# Patent

## Title
Power-on detecting circuit

## Abstract

A circuit for detecting a power-on voltage of power supply encompasses a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply, and a detecting circuit connected between the first power supply and the second power supply. The voltage divider includes a series circuit encompassing a diode, a first dividing resistor connected to the diode and a second dividing resistor connected between the first dividing resistor and the second power supply. The detecting circuit includes a pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor, a source resistor connected between the first power supply and the source electrode of the pMOS transistor and a drain resistor connected to the drain electrode of the pMOS transistor and the second power supply.

## Background
CROSS REFERENCE TO RELATED APPLICATION AND INCORPORATION BY REFERENCE
This application is based upon and claims the benefit of priority from prior Japanese Patent Application P2005-334370 filed on Nov. 18, 2005; the entire contents of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power-on detection circuit for detecting a state that a supplied voltage has reached to a constant potential, in a period when a power supply is initiated.
2. Description of the Related Art
A circuit for detecting a power-on voltage of power supply (hereinafter called “the power-on detection circuit”) has been widely used in a semiconductor device including a memory, for example, as described in Japanese Laid Open Patent Application (JP-P 2001-127609A).
A conventional power-on detection circuit, which corresponds to a circuit portion surrounded with a box of dashed line in FIG. 1, is known. That is, in the circuit configuration where a series circuit consisting of a first dividing resistor R1 and a second dividing resistor R2 is inserted between a first power supply Vdd and a second power supply (ground wiring) Vss of, the second power supply provides a lower potential Vss than the potential Vdd of the first power supply, by feeding a current so as to flow through the first dividing resistor R1 and the second dividing resistor R2, a certain desirable potential is obtained from a node between the first dividing resistor R1 and the second dividing resistor R2, by voltage division across the first dividing resistor R1 and the second dividing resistor R2. In order to attain a smaller current which can satisfy a circuit requirement, the resistance values of the first dividing resistor R1 and the second dividing resistor R2 inevitably have the large values determined from the circuit requirement for a semiconductor chip on which a plurality of semiconductor elements are integrated.
Moreover, as shown in the circuit portion surrounded with the box of dashed line of FIG. 1, the node between the first dividing resistor R1 and the second dividing resistor R2 is connected to a gate electrode of a pMOS transistor whose source electrode is connected to the first power supply Vdd. On the other hand, a drain resistor R4 is inserted between a drain electrode of the pMOS transistor and the second power supply (ground wiring) Vss. The potential determined by the voltage division across the first dividing resistor R1 and the second dividing resistor R2 is applied to the gate electrode of the pMOS transistor. Thus, as the potential Vdd of the first power supply increases, a potential difference across the source electrode and the gate electrode becomes larger. Then, at a certain potential Vdd, the pMOS transistor is turned on, thereby detecting the power-on potential.
However, the threshold voltage of the pMOS transistor has temperature dependence. Also, even under a constant temperature, the threshold voltages vary in different semiconductor chips and for different wafers. Thus, there was a problem that, when the power-on was detected, the power-on potential of the first power supply Vdd scatters with semiconductor chips and wafers. Moreover, there was a problem that, when a ratio of a resistance value between the first dividing resistor R1 and the second dividing resistor R2 was, for example, 1:2, the variation in the threshold voltage of the pMOS transistor was amplified to three times, which affects the power-on potential.
SUMMARY OF THE INVENTION
An aspect of the present invention inheres in a circuit for detecting a power-on voltage of power supply, encompassing (a) a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply, and (b) a detecting circuit connected between the first power supply and the second power supply. The voltage divider includes a series circuit encompassing a diode, a first dividing resistor connected to the diode and a second dividing resistor connected between the first dividing resistor and the second power supply. The detecting circuit includes a pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor, a source resistor connected between the first power supply and the source electrode of the pMOS transistor and a drain resistor connected to the drain electrode of the pMOS transistor and the second power supply.
Another aspect of the present invention inheres in a circuit for detecting a power-on voltage of power supply, encompassing (a) a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply; and (b) a detecting circuit connected between the first power supply and the second power supply. The voltage divider includes a series circuit encompassing a diode-connected first pMOS transistor, a first dividing resistor connected to the first pMOS transistor and a second dividing resistor connected between the first dividing resistor and the second power supply. The detecting circuit includes a second pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor, and a drain resistor connected to the drain electrode of the second pMOS transistor and the second power supply.
A still another aspect of the present invention inheres in a circuit for detecting a power-on voltage of power supply, encompassing (a) a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply; and (b) a detecting circuit connected between the first power supply and the second power supply. The voltage divider includes a series circuit encompassing a diode-connected first divider pMOS transistor, a diode-connected second divider pMOS transistor connected to the first divider pMOS transistor, a first dividing resistor connected to the second divider pMOS transistor and a second dividing resistor connected between the first dividing resistor and the second power supply. The detecting circuit includes a detecting pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor, a source resistor connected between the first power supply and the source electrode of the detecting pMOS transistor, and a drain resistor connected to the drain electrode of the detecting pMOS transistor and the second power supply.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a power-on detection circuit according to a first embodiment of the present invention;
FIG. 2 is a graph showing a temperature dependence of a current voltage characteristic of a diode used in the power-on detection circuit of the first embodiment of the present invention, whereas the ordinate and the abscissa are shown on linear scales;
FIG. 3 is a graph showing a temperature dependence of current voltage characteristics and threshold voltages of transistors used in the power-on detection circuits according to the first embodiment of the present invention, whereas the ordinate and the abscissa are shown on linear scales;
FIG. 4 is a circuit diagram of a power-on detection circuit according to a second embodiment of the present invention;
FIG. 5 is a circuit diagram of a power-on detection circuit according to a third embodiment of the present invention;
FIG. 6 is a circuit diagram of a power-on detection circuit according to a fourth embodiment of the present invention;
FIG. 7 is a graph showing a temperature dependence of gate electrode-voltage vs. drain-current characteristics of the detecting pMOS transistor Q1 and the divider pMOS transistor Q2 shown in FIG. 6, with gate width W and gate length L, the ordinate shows on a logarithmic scale the drain current ID1 per unit W/L flowing through the detecting pMOS transistor Q1 and the drain currents ID2 per unit W/L flowing through the divider pMOS transistor Q2, the abscissa shows on a linear scale the gate-source voltage VGS1 of the detecting pMOS transistor Q1 and the gate-source voltage VGS2 of the divider pMOS transistor Q2;
FIG. 8 is a graph showing a temperature dependence of gate electrode-source voltage vs. source-drain current characteristics of pMOS transistors Q1 and Q2, the ordinate shows on a logarithmic scale the drain currents ID1 and ID2, the abscissa shows on a linear scale the gate-source voltages VGS1 and VGS2;
FIG. 9 is a circuit diagram of a power-on detection circuit according to a fifth embodiment of the present invention;
FIG. 10 is a circuit diagram of a power-on detection circuit according to a sixth embodiment of the present invention; and
FIG. 11 is a circuit diagram of a power-on detection circuit according to a seventh embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified. In the following descriptions, numerous specific details are set fourth such as specific signal values, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details.
First Embodiment
As shown in FIG. 1, a power-on detection circuit according to the first embodiment of the present invention includes a voltage divider connected between a first power supply Vdd and a second power supply Vss, the potential Vss of the second power supply is lower than the potential Vdd of the first power supply. The voltage divider is implemented by a diode D1, a first dividing resistor R1 and a second dividing resistor R2 connected in series.
The power-on detection circuit further includes a detecting circuit between the first power supply Vdd and the second power supply Vss. The detecting circuit is implemented by a series connection of a pMOS transistor Q1 whose gate electrode is connected to a connection node between the first dividing resistor R1 and the second dividing resistor R2; a source resistor R3 connected between the first power supply Vdd and the source electrode of the pMOS transistor Q1; and a drain resistor R4 connected to the drain electrode of the pMOS transistor Q1 and the second power supply Vss. An inverter INV is connected to a connection node between the drain electrode of the pMOS transistor Q1 implementing the detecting circuit and the drain resistor R4. Then, a potential of the connection node between the drain electrode of the pMOS transistor Q1 and the drain resistor R4 is transferred through the inverter INV.
The circuit portion surrounded with a box of dashed line in FIG. 1 corresponds to the conventional circuit explained in the beginning of the present Specification. As compared with the conventional circuit, according to the power-on detection circuit of the first embodiment, the diode D1 is further inserted between the first power supply Vdd and the series circuit of the first dividing resistor R1 and the second dividing resistor R2. That is, an anode side (p-side) of the diode D1 is connected to the first power supply Vdd and the series circuit of the first dividing resistor R1 and the second dividing resistor R2 is inserted between a cathode side (n-side) of the diode D1 and the second power supply Vss.
An on-state voltage that is a potential difference established across the anode (p-side) and the cathode (n-side) when the diode D1 is in a conductive (on) state is referred to as Vdio. As shown in FIG. 2, the on-state voltage Vdio is known to have a temperature dependence dVdio/dT of about −2 mV/° C. On the other hand, a threshold voltage Vth of the pMOS transistor Q1 has a temperature dependence dVth/dT, which is usually smaller in absolute value than the temperature dependence dVdio/dT of the diode D1, as shown in FIG. 3. In FIGS. 2 and 3, both the ordinate and the abscissa are shown on linear scales.
Thus, as shown in FIG. 1, in the circuit configuration that the diode D1, the first dividing resistor R1 and the second dividing resistor R2 are connected in series, with resistance values of the first dividing resistor R1 and the second dividing resistor R2 determined by Eq. (1), a potential VG1 determined by the voltage division across the first dividing resistor R1 and the second dividing resistor R2 can have a temperature dependence equal to the temperature dependence of the threshold voltage Vth of the pMOS transistor Q1:

R2/(R1+R2)=(dVth/dT×R4/(R3+R4))/(−2×10−3(V/° C.)).  (1)

In the first embodiment, an attention should be paid to the necessity that a source resistor R3 is connected to the source electrode side of the pMOS transistor Q1 so that a source potential Vs1 of the pMOS transistor Q1 is decreased from the potential Vdd of the first power supply. This is, because the on-state voltage Vdio of the diode D1 is typically considered to be higher than the absolute value of the threshold voltage Vth of the pMOS transistor Q1. A gate potential VG1 of the pMOS transistor Q1 is calculated from Eq. (2):

VG1=(Vdd−Vdio)×R2/(R1+R2).  (2)

Also, the source potential Vs1 of the pMOS transistor Q1 is determined by Eq. (3):

Vs1−VG1=Vth  (3)

From the above explanation, the resistance values of the first dividing resistor R1, the second dividing resistor R2, the source resistor R3 and the drain resistor R4 shall be set so as to satisfy Eqs. (1), (2) and (3) simultaneously. If these resistance values cannot be managed precisely to be coincident with the resistance values determined by Eqs. (1), (2) and (3) simultaneously, it is preferable that the resistance values of the first dividing resistor R1, the second dividing resistor R2, the source resistor R3 and the drain resistor R4 are set so as to approximately satisfy Eq. (1) within the range of the requirement, after they has been set to satisfy Eqs. (2) and (3).
According to the power-on detection circuit of the first embodiment, even if the temperature is varied, the power-on potential can be kept so as not to vary. According to the power-on detection circuit of the first embodiment, the power-on potential can be detected at a constant potential, independently of the variation of temperature.
Second Embodiment
FIG. 4 shows a power-on detection circuit according to a second embodiment of the present invention. The power-on detection circuit according to the second embodiment corresponds to a modification of the power-on detection circuit explained in the first embodiment. The difference from the power-on detection circuit explained in the first embodiment lies in the circuit configuration that a divider series resistor R5 is further connected between the first power supply Vdd and the diode D1, and a second source resistor R6 is further connected between the first power supply Vdd and the source resistor (first source resistor) R3.
According to the power-on detection circuit of the second embodiment, even for the case that the power-on potential is considered to be excessively low for the circuit configuration shown in the first embodiment, the desirable power-on supply voltage can be obtained, because an increase of the potential is achieved by the voltage drop across the divider series resistor R5 and the second source resistor R6.
According to the power-on detection circuit of the second embodiment, similarly to the power-on detection circuit according to the first embodiment, the power-on potential can be detected at a constant potential, independently of the variation of temperature.
Third Embodiment
FIG. 5 shows a power-on detection circuit according to a third embodiment of the present invention. The power-on detection circuit according to the third embodiment corresponds to a modification of the power-on detection circuit explained in the second embodiment. The difference from the power-on detection circuit explained in the second embodiment lies in a circuit configuration that the inserted position of the divider series resistor R5 is different.
According to the power-on detection circuit of the third embodiment, similarly to the second embodiment, even in the case that the power-on potential is estimated to be excessively low, the desirable power-on supply voltage can be obtained, because an increase of the potential is achieved by the voltage drop across the divider series resistor R5 and the second source resistor R6.
When the resistance values of the first dividing resistor R1, the second dividing resistor R2 and the divider series resistor R5 are managed to satisfy Eq. (4), the potential VG1 determined by the voltage division across the first dividing resistor R1, the second dividing resistor R2 and the divider series resistor R5 can have the temperature dependence equal to the threshold voltage Vth of the pMOS transistor Q1.

R2/(R1+R2+R5)=(dVth/dT×R4/(R3+R6+R4))/(−2×10−3(V/° C.))  (4)

According to the power-on detection circuit of the third embodiment, the source resistor (first source resistor) R3 and the second source resistor R6 are inserted into the source electrode side of the pMOS transistor Q1, and the source potential Vs1 of the pMOS transistor Q1 is decreased from the potential Vdd of the first power supply. The gate potential VG1 of the pMOS transistor Q1 is calculated from Eq. (5).

VG1=(Vdd−Vdio)×R2/(R1+R2+R5)  (5)

Also, the source potential Vs1 of the pMOS transistor Q1 is represented by Eq. (6).

Vs1−VG1=Vth  (6)

From the above explanation, the resistance values of the first dividing resistor R1, the second dividing resistor R2, the source resistor R3, the drain resistor R4 and the divider series resistor R5 shall be set so as to satisfy Eqs. (4), (5) and (6) simultaneously. If these the resistance values cannot be managed to precisely coincident with the resistance values determined by Eqs. (4), (5) and (6) simultaneously, it is preferable that the resistance values of the first dividing resistor R1, the second dividing resistor R2, the source resistor R3, the drain resistor R4 and the divider series resistor R5 are set so as to approximately satisfy Eq. (4) within the range of the requirement, after they have made to satisfy Eqs. (5) and (6).
According to the power-on detection circuit of the third embodiment, even if the temperature is varied, the power-on potential can be kept so as not to vary, and the desirable power-on supply voltage can be obtained because an increase of the potential is achieved by the voltage drop across the divider series resistor R5 and the second source resistor R6. According to the power-on detection circuit of the third embodiment, the power-on potential can be detected at a constant potential, independently of the variation of temperature.
Fourth Embodiment
FIG. 6 shows a power-on detection circuit according to a fourth embodiment of the present invention. The power-on detection circuit according to the fourth embodiment is another modification of the power-on detection circuit explained in the first embodiment. In the power-on detection circuit of the first embodiment, the circuit configuration has been explained for detecting a constant power-on potential independently of the variation of temperature even if the threshold voltage Vth of the pMOS transistor Q1 has the temperature dependence, by establishing the voltage divider with the diode D1 inserted between the first power supply Vdd and the series circuit, which is implemented by the first dividing resistor R1 and the second dividing resistor R2. However, if the threshold voltage Vth itself of the pMOS transistor Q1 has the variation, the variation will cause a problem that the shift of the threshold voltage Vth affects the value of the power-on potential. A subject of the power-on detection circuit according to the fourth embodiment is to relax such a problem.
The power-on detection circuit according to the fourth embodiment of the present invention differs from the power-on detection circuit already explained in the first embodiment in that the voltage divider encompasses a series circuit connected between the first power supply Vdd and the second power supply Vss, the potential Vss of the second power supply is lower than the potential Vdd of the first power supply, the series circuit is implemented by a diode-connected “divider pMOS transistor (first pMOS transistor)” Q2, a first dividing resistor R1 and a second dividing resistor R2.
Also, the circuit configuration of the detecting circuit is similar to the power-on detection circuit according to the first embodiment, until the structure that this has a second pMOS transistor (hereinafter may be called “the detecting pMOS transistor”) Q1 in which the potential VG1 of the connection node between the first dividing resistor R1 and the second dividing resistor R2 is applied to a gate electrode, and the drain electrode of the detecting pMOS transistor Q1 is connected through the drain resistor R4 to the second power supply Vss. However, the topology that the source electrode of the detecting pMOS transistor Q1 is directly connected to the first power supply Vdd differs from the power-on detection circuit already explained in the first embodiment (in the power-on detection circuit of the first embodiment shown in FIG. 1, the source resistor R3 was connected between the first power supply Vdd and the source electrode of the pMOS transistor Q1).
The circuit configuration that the inverter INV is connected to the connection node between the drain electrode of the pMOS transistor Q1 implementing the detecting circuit and the drain resistor R4, and that the potential of the connection node between the drain electrode of the pMOS transistor Q1 and the drain resistor R4 is transferred through the inverter INV is similar to the power-on detection circuit already explained in the first embodiment.
With gate width W and gate length L, FIG. 7 indicates the drain current ID1 per unit W/L flowing through the detecting pMOS transistor Q1 and the drain currents ID2 per unit W/L flowing through the divider pMOS transistor Q2 as the ordinate on a logarithmic scale, and the gate-source voltage VGS1 of the detecting pMOS transistor Q1 and the gate-source voltage VGS2 of the divider pMOS transistor Q2 as the abscissa on a linear scale. For example, the drain currents ID2 per unit W/L may set to be smaller than the drain currents ID1 per unit W/L, such that the gate width W of the divider pMOS transistor Q2 shall be set to be 100 times the gate width W of the detecting pMOS transistor Q1, while the flowing drain currents ID1, ID2 are kept to be the substantially same values. If so, a temperature dependence dVGS2/dT of a gate-source voltage VGS2 of the divider pMOS transistor Q2 can be made larger than a temperature dependence dVGS1/dT of a gate-source voltage VGS1 of the detecting pMOS transistor Q1.
FIG. 8 indicates the drain current ID1 and ID2 as the ordinate on a logarithmic scale, and the gate-source voltage VGS1 and VGS2 as the abscissa on a linear scale. As shown in FIG. 8, in the case that the ratio W/L of the divider pMOS transistor Q2 is larger than the W/L of the detecting pMOS transistor Q1, even if the flowing drain currents ID1, ID2 have the same values, the dVGS2/dT can be made larger than the dVGS1/dT. That is, the drain currents ID1, ID2 per unit W/L shall be set such that the value of the divider pMOS transistor Q2 is smaller than the value of the detecting pMOS transistor Q1.
In the circuit configuration shown in FIG. 6, as can be understood from FIG. 7, there is a relationship of Eq. (7) between the gate-source voltage VGS2 of the divider pMOS transistor Q2 and the gate-source voltage VGS1 of the detecting pMOS transistor Q1.

Vdd−{Vdd−|VGS2|}×R2/(R1+R2)=|VGS1|  (7)

Also, when the temperature dependence of the gate-source voltage VGS2 of the divider pMOS transistor Q2 is represented by dVGS2/dT and the temperature dependence of the gate-source voltage VGS1 of the detecting pMOS transistor Q1 is represented by dVGS1/dT, there is a relationship of Eq. (8) as can be understood from FIG. 8.

|dVGS2/dT|>|dVGS1/dT|  (8)

Thus, with Vs1=Vdd in Eq (6), the potential Vdd of the first power supply in a period of power-on stage and the resistance values of the first dividing resistor R1 and the second dividing resistor R2 are determined so as to satisfy Eq (9).

|dVGS2/dT|×R2/(R1+R2)=|dVGS1/dT|  (9)

Also, the absolute values of the first dividing resistor R1 and the second dividing resistor R2 are determined by the requirement for the current ID2 flowing through the series connected resistors, which implement the voltage divider.
Also, different from the first to third embodiments, instead of the diode D1, the diode-connected divider pMOS transistor Q2, having a gate length L equal to the gate length L of the detecting pMOS transistor Q1, is replaced in the voltage divider. Thus, if the detecting pMOS transistor Q1 and the divider pMOS transistor Q2 are integrated on the same semiconductor chip, the threshold voltage variation of the divider pMOS transistor Q2 can be assumed to vary similarly with the threshold voltage variation of the detecting pMOS transistor Q1.
That is, according to the power-on detection circuit of the fourth embodiment, since the gate lengths L of the divider pMOS transistor Q2 and the detecting pMOS transistor Q1 are set to be equal, both of the threshold voltages Vth are considered to scatter with a value ΔVth, which is equal to each other. In short, since the threshold voltage Vth of the divider pMOS transistor Q2 scatters with ΔVth, the gate-source voltage VGS1 of the detecting pMOS transistor Q1 scatters with ΔVGS1 shown in Eq. (10).

ΔVGS1=ΔVth×R2/(R1+R2)  (10)

That is, even if the threshold voltage Vth of the detecting pMOS transistor Q1 scatters with ΔVth, the amount of the variation of gate-source voltage VGS1 of the detecting pMOS transistor Q1 is reduced to a value corresponding to the value of ΔVGS1, which is prescribed in Eq. (10). Hence, although not perfect, it is possible to remove the temperature dependence and relax the influence caused by the threshold voltage variation.
According to the power-on detection circuit of the fourth embodiment, in addition to the effect of the variation in characteristics caused by temperature, the effect of the variation of threshold voltage itself of the detecting pMOS transistor Q1 can be minimized so as to relax the variation in the power-on potential.
Fifth Embodiment
FIG. 9 shows a power-on detection circuit according to a fifth embodiment of the present invention. In the fourth embodiment, the potential Vdd of the first power supply in a period of power-on stage and the resistance values of the first dividing resistor R1 and the second dividing resistor R2 are determined so as to satisfy Eqs. (6) and (9) with Vs1=Vdd. Thus, in a situation such that the potential Vdd of the first power supply in a period of power-on stage is controlled to have no temperature dependence, it was impossible to freely adjust the potential Vdd of the first power supply in a period of power-on stage.
The purpose of the power-on detection circuit according to the fifth embodiment is to freely set the potential Vdd of the first power supply in a period of power-on stage, while forcing the potential Vdd of the first power supply in a period of power-on stage to have no temperature dependence.
For this reason, the power-on detection circuit according to the fifth embodiment differs from the power-on detection circuit already explained in the fourth embodiment in that the divider series resistor R5 is further inserted between the first power supply Vdd and the divider pMOS transistor Q2, and the source resistor R3 is further inserted between the first power supply Vdd and the source electrode of the detecting pMOS transistor Q1.
Even in a case that the power-on potential is estimated to be excessively low for the power-on detection circuit according to the fourth embodiment, the desirable power-on supply voltage can be obtained in the fifth embodiment, because an increase of the potential is achieved by the voltage drop across the divider series resistor R5 and the second source resistor R6.
According to the power-on detection circuit of the fifth embodiment, in addition to the effect of the variation in characteristics caused by temperature, the effect of the variation of threshold voltage itself of the detecting pMOS transistor Q1 can be minimized so as to relax the variation in the power-on potential.
Sixth Embodiment
FIG. 10 shows a power-on detection circuit according to a sixth embodiment of the present invention. The power-on detection circuit according to the sixth embodiment corresponds to a modification of the power-on detection circuit explained in the fifth embodiment. The difference from the power-on detection circuit explained in the fifth embodiment lies in the location where the divider series resistor R5 is inserted in a circuit configuration.
According to the power-on detection circuit of the sixth embodiment, similarly to the fifth embodiment, even in the case that the power-on potential is estimated to be excessively low for the fourth embodiment, the desirable power-on supply voltage can be obtained, because an increase of the potential is achieved by the voltage drop across the divider series resistor R5 and the source resistor R3.
When the resistance values of the first dividing resistor R1, the second dividing resistor R2 and the divider series resistor R5 are managed to satisfy Eq. (11), the potential VG1 determined by the voltage division across the first dividing resistor R1, the second dividing resistor R2 and the divider series resistor R5 can have the temperature dependence equal to the temperature dependence dVth(Q1)/dT of the threshold voltage Vth(Q1) of the detecting pMOS transistor Q1.

R2/(R1+R2+R5)=(dVth(Q1)/dT×R4/(R6+R4))/(dVth(Q2)/dT)  (11)

Also, according to the power-on detection circuit of the sixth embodiment, it is necessary to insert the source resistor R3 into the source electrode side of the detecting pMOS transistor Q1 and decrease the source potential Vs1 of the detecting pMOS transistor Q1 from the first power supply Vdd. The gate potential VG1 of the detecting pMOS transistor Q1 is calculated from Eq. (12).

VG1=(Vdd−Vdio)×R2/(R1+R2+R5)  (12)

Also, the source potential Vs1 is represented by Eq. (13).

Vs1−VG1=Vth(Q1)  (13)

From the above explanation, the resistance values of the first dividing resistor R1, the second dividing resistor R2, the source resistor R3, the drain resistor R4 and the divider series resistor R5 shall be set so as to satisfy Eqs. (11), (12) and (13) simultaneously. If these resistance values cannot be managed to precisely coincident with the resistance values defined by Eqs. (11), (12) and (13) simultaneously, it is preferable that the resistance values of the first dividing resistor R1, the second dividing resistor R2, the source resistor R3, the drain resistor R4 and the divider series resistor R5 are set so as to approximately satisfy Eq. (11) within the range of the requirement, after they have made to satisfy Eqs. (12) and (13).
Thus, the resistance values of the drain resistor R4 and the source resistor R3 are determined such that the source potential Vs1 of the detecting pMOS transistor Q1 has the value represented by Eq. (13). According to the power-on detection circuit of the sixth embodiment, even if the temperature is varied, the power-on potential can be kept so as not to vary. Also, according to the power-on detection circuit of the sixth embodiment, in addition to the effect of the variation in characteristics caused by temperature, the effect of the variation of threshold voltage itself of the detecting pMOS transistor Q1 can be minimized so as to relax the variation in the power-on potential.
Seventh Embodiment
FIG. 11 shows a power-on detection circuit according to a seventh embodiment of the present invention. In the fourth embodiment, the gate width W of the divider pMOS transistor Q2 has been set to be 100 times the gate width W of the detecting pMOS transistor Q1, the flowing currents ID1, ID2 were set to the same values, so that the temperature dependence of the power-on potential can be minimized, and that the threshold voltage variation dependence ca be reduced. The power-on detection circuit according to the seventh embodiment corresponds to a modification of the power-on detection circuit explained in the fourth embodiment.
The power-on detection circuit according to the seventh embodiment is directed to a scheme for minimizing simultaneously both the temperature dependent variation of the power-on potential and the dependence on the variation of threshold voltages, with a circuit configuration including a detecting pMOS transistor Q1, a first divider pMOS transistor Q2 and a second divider pMOS transistor Q3, all of the pMOS transistors Q1, Q2 and Q3 having the same ratio (W/L), which is defined as the ratio of the gate width W against to the gate length L.
As shown in FIG. 11, the power-on detection circuit according to the seventh embodiment of the present invention includes a voltage divider connected between a first power supply Vdd and a second power supply Vss, which provide lower potential Vss than the potential Vdd of the first power supply. A first divider pMOS transistor Q2, a second divider pMOS transistor Q3, a first dividing resistor R1 and a second dividing resistor R2 are connected in series so as to implement the voltage divider. The first divider pMOS transistor Q2 and the second divider pMOS transistor Q3 are respectively diode-connected and are equal in the ratio W/L, or the ratio of the gate width W against to the gate length L. The first dividing resistor R1 and the second dividing resistor R2 are equal in resistance value.
On the other hand, the detecting circuit is implemented by a series connection of a detecting pMOS transistor Q1 whose gate electrode is connected to the connection node between the first dividing resistor R1 and the second dividing resistor R2; a source resistor R3 connected between the first power supply Vdd and a source electrode of the detecting pMOS transistor Q1; and a drain resistor R4 connected between a drain electrode of the detecting pMOS transistor Q1 and the second power supply Vss. Here, the detecting pMOS transistor Q1 and the first divider pMOS transistor Q3 are equal in the ratio W/L of the gate width W to the gate length L.
The power-on detection circuit according to the seventh embodiment of the present invention is required to satisfy Eq. (14), with the potential Vdd of the first power supply, the gate-source voltage VGS2 of the first divider pMOS transistor Q2, the gate-source voltage VGS3 of the second divider pMOS transistor Q3, and the gate-source voltage VGS1 of the detecting pMOS transistor Q1.

Vdd−{Vdd−|VGS2|−VGS3|}×R2/(R1+R2)=|VGS1|  (14)

Vdd−{Vdd−|VGS2|−|VGS3|}/2=|VGS1|  (15)

Here, as shown in Eq. (15), if we assume that the resistance values of the first dividing resistor R1 and the second dividing resistor R2 are equal, because the temperature dependences of the threshold voltages Vth of the detecting pMOS transistor Q1, the first divider pMOS transistor Q2 and the second divider pMOS transistor Q3 are approximately equal, the temperature dependence of the potential VG1 of the detecting pMOS transistor Q1 becomes substantially equal to the temperature dependence of the threshold voltage Vth of the detecting pMOS transistor Q1. Hence, if the resistance values of the source resistor R3 and the drain resistor R4 are determined so as to satisfy Eq. (15), it is possible to design the power-on detection circuit, which does not depend on the temperature and the threshold voltage variation of the detecting pMOS transistor.
According to the power-on detection circuit of the seventh embodiment, in addition to the effect of the variation in characteristics caused by temperature, the effect of the variation of threshold voltage itself of the detecting pMOS transistor Q1 can be minimized so as to relax the variation in the power-on potential.
Other Embodiments
Various modifications will become possible for those skilled in the art after receiving the teaching of the present disclosure without departing from the scope thereof. The technical scope of the present invention shall not be limited to the power on detection circuits disclosed in the first to seventh embodiments, because the circuit configurations and the related explanations disclosed in the first to seventh embodiments are mere examples so that any person skilled in the art can carry out the present invention.
Various modification of the present invention can be enabled without departing from the technical ideas and features disclosed in the first to seventh embodiments. In other words, the changes, the improvements or the replacement of one of the constituent elements may be possible within a scope of the present invention. Thus, the present invention of course includes various embodiments and modifications and the like which are not detailed above. Therefore, the scope of the present invention will be defined in the following claims.

## Claims

1. A circuit for detecting a power-on voltage of a power supply, comprising:
a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply, the voltage divider including a series circuit which includes:
a diode;
a first dividing resistor connected to the diode; and
a second dividing resistor connected between the first dividing resistor and the second power supply; and

a detecting circuit connected between the first power supply and the second power supply, the detecting circuit including:
a pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor;
a source resistor connected between the first power supply and the source electrode of the pMOS transistor; and
a drain resistor connected to the drain electrode of the pMOS transistor and the second power supply,

wherein a resistance value R1 of the first dividing resistor, a resistance value R2 of the second dividing resistor, a resistance value R3 of the source resistor R3 and a resistance value R4 of the drain resistor satisfy:


R2/(R1+R2)=(dVth/dT×R4/(R3+R4))/(dVdio/dT),



where dVth/dT is a temperature dependence of a threshold voltage of the pMOS transistor, and dVdio/dT is a temperature dependence of an on-state voltage of the diode.

2. The circuit of claim 1, further comprising a divider series resistor connected between the first power supply and the diode.

3. The circuit of claim 2, further comprising a second source resistor connected between the first power supply and the source resistor.

4. The circuit of claim 1, further comprising a divider series resistor connected between the diode and the first dividing resistor.

5. The circuit of claim 4, further comprising a second source resistor connected between the first power supply and the source resistor.

6. A circuit for detecting a power-on voltage of a power supply, comprising:
a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply, the voltage divider including a series circuit which includes:
a diode-connected first pMOS transistor;
a first dividing resistor connected to the first pMOS transistor; and
a second dividing resistor connected between the first dividing resistor and the second power supply; and

a detecting circuit connected between the first power supply and the second power supply, the detecting circuit including:
a second pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor; and
a drain resistor connected to the drain electrode of the second pMOS transistor and the second power supply,

wherein a resistance value R1 of the first dividing resistor and a resistance value R2 of the second dividing resistor satisfy:


|dVGS1/dT|×R2/(R1+R2)=|dVGS2/dT|



where dVGS1/dT is a temperature dependence of gate-source voltage of the first pMOS transistor, and dVGS2/dT is a temperature dependence of gate-source voltage of the second pMOS transistor.

7. The circuit of claim 6, further comprising a divider series resistor connected between the first power supply and the first pMOS transistor.

8. The circuit of claim 7, further comprising a source resistor connected between the first power supply and the source electrode of the second pMOS transistor.

9. The circuit of claim 6, further comprising a divider series resistor connected between the first pMOS transistor and the first dividing resistor.

10. The circuit of claim 9, further comprising a source resistor connected between the first power supply and the source electrode of the second pMOS transistor.

11. A circuit for detecting a power-on voltage of power supply, comprising:
a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply, the voltage divider including a series circuit comprising:
a diode-connected first divider pMOS transistor;
a diode-connected second divider pMOS transistor connected to the first divider pMOS transistor;
a first dividing resistor connected to the second divider pMOS transistor; and
a second dividing resistor connected between the first dividing resistor and the second power supply; and

a detecting circuit connected between the first power supply and the second power supply, the detecting circuit including:
a detecting pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor;
a source resistor connected between the first power supply and the source electrode of the detecting pMOS transistor; and
a drain resistor connected to the drain electrode of the detecting pMOS transistor and the second power supply

wherein, with gate width W and gate length L, ratios W/L in the first and second divider pMOS transistors are equal,
wherein resistance values of the first and second dividing resistors are equal.

12. A circuit for detecting a power-on voltage of a power supply, comprising:
a voltage divider connected between a first power supply and a second power supply, the potential of the second power supply is lower than the potential of the first power supply, the voltage divider including a series circuit comprising:
a diode-connected first divider pMOS transistor;
a diode-connected second divider pMOS transistor connected to the first divider pMOS transistor;
a first dividing resistor connected to the second divider pMOS transistor; and
a second dividing resistor connected between the first dividing resistor and the second power supply; and

a detecting circuit connected between the first power supply and the second power supply, the detecting circuit including:
a detecting pMOS transistor whose gate electrode is connected to a connection node between the first dividing resistor and the second dividing resistor;
a source resistor connected between the first power supply and the source electrode of the detecting pMOS transistor; and
a drain resistor connected to the drain electrode of the detecting pMOS transistor and the second power supply,

wherein a relationship given by:


Vdd−{Vdd−|VGS1|−|VGS2|}/2=|VGS3|



is satisfied, where Vdd is a potential supplied by the first power supply, VGS1 is a gate-source voltage of the first divider pMOS transistor, VGS2 is a gate-source voltage of the second divider pMOS transistor and VGS3 is a gate-source voltage of the detecting pMOS transistor.