable period cannot be output. There was a problem.

  In view of the above, the main object of the present invention is to provide an oscillation circuit in which deterioration of the characteristics of the period of the clock signal is reduced even when the power supply voltage is low.

  The main features of the present invention are as follows. That is, the oscillation circuit of the present invention includes a ring oscillator that outputs a clock signal, a plurality of inverters connected in series, a first current generation circuit that generates a first bias current that depends on a power supply voltage, and a power supply voltage. A second current generating circuit that generates a second bias current that does not depend on the first bias current when the power supply voltage is larger than the reference voltage, and when the power supply voltage is smaller than the reference voltage; A selection circuit that selects the second bias current, and a current supply transistor that supplies the first bias current or the second bias current selected by the selection circuit to the inverter. .

  The oscillation circuit of the present invention is characterized in that the second current generation circuit includes a constant current generation circuit.

  The oscillation circuit of the present invention includes a first current generation circuit in which a bias current to be output (first bias current) depends on a power supply voltage, and a first current generation circuit in which an output bias current (second bias current) does not depend on a power supply voltage. Two current generation circuits are arranged, and one of the bias currents is supplied to the inverter by the selection circuit according to the state of the power supply voltage. Therefore, it is possible to output a clock signal having a stable cycle in a region where the power supply voltage is high and to prevent the cycle from rapidly deteriorating even in a region where the power supply voltage is low.

  Next, a first embodiment of the present invention will be described with reference to the drawings. In addition, about the structure similar to a conventional structure, the same code | symbol is shown and the description is abbreviate | omitted or simplified.

  FIG. 1 is a circuit diagram showing an example of the configuration of the oscillation circuit 10 according to the present embodiment. The oscillation circuit 10 includes a ring oscillator circuit 101, first and second current generation circuits 20, 30, a selection circuit 40, a power supply voltage detection circuit 50, a PMOS 103, and an NMOS 104.

  The ring oscillator 101 has an odd number of stages, and three or more stages of inverters 105a to 105x are connected in series in a ring shape. The output terminal Out of the inverter 105x is the output terminal of the oscillation circuit 10, and the clock signal CLK is output from the output terminal Out.

  The inverters 105a to 105x are CMOS inverters in which a PMOS 107 and an NMOS 108 are connected in series between a power supply terminal 106 and a ground terminal GND. The power supply terminal 106 is a terminal that supplies a power supply voltage VCC, and the ground terminal GND is a terminal that supplies a ground voltage.

  One end of the capacitive element C is connected to the connection point of the inverters 105a to 105x. The other terminal of each capacitive element C is connected to the ground terminal GND.

  Further, between the drain of the PMOS 107 and the power supply terminal 106, a current I flowing through each of the inverters 105a to 105x is supplied via one of first or second current generation circuits 20 and 30 described later. The PMOS 103 (first current supply transistor) is connected. Further, between the drain of the NMOS 108 and the ground terminal GND, an NMOS 104 (second circuit) for supplying a current flowing from each of the inverters 105a to 105x to the ground terminal GND through the first or second current generating circuits 20 and 30. Current supply transistor) is connected.

  Next, the first and second current generation circuits 20 and 30 that generate the current I flowing through the inverters 105a to 105x will be described.

  The first output terminal Vp1 of the first current generating circuit 20 is connected to the gate of the PMOS 103 via the first switching element SW1, and the second output terminal Vn1 is connected to the gate of the NMOS 104 via the third switching element SW3. Connected with.

  In the first current generation circuit 20, the output bias current (first bias current) changes in response to the change in the power supply voltage VCC. That is, when the power supply voltage VCC increases, the bias current increases, and when the power supply voltage VCC decreases, the bias current also decreases. Therefore, the first current generation circuit 20 supplies a bias current depending on the power supply voltage VCC to the inverters 105a to 105x. Then, in a state where the power supply voltage VCC is higher than the reference voltage, control is performed so as to keep the cycle T of the clock signal CLK constant.

  A specific configuration example of the first current generation circuit 20 will be described with reference to FIG. The first current generation circuit 20 of this embodiment includes NMOSs 110 and 111, a resistor 112, and a PMOS 113. The drain and gate of the PMOS 113 are connected to the first output terminal Vp1, and a bias current is supplied from the output terminal Vp1 to the gate of the PMOS 103 via the first switching element SW1.

  Further, the NMOS 110 and the NMOS 111 constitute a current mirror circuit 114. A connection point between the NMOS 110 and the NMOS 111 is connected to the second output terminal Vn1, and a bias current is supplied from the second output terminal Vn2 to the gate of the NMOS 104 via the third switching element SW3. The other circuit configuration is the same as that of the current generation circuit 102 of FIG.

  When a bias current is supplied from the first current generation circuit 20 to the gates of the PMOS 103 and the NMOS 104, the period T of charging or discharging time per stage of the inverters 105a to 105x is T = C / R (1 -Vt / VCC) ... It is represented by Formula (3). Further, the cycle T and the power supply voltage VCC have a relationship that the cycle T is substantially constant in a region where the power supply voltage VCC is equal to or higher than a certain voltage, and the cycle T increases exponentially below a certain voltage (see FIG. 7).

  As shown in FIG. 1, the first output terminal Vp2 of the second current generation circuit 30 is connected to the gate of the PMOS 103 via the second switching element SW2, and the second output terminal Vn2 is the fourth switching element. It is connected to the gate of the NMOS 104 via SW4.

  The second current generation circuit 30 has a characteristic that the output bias current (second bias current) does not substantially depend on the power supply voltage VCC. The second bias current is preferably constant without depending on the power supply voltage VCC. In other words, the second current generation circuit 30 has a smaller change rate of the bias current output with respect to the change of the power supply voltage VCC than the first current generation circuit 20. A constant current that does not easily depend on the power supply voltage VCC is supplied to the inverters 105a to 105x.

  A specific configuration example of the second current generation circuit 30 will be described with reference to FIG. The second current generation circuit 30 of this embodiment includes PMOSs 31 and 32, a constant current generation circuit 33, and an NMOS 34.

  The sources of the PMOSs 31 and 32 are connected to the power supply terminal 106. The gate and drain of the PMOS 31 are short-circuited, and a constant current generating circuit 33 is connected between the connection point and the ground terminal GND. The constant current generating circuit 33 is a circuit that can maintain an output current substantially constant with respect to an applied voltage, and is configured by a current mirror circuit including a pair of MOS transistors, for example.

  The gate of the PMOS 31 is connected to the gate of the PMOS 32 to constitute a current mirror circuit 35. A connection point between the PMOS 31 and the PMOS 32 is connected to the first output terminal Vp2, and a bias current is supplied from the first output terminal Vp2 to each gate of the PMOS 103 via the second switching element SW2. That is, the PMOS 31 and each PMOS 103 constitute a current mirror circuit.

  An NMOS 34 whose gate and drain are short-circuited is connected in series between the drain of the PMOS 32 and the ground terminal GND. A connection point between the PMOS 32 and the NMOS 34 is connected to the second output terminal Vn2, and a bias current is supplied from the second output terminal Vn2 to each gate of the NMOS 104 via the fourth switching element SW4. That is, the NMOS 34 and each NMOS 104 constitute a current mirror circuit.

The current value of the current I ₂ flowing through the second current generation circuit 30 of this configuration example is kept almost constant without depending on the change of the power supply voltage VCC due to the action of the constant current generation circuit 33. Then, from the characteristic of the current mirror circuit, a current I flowing through this current _{I 2} to the inverter 105a~105x are equal.

When a bias current is supplied from the second current generation circuit 30 to the gates of the PMOS 103 and the NMOS 104, the period T of charging or discharging time per stage of the inverters 105a to 105x is T = C × as in the equation (2). VCC / I ₂ ... represented by the formula (4). Here, C and I ₂ in the present embodiment are constant values. Therefore, the cycle T shows a relationship proportional to the power supply voltage VCC as shown in FIG.

  Further, as shown in FIG. 1, either the first current generation circuit 20 or the second current generation circuit 30 is selectively connected to the PMOS 103 and the NMOS 104, and the first bias current or the second bias current is connected. A selection circuit 40 for selecting one of the currents is provided. The selection circuit 40 includes the above-described first to fourth switching elements SW1, SW2, SW3, SW4. For example, when a high-level selection signal Φ (H) is applied to the selection circuit 40, the first and third switching elements SW1 and SW3 are turned on and the second and fourth switching elements SW2 and SW4 are turned off. To do. When the low level selection signal Φ (L) is applied to the selection circuit 40, the opposite is true.

  The selection signal Φ is generated by the power supply voltage detection circuit 50. The power supply voltage detection circuit 50 is connected to the power supply terminal 106 and monitors the level of the power supply voltage VCC. When the power supply voltage VCC is larger than a voltage (referred to as a reference voltage Vx) determined in advance by design (hereinafter referred to as a high voltage state), a high level selection signal Φ (H) is output to output the first voltage. The first bias current from the current generation circuit 20 is supplied to each of the inverters 105a to 105x. On the other hand, when the power supply voltage VCC is lower than the reference voltage Vx (hereinafter referred to as a low voltage state), a low level selection signal Φ (L) is output and the second bias current generated by the second current generation circuit 30 is output. Is supplied to each of the inverters 105a to 105x.

As described above, in the present embodiment, the first current generation circuit 20 in which the bias current to be output changes with respect to the magnitude of the power supply voltage VCC, and the current that flows in the inverters 105a to 105x without depending on the change in the power supply voltage VCC. The second current generation circuit 30 that controls the current substantially constant can be arranged to select which bias current is supplied to each of the inverters 105a to 105x with the reference voltage Vx as a boundary. Then, by adjusting the switching point (reference voltage Vx), the ring oscillator is operated with the current I ₁ depending on the power supply voltage VCC as the first bias current in the high voltage state, and the constant current generating circuit 33 in the low voltage state. The ring oscillator is operated by using the current I ₂ utilizing the current I ₂ as the second bias current.

  The period T and the power supply voltage VCC according to the configuration of the present embodiment have a relationship obtained by combining a part of the graph shown in FIG. 4 and a part of the graph shown in FIG. 7, as shown by the solid line in FIG. As can be seen from FIG. 5, by switching from the first current generation circuit 20 to the second current generation circuit 30 before the power supply voltage VCC is low and the period T exponentially changes, the conventional method (see FIG. 7). ), The change in the period T in the low voltage state is moderate. On the contrary, when the power supply voltage VCC becomes higher than the reference voltage Vx, the cycle T becomes constant by switching from the second current generation circuit 30 to the connection with the first current generation circuit 20.

  As described above, according to the configuration of the present embodiment, it is possible to output a clock signal having a stable cycle that does not depend on a change in the power supply voltage VCC in a high voltage state. And even if it is a low voltage state, it can prevent that the period of a clock signal changes rapidly. In addition, the range of the power supply voltage that can be guaranteed for the operation of the oscillation circuit can be expanded to a voltage lower than the conventional one.

  Needless to say, the present invention is not limited to the above-described embodiment, and the design can be changed without departing from the gist thereof. For example, the switching point between the first current generation circuit 20 and the second current generation circuit 30 can be arbitrarily set in consideration of the range of the period T that is allowable in the specification. The present invention is widely applicable to ring oscillator type oscillation circuits.

It is a circuit diagram showing an oscillation circuit concerning an embodiment of the present invention. It is a circuit diagram showing an example of the 1st current generating circuit concerning the embodiment of the present invention. It is a circuit diagram showing an example of the 2nd current generating circuit concerning the embodiment of the present invention. It is a graph which shows the relationship between the period T at the time of supplying the bias current by a 2nd electric current generation circuit to a ring oscillator, and the power supply voltage VCC. It is a graph which shows the relationship between the period T by the oscillation circuit which concerns on this embodiment, and the power supply voltage VCC. It is a circuit diagram which shows the conventional oscillation circuit. It is a graph which shows the relationship between the period T by the conventional oscillation circuit, and the power supply voltage VCC.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Oscillation circuit 20 1st current generation circuit 30 2nd current generation circuit 31, 32 P channel type MOS transistor 33 Constant current generation circuit 34 N channel type MOS transistor 35 Current mirror circuit 40 Selection circuit 50 Power supply voltage detection circuit 101 Ring Oscillator 102 Current generation circuit 103 P-channel MOS transistor
104 N-channel MOS transistors 105a to 105x Inverter 106 Power supply terminal 107, 113 P-channel MOS transistors 108, 110, 111 N-channel MOS transistor 112 Resistor 114 Current mirror circuit
SW1 to SW4 First to fourth switching elements OUT Output terminal VCC Power supply voltage Vp1 (First current generation circuit) First output terminal Vn1 (First current generation circuit) Second output terminal Vp2 (Second First output terminal Vn2 (of the current generation circuit) Second output terminal (of the second current generation circuit) GND Ground terminal C Capacitance element VCC Power supply voltage Φ Selection signal CLK Clock signal

Claims (6)

A ring oscillator in which a plurality of inverters are connected in series and outputs a clock signal;
A first current generating circuit for generating a first bias current depending on a power supply voltage;
A second current generating circuit for generating a second bias current independent of the power supply voltage;
A selection circuit that selects the first bias current when a power supply voltage is higher than a reference voltage, and that selects the second bias current when the power supply voltage is lower than a reference voltage;
An oscillation circuit comprising: a current supply transistor that supplies the first bias current or the second bias current selected by the selection circuit to the inverter. The oscillation circuit according to claim 1, wherein the second current generation circuit includes a constant current generation circuit. The current supply transistor is:
A first current supply transistor connected to the positive electrode terminal of the inverter;
3. The oscillation circuit according to claim 1, further comprising a second current supply transistor connected to the negative electrode terminal of the inverter. The first current generation circuit includes:
A first transistor constituting a current mirror circuit with the second current supply transistor;
A resistor connected between a power supply terminal and the first transistor;
A second transistor constituting a current mirror circuit with the first transistor;
4. The device according to claim 3, further comprising a third transistor that is connected in series between the power supply terminal and the second transistor, and that forms a current mirror circuit with the first current supply transistor. Oscillator circuit. The second current generation circuit includes a fourth transistor that forms a current mirror circuit with the first current supply transistor;
The oscillation circuit according to claim 3, further comprising: a constant current generation circuit connected in series with the fourth transistor. A fifth transistor that forms a current mirror circuit with the fourth transistor; and a sixth transistor that is connected in series with the fifth transistor and forms a current mirror circuit with the second current supply transistor. The oscillation circuit according to claim 5.

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