Patent Application
20090115502
The present invention relates to a reference current circuit, a reference voltage circuit, and a startup circuit which use a band gap reference circuit.
In general, various semiconductor devices including an analog circuit use a reference voltage circuit using a band gap reference (hereinafter referred to it as BGR) circuit for suppressing variation in characteristics caused due to power supply voltage variation or the temperature variation. It is known that reference voltage that the BGR type reference voltage circuit generates less depends on the power supply voltage variation and the temperature variation.
Similarly, a reference current having less power supply voltage dependency and less temperature dependency can be obtained generally by converting a voltage obtained in a BGR type reference voltage circuit as above to a current. As well, a BGR type reference current circuit is known recently which directly obtains a constant reference current (see Patent Document 1, for example).
The BGR type reference current circuit of Patent Document 1 will now be described with reference to
A conventional circuit shown in
The two pMOS transistors MP1, MP2 are so designed to have a current ratio (a transistor size ratio) of 1:m (where m is an integer equal to or larger than 1, for example). Respective pairs of two resistors R2, R3 and the diodes D1, D2 are so designed that the respective current ratios of the currents flowing therein become 1:m, as well. The two diodes (D1, D2) are so set that the ratio of the reverse saturation currents thereof (a size ratio of the diodes) becomes n:1 (where n is an integer larger than 1, for example). Further, the resistances of R1 to R3, the diode ratio n, and the current ratio m are so designed that the current variation caused due to the temperature variation becomes small.
More specifically, the conventional circuit is designed as follows, for example.
The resistance of the resistor R3 is so designed to have a ratio to the resistance of the resistor R2 equal to the above current ratio, 1:m (Expression 21 indicated in combination in
On the other hand, the relationship between the current I1 flowing in the diode D1 and the anode potential V1 is expressed by Expression 23. Further, the current I3 is m times the current I1, as described above, and accordingly, the relationship between the current I3 (=m·I1) and the anode potential Vd of the diode D2 is expressed by Expression 24, where Is is a reverse saturation current of the diode D2, n is a ratio of the reverse saturation currents of the diodes D1, D2, k is a Boltzmann constant, T is an absolute temperature, and q is an electron charge amount.
Accordingly, the voltage across the resistor R1 and the current I1 flowing therein are expressed by Expressions 25 and 26, respectively.
The current I2 flowing in the resistor R2 is expressed by Expression 27, and therefore, the current Iout flowing in the pMOS transistor MP1 can be obtained by adding Expressions 26 and 27 (Expression 28). Referring to Expression 28, with no VDD depending terms, the current Iout of an ideal element will not vary by the power supply voltage variation.
The temperature coefficient of the current Iout with respect to the absolute temperature is obtained by partially differentiating Expression 28 by the absolute temperature T (Expression 29). In Expression 29, the first term is a positive value of a coefficient with respect to the absolute temperature T while the gradient (∂Vd/∂T) of Vd with respect to the absolute temperature in the second term is a negative value. Accordingly, when the values of R1, R2, m, and n are so set that the value of Expression 29 as the temperature coefficient of the current Iout becomes zero, the current Iout becomes independent from temperature.
Specifically, given that the value A is defined as in Expression 30, the resistor R2 is set as in Expression 31 with respect to the resistor R1. As well, the resistor R3 is set as in Expression 32 from this and Expression 21.
The total resistance Rall necessary for forming the above reference current circuit is expressed by Expression 33 in
Accordingly, R2=R3≈7·R1 (Expression 35) and the total resistance Rall≈15·R1 (Expression 36) are lead, which means that a resistance of approximately 15 times the resistance of the resistor R1 as a reference is necessary.
Consequently, in order to obtain smaller current consumption and a smaller current Iout, larger resistances of R1, R2 are necessary as indicated by Expression 28. In contrast, the sheet resistance of resistors formed on a semiconductor integrated circuit is 200 to 300Ω at the most. For this reason, the resistors will occupy almost of all the area of a BGR type reference current circuit having a total current value of 1 μA or lower, for example. This means that in order to realize a BGR type reference current circuit having a smaller area, the total resistance necessary for realizing the circuit must be further reduced.
Patent Document 1: Japanese Unexamined Patent Application Publication 11-45125 (FIG. 5 and the like)
In the conventional reference current circuit, however, a resistance of approximately 15 times the resistance R1 as a reference is necessary, as described above, and therefore, it is difficult to reduce the area of a semiconductor integrated circuit by remarkably reducing the total resistance.
In view of the foregoing, the present invention has its object of effortlessly reducing the area of a semiconductor integrated circuit by reducing the resistance necessary for a BGR type reference current circuit and the like.
In order to solve the above problems, an embodiment in accordance with the present invention provides a reference current circuit includes: a current mirror circuit connected to a first power supply and generating first and second mirror currents at a predetermined current ratio, the first mirror current being distributed to first and second distributed currents while the second mirror current being distributed to third and fourth distributed currents; a first diode provided in a current path through which the first distributed current flows to a second power supply; a second diode provided in a current path through which the third distributed current flows to the second power supply; a first resistor provided in at least one of the current paths in which the first and third distributed currents flow; a second resistor provided in a current path in which an added current of the second and fourth distributed currents flows added and having one end connected to the second power supply; first to fourth transistors including drain electrodes on the first power supply side and source electrodes on the second power supply side of current paths in which the first to fourth distributed currents flow, respectively, wherein the first and third transistors control the first and third distributed currents to equalize a difference of potential differences between respective ends of the first and second diodes to a potential difference of ends of the first resistor, and the second and fourth transistors control a current ratio between the second and fourth distributed currents to be the predetermined current ratio.
With the above arrangement, the first and second mirror currents are kept at the predetermined current ratio by the current mirror circuit. The first mirror current is distributed to the first and second distributed currents while the second mirror current is distributed to the third and fourth distributed currents. The second and fourth distributed currents are kept at the predetermined current ratio by the second and fourth transistors. Herein, the terms, current distribution and addition are distinguished from each other according to the direction of the current flow for convenience in general and have the same meaning on the assumption that the positive and negative currents are symmetric.
Accordingly, the first and third distributed currents have the same current ratio. This means that the difference between the potential difference between the ends of the first diode in which the first distributed current flows and the potential difference between the ends of the second diode in which the third distributed current flows is in proportion to the absolute temperature. Further, the difference between the potential differences is controlled by the first and third transistors so as to be equal to the potential difference between the ends of the first resistor, and accordingly, the current flowing in the first resistor is in proportion to the absolute temperature and has a positive temperature coefficient.
On the other hand, the potential difference between the ends of the second resistor is approximately equal to the potential differences between the respective ends of the first and second diodes and has a negative temperature coefficient with respect to the absolute temperature. Therefore, the current flowing in the second resistor has a negative temperature coefficient with respect to the absolute temperature. Accordingly, appropriate setting of the resistances of the first and second resistors leads to offset of influence of the positive and negative temperature coefficients, so that variation caused due to the temperature variation of the mirror currents and the like can be suppressed or reduced in the first and second mirror currents before the second and fourth distributed currents are distributed.
Moreover, the second distributed current to which the first mirror current is distributed and the fourth distributed current to which the second mirror current is distributed are added and are allowed to flow into the second resistor. Accordingly, when the current ratio is 1:1, for example, required is only one resistor as the second resistor in which current of double flows, when compared with the case using two resistors in which the second and fourth distributed currents are allowed to flow individually. This means that the necessary resistance can be effortlessly reduced to one fourth.
According to the present invention, the resistance necessary for a BGR type reference current circuit and the like can be reduced, thereby reducing the area of a semiconductor integrated circuit.
Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In each of the following embodiments, the same reference numerals are assigned to elements having the same functions as in the other embodiments for omitting the description thereof.
A reference current circuit of Embodiment 1 includes, as shown in
The gate electrodes of the pMOS transistors MP1, MP2 and the drain electrode of the pMOS transistor MP1 are connected to one another, and the potential of the electrodes is output as constant current bias. Further, they are formed to have a current ratio (a transistor size ratio) of 1:m, where m is a real number or an integer equal to or larger than 1, for example. A more specific approach is that the gate lengths thereof are set equal to each other while the gate width of one of them is set m times that of the other, for example. This achieves control of one of output currents to be output at the current ratio of 1:m (Iout:m·Iout) according to the other output current Iout when both the pMOS transistors MP1, MP2 operate in the saturation region, for example.
Furthermore, the nMOS transistors MN1, MN3 and the nMOS transistors MN2, MN4 are so formed respectively to have a transistor size ratio of 1:m. The gate electrodes of the nMOS transistors MN1 to MN4 are connected to one another and are connected to the drain electrode of the nMOS transistor MN3. There is no specific limitation on the ratio between the currents flowing in or the transistor size of the nMOS transistors MN1, MN3 and the currents flowing in or the transistor size of the nMOS transistors MN2, MN4. In other words, the ratio of the currents flowing in the nMOS transistors MN1, MN3, MN2, and MN4 may be 1:m:α:mα where α is an arbitrary constant, for example.
The two diodes (D1, D2) is so set that the ratio of the reverse saturation currents (diode size ratio) becomes n: 1, where n is a real number or an integer larger than 1, for example. The resistances of the resistors R1, R2 are set so as to suppress or reduce variation of the current Iout caused due to the temperature variation, which will be described later.
In the thus configured reference current circuit, the ratio of the currents output from the current mirror circuit 10, that is, the ratio between the sum of the currents flowing in the nMOS transistors MN1, MN2 and the sum of the currents flowing in the nMOS transistors MN3, MN4 is 1:m (Iout:m·Iout), as above.
In addition, the nMOS transistors MN2, MN4 have a transistor size ratio of 1:m and have gate potentials equal to each other and source potentials equal to each other, which means that the ratio of the currents flowing in the nMOS transistors MN2, MN4 is 1:m (I2:m·I2).
Accordingly, each current Iout and m·Iout from the current mirror circuit 10 is distributed so that the ratio of the currents flowing in the nMOS transistors MN1, MN3 (and the diodes D1, D2) becomes 1:m (I1:m·I1). In this case, also, the transistor size ratio of the nMOS transistors MN1, MN3 is 1:m and the gate potentials thereof are equal to each other, which means that the source potentials thereof are equal to each other, Vd.
On the other hand, the relationship between the current I1 flowing in the diode D1 and the anode potential V1 is expressed by Expression 1 indicated in combination in
The source potentials of the nMOS transistors MN1, MN3 are each Vd, and therefore, the voltage across the resistor R1 is equal to the voltage difference between the voltages across the diodes D1, D2. Further, the current I1 flowing in the resistor R1 is equal to a value obtained by dividing this voltage difference by the resistance of the resistor R1 (Expression 3 and Expression 4).
If the mutual conductance of the nMOS transistors MN2, MN4 is sufficiently large, the source potentials thereof can be deemed to Vd, the same as those of the nMOS transistors MN1, MN3, and accordingly, the current (sum of I2 and m·I2) flowing in the resistor R2 is Vd/R2. This leads to Expression 5 for the drain current I2 of the nMOS transistor MN2, and therefore, the current Iout flowing in the pMOS transistor MP1 can be obtained by addition of Expressions 4 and 5 (Expression 6). With no VDD depending term in Expression 6, an ideal element causes no variation of the current Iout caused due to the power supply voltage variation.
The temperature coefficient of the current Iout with respect to the absolute temperature can be obtained by partially differentiating Expression 6 by the absolute temperature T (Expression 7). In Expression 7, the first term is a positive value of a coefficient with respect to the absolute temperature T while the gradient (∂Vd/∂T) of Vd with respect to the absolute temperature in the second term is a negative value. Accordingly, when the values of R1, R2, m, and n are set so that the value of Expression 7 as the temperature coefficient of the current Iout becomes zero, the current Iout and the like become independent from temperature.
Specifically, given that the value A is defined by Expression 8, the resistor R2 is set by Expression 9 with respect to the resistor R1.
The total resistance Rall necessary for forming the above reference current circuit is the sum of R1 and R2, namely, is expressed by Expression 10 in
Accordingly, R2≈3.5·R1 (Expression 12) and the total resistance Rall≈4.5·R1 (Expression 13) are lead, and accordingly, the total resistance can be suppressed to approximately 4.5 time the resistance of R1 as a reference resistance.
In other words, when the resistance of the resistor R1 is set equal to that of the circuit shown in
The resistors dominantly occupies the circuit area specifically in a BGR type reference current circuit consuming extremely small current and the like, and therefore, the area of the semiconductor integrated circuit can be reduced effortlessly and remarkably.
Although the above example refers to the case where the nMOS transistors MN1 to MN4 and the pMOS transistors MP1, MP2 are provided, the present invention is not limited thereto and may use a current mirror circuit configuration in a cascade structure for suppressing current variation caused due to drain voltage variation small, for example.
Referring to the connection order of the resistor R1 and the diode D1, the diode D1 is usually provided on the VSS side in general in view of the semiconductor process technology. Even when the order is reversed, however, the same effects can be obtained, namely, the temperature dependency can be suppressed or reduced with a small total resistance Rall.
Further, the temperature dependency may not necessarily be suppressed completely, and constants and the like may be set so as to obtain characteristics according to the accuracy and the specification required in a device.
The values of m and n are usually set to integers but is not limited thereto. The values may be set to real numbers other than integers.
The constant current bias outputting method is not limited to the aforementioned one as far as the currents in proportion to I1+I2 are current-mirrored.
A reference current circuit of Embodiment 2 includes, as shown in
The current mirror circuit 11 is different from that in Embodiment 1 in that the gate electrodes of the pMOS transistors MP1, MP2 connected to each other are connected to the output terminal of the operation amplifier 20 rather than the drain electrode of the pMOS transistor MP1.
The operation amplifier 20 keeps the current ratio of the output currents of the pMOS transistors MP1, MP2 at 1:m by feedback-controlling the drain potentials thereof to be equal to each other.
The thus configured reference current circuit has the same mechanism as in Embodiment 1 that the temperature dependency of the current Iout and the like are suppressed or reduced by controlling the currents Iout, m·Iout output from the current mirror circuit 11, the currents I1, I2, m·I1, and m·I2 of the respective parts, the potential Vd, and the like and the same mechanism as in Embodiment 1 that the total resistance Rall can be reduced effortlessly by allowing the currents flowing in the nMOS transistors MN2, MN4 to be added and flow into the resistor R2.
Further, when the operation amplifier 20 controls the pMOS transistors MP1, MP2 to have drain potentials equal to each other, the current can be distributed at a current ratio of 1:m when MP1 and MP2 operate even in the linear region as well as in the saturation region. In consequence, operation with further lower power supply voltage VDD can be achieved effortlessly when compared with Embodiment 1 (
A reference current circuit of Embodiment 3 has, as that shown in
The gate electrode of the nMOS transistor MN1 is connected to the drain electrode of itself rather than the gate electrode of the nMOS transistor MN3 to prevent the drain potential of the pMOS transistor MP1 from being unstable.
The thus configured reference current circuit is different from those of Embodiments 1 and 2 in that the operation amplifier 20 performs feedback control for equalizing the potentials Vd of the source electrodes of the nMOS transistors MN1, MN3 to each other. The resultant mechanism that the temperature dependency of the current Iout and the like are suppressed or reduced and the like are the same as those in Embodiments 1 and 2. The control by the operation amplifier 20 on the gate potential of the pMOS transistors MP1, MP2 effortlessly allows the reference current circuit to operate with further lower power supply voltage VDD when compared with that in Embodiment 1 (
The gate electrode of the nMOS transistor MN2 may be connected to the gate electrode of MN1 rather than the gate electrodes of the nMOS transistors MN3, MN4 in common.
In order to equalize the difference between voltages across the diodes D1, D2 to the voltage across the resistor R1, the difference in the gate potentials of the nMOS transistors MN1, MN3 and the difference in the source potentials thereof may be set equal to each other rather than setting of the respective potentials and the respective source potentials thereof equal to each other. Specifically, as shown in
In this case, the currents flowing in the nMOS transistors MN1, MN3 become equal to each other, I1, as in Embodiment 1. While on the other hand, the gate potential of the nMOS transistor MN1 is I1 R1 lower than the gate potential of the nMOS transistor MN3. When the source potential of the nMOS transistor MN3 is Vd, the source potential of the nMOS transistor MN1 is Vd−I1·R1 and the voltage difference between voltages across of the diodes D1, D2 is equal to the voltage across the resistor R1.
In consequence, when both the nMOS transistors MN1, MN3 operate in the saturation region and the body effect of the nMOS transistors MN1, MN3 is small enough to be ignored, the expressions for obtaining the current Iout and the total resistance Rall are the same as those indicated in
The gate electrodes of the nMOS transistors MN2, MN4 may be connected to the gate electrode of the nMOS transistor MN1, as shown
The configuration in
In addition, the above example refers to the case where m=1 (the current ratio is 1:1), but m may be larger than 1 (m>1). In this case, the resistance on the drain side of MN3 can be reduced to 1/m.
In each of the above embodiments, the gate potential of the pMOS transistors MP1, MP2 composing the current mirror circuit 10, 11 are output as constant current bias, but the present invention is not limited thereto as far as a current in proportion to the current Iout output from the current mirror circuit 10, 11 can be obtained. For example, as shown in
In the example shown in
The configuration provided with the above pMOS transistor MP3 is applicable not only to the configuration as in
In the above description, the current mirror circuit 10, 11 is connected to the high-potential power supply (VDD). In reverse, each polarity of the nMOS transistors and the pMOS transistors and each polarity of the anodes and the cathodes of the diodes D1, D2 may be reversed with the high-potential power supply (VDD) replaced by the low-potential power supply (VSS), as shown in
A reference voltage circuit utilizing constant current bias may be configured by using any of various BGR type reference current circuits causing less (or no) influence of the temperature variation and the power supply voltage variation as above. For example, as shown in
In other words, when a resistor having sufficiently small temperature variation relative to the necessary accuracy is used, a small reference voltage Vout having less influence of the temperature variation and the power supply voltage variation can be generated effortlessly. The configuration for obtaining this reference voltage is applicable to any of the configurations shown in
A BGR circuit applied to any of the above reference current circuits and the above reference voltage circuits involves a bi-stable problem intrinsically and necessitates, therefore, a startup circuits in general. The bi-stable problem is a problem that an abnormal stable operation state is present besides a normal stable operation state and the circuit once falling in the abnormal stable state cannot be automatically reset to the normal stable state.
Specifically, in the reference current circuit (
To tackle this problem, a startup circuit is used in general for avoiding the abnormal stable state at operation start of a BGR circuit. Specifically, the abnormal stable state is transferred to the normal stable state by lowering the gate potential of the pMOS transistor(s) MP1 (and MP2) or raising the gate potential(s) of the nMOS transistor(s) MN3 (and MN1) by applying a predetermined initializing pulse.
The above startup circuit, however, necessitates a special initialization control signal indicating operation start of the BGR circuit, and therefore, the BGR circuit is liable to have less flexibility. Further, in the case where some trigger once causes the abnormal stable state in the normal stable operation state, the above startup circuit cannot reset the circuit to the normal stable operation state gain.
A startup circuit will be described below which does not necessitate the above special initialization control signal and the like and can reset a circuit to the stable state at any time.
A BGR circuit 30 for the startup circuit 60 includes a state potential output node and a startup control node as follows. Namely, the state potential output node outputs an abnormal state potential Vout0 (for example, Vout0≈VSS) in the abnormal stable state while outputting a normal state potential Vout1 (for example, Vout1 is a voltage higher than the threshold voltage of the inverter circuit INV1) in the normal stable state. On the other hand, the startup control node is capable of resetting the BGR circuit 30 to the normal stable state by being connected to the power supply VSS or the like for allowing a startup current to flow.
The startup circuit 60 includes a control circuit 40 and an inverter power supply circuit 50.
The control circuit 40 includes an inverter circuit INV1 composed of a pMOS transistor and an nMOS transistor (both not shown) and an nMOS transistor MN5 ON/OFF switched according to the output of the inverter circuit INV1. The control circuit 40 connects the startup control node to the power supply VSS when the abnormal state potential Vout0 is output from the BGR circuit 30.
The inverter power supply circuit 50 outputs an inverter power supply voltage IVDD for the inverter circuit INV1. The inverter power supply voltage IVDD is set equal to or larger than the threshold voltage Vtn of the nMOS transistor MN5 (IVDD≧Vtn) and equal to or lower than the voltage obtained by adding the threshold voltage Vtp of the pMOS transistor (not shown) in the inverter INV1 to the normal state potential Vout1 output from the BGR circuit 30 (IVDD≦Vout1+Vtp). The inverter power supply voltage IVDD is preferably set within the range of Vout1±0.2 V, more preferably, set nearly equal to Vout1 (IVDD≈Vout1).
With the above arrangement, the abnormal state potential Vout0 (Vout0≈Vss) is output when the BGR circuit 30 falls in the abnormal state, so that the pMOS transistor and the nMOS transistor in the inverter circuit INV1 are turned ON and OFF, respectively, to allow the inverter circuit INV1 to output the supplied inverter power supply voltage IVDD directly. The inverter power supply voltage IVDD is set equal to or larger than Vtn (IVDD≧Vtn) as described above, for example, and accordingly, the nMOS transistor MN5 is turned ON. This allows the startup current to flow from the startup control node to the power supply VSS to reset the BGR circuit 30 to the normal stable state.
In contrast, in the normal state, the BGR circuit 30 outputs the normal state potential Vout1 (a voltage higher than the threshold voltage of the inverter circuit INV1), so that the output potential of the inverter circuit INV1 is VSS to turn the nMOS transistor MN5 OFF. Therefore, the startup circuit 60 does not affect the normal operation of the BGR circuit 30. When the inverter power supply voltage IVDD is set as above, namely, IVDD≦Vout1+Vtp (or IVDD≈Vout1 or so), for example, the pMOS transistor in the inverter circuit INV1 is turned OFF, which definitely prevents through current of the inverter circuit INV1, thereby inviting no increase in current consumption.
A BGR circuit to which the above startup circuit is applicable may be the reference current circuit described in Embodiment 1 (
In the above case, the inverter power supply circuit 50 may divide the voltage between the power supplies VDD and VSS to a resistor R5 and a diode D3, for example, to output the inverter power supply voltage IVDD. The power supply voltage IVDD is a forward voltage of the diode D3 and the normal state potential Vout1 is nearly equal to the forward voltage Vd of the diode D2. This means that the power supply voltage IVDD is set nearly equal to Vd (IVDD≈Vd), as described above. Further, since the threshold voltage of the nMOS transistor MN5 is generally lower than the forward voltage of a diode, the condition that IVDD≧Vtn is satisfied.
In consequence, the abnormal state potential Vout0 (Vout0≈VSS) is output from the state potential output node in the abnormal stable state to turn the nMOS transistor MN5 ON, as described above, thereby allowing the startup current to flow. This causes the gate potential of the pMOS transistors MP1, MP2 to be VSS to allow the current to flow through MP2. When the gate potential of the nMOS transistor MN3 increases according thereto to cause the current to flow through the resistor R2, the potential of Vout increases to reset the BGR circuit 31 to the normal stable state.
While in the normal stable state, the normal state potential Vout1 (Vout1≈Vd) is output to turn the nMOS transistor MN5 OFF, as described above, causing no influence on the BGR circuit 31, as described above. In addition, the through current does not flow through the inverter circuit INV1.
The above startup circuit is applicable also to the reference voltage circuit described in Embodiment 7 (
In addition, the above startup circuit is applicable to the reference current circuit (or the reference voltage circuit) shown in
As described above, the startup circuit shown in
In the above example, the inverter circuit INV1 has a one-stage configuration composed of the pMOS transistor and the nMOS transistor, but the present invention is not limited thereto. Any inverter circuit having an odd-numbered stage configuration of three-stage or five-stage configuration can be used as far as the logical conformity is met.
In the above example, the nMOS transistor MN5 is turned ON to allow the gate electrode of the pMOS transistor MP2 and the like in
The elements described in the above embodiments including the modified examples may be combined variously within a theoretically capable range.
The reference current circuits, the reference voltage circuits, and the startup circuits in accordance with the present embodiment exhibit the effects of reducing the necessary resistance to reduce the area of a semiconductor integrated circuit effortlessly, and therefore, are useful as reference current circuits, reference voltage circuits, startup circuits, and the like using a bandgap reference circuit.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-247818 | Sep 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/067212 | 9/4/2007 | WO | 00 | 5/12/2008 |
