# Patent

## Title
Power-on reset circuit

## Abstract

When the value of a power supply voltage (VDD) becomes a first threshold value or higher, a first start-up circuit (20) causes a band gap reference circuit (10) to start a stable operation and a first voltage value (VA) is output from the band gap reference circuit (10). When the value of the power supply voltage becomes a second threshold value or higher which is greater than the first threshold, a second start-up circuit (40) turns on a PMOS transistor (MP3) of a voltage dividing circuit (30), and a second voltage value (VB) output from the voltage dividing circuit (30) becomes a value, which is derived by dividing the value of the power supply voltage according to the resistance ratio of resistors (R31, R32). From a voltage comparison circuit (50), a reset level voltage value is output when the second voltage value (VB) is smaller than the first voltage value (VA), and a power-supply voltage level voltage value is output if the second voltage value (VB) becomes the first voltage value (VA) or higher.

## Background
TECHNICAL FIELD
The invention relates to power-on reset circuits.
BACKGROUND ART
A power-on reset circuit is used in order to secure the stable operation after starting to supply a power supply voltage in. various electronic devices. That is, the value of a power supply voltage supplied to an electronic device gradually increases from a ground potential level at the start of supply and will soon reach a constant level. If such a gradually-increasing power supply voltage value is directly provided to each circuit in an electronic device, the electronic device may not normally operate. Then, the power-on reset circuit, in the course of increasing of the power supply voltage value, provides a reset level voltage value to each circuit in the electronic device when the power supply voltage value is less than a threshold value, and will provide a power-supply voltage level voltage value to each circuit in the electronic device when the power supply voltage value becomes the threshold value or higher, thereby securing the stable operation of the electronic device.
Such a power-on reset circuit generally includes a bandgap reference circuit, a voltage dividing circuit, and a voltage comparison circuit as disclosed in Patent Documents 1 and 2. Even if there is a variation in the value of a power supply voltage to be supplied or a temperature variation, the bandgap reference circuit can output a voltage value with a small variation (hereinafter, referred to as a “first voltage value”).
On the other hand, the voltage dividing circuit includes a first resistor and a second resistor connected in series between a power supply voltage terminal to which the power supply voltage is supplied and a ground terminal, and outputs a voltage value, which is derived by dividing a power supply voltage value according to a ratio of the respective resistance values of the first resistor and second resistor, from a connection point between the first resistor and the second resistor. That is, the voltage dividing circuit outputs a voltage value (hereinafter, referred to as a “second voltage value”) proportional to the power supply voltage value.
Then, the voltage comparison circuit receives the first voltage value output from the bandgap reference circuit and the second voltage value output from the voltage dividing circuit, and outputs the reset level voltage value when the second voltage value is smaller than the first voltage value, and outputs the power-supply voltage level voltage value if the second voltage value becomes the first voltage value or higher.
CITATION LIST
Patent Literature
Patent Document 1: U.S. Pat. No. 5,867,047
Patent Document 2: U.S. Pat. No. 6,847,240
SUMMARY OF INVENTION
Technical Problem
In the power-on reset circuit, in the course of increasing of the power supply voltage value after starting to supply the power supply voltage, at the beginning the second voltage value is greater than the first voltage value, and thereafter, for a certain period of time, the second voltage value becomes smaller than the first voltage value, and further thereafter the second voltage value becomes the first voltage value or higher. The power-on reset circuit, during a period when the second voltage value is smaller than the first voltage value (hereinafter, referred to as a “reset period”), outputs a reset level voltage value to reset each circuit in an electronic device, thereby securing a stable operation of the electronic device thereafter.
However, in the conventional power-on reset circuits including those disclosed in Patent Documents 1, 2, the length of the reset period is unstable. Without a sufficient length of reset period, the stable operation of the electronic device may not be obtained.
The invention has been made in order to resolve the above-described problem and provides a power-on reset circuit capable of stably providing a sufficient length of reset period.
Solution to Problem
A power-on reset circuit according to the invention comprises: (1) a bandgap reference circuit to which a power supply voltage is supplied, and which outputs a predetermined first voltage value; (2) a first start-up circuit causing the bandgap reference circuit to start a stable operation when a value of the power supply voltage becomes a first threshold value or higher; (3) a voltage dividing circuit including: a switch and a first resistor provided in series between a power supply voltage terminal to which the power supply voltage is supplied and an output terminal; and a second resistor provided between the output terminal and a ground terminal, the voltage dividing circuit outputting a second voltage value from the output terminal; (4) a second start-up circuit which causes the switch of the voltage dividing circuit to close when the value of the power supply voltage becomes greater a second threshold value or higher which is greater than the first threshold value; (5) a voltage comparison circuit, which receives a first voltage value output from the bandgap reference circuit and a second voltage value output from the voltage dividing circuit, and outputs a reset level voltage value when the second voltage value is smaller than the first voltage value, and outputs the power-supply voltage level voltage value if the second voltage value becomes the first voltage value or higher.
In the power-on reset circuit according to the invention, when the value of the power supply voltage becomes the first threshold or higher, the first start-up circuit causes the band gap reference circuit to start a stable operation and the predetermined first voltage value is output from the band gap reference circuit. On the other hand, when the value of the power supply voltage becomes the second threshold or higher which is greater than the first threshold, the second start-up circuit causes the switch of the voltage dividing circuit to close, and the second voltage value output from the voltage dividing circuit is thus a value which is derived by dividing the power supply voltage according to a ratio of the respective resistance values of the first resistor and the second resistor. The first voltage value output from the bandgap reference circuit and the second voltage value output from the voltage dividing circuit are input to the voltage comparison circuit. Then, the voltage comparison circuit outputs the reset level voltage value when the second voltage value is smaller than the first voltage value, and outputs the power-supply voltage level voltage value if the second voltage value becomes the first voltage value or higher.
Advantageous Effects of Invention
According to the invention, a sufficient length of reset period can be stably obtained.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a circuit diagram of a power-on reset circuit 1 according to an embodiment.
FIG. 2 is a graph showing the temporal variation of each voltage value in the power-on reset circuit 1 according to the embodiment.
FIG. 3 is a graph schematically showing the process condition dependence of each of a threshold voltage Vthp of a PMOS transistor MP22, a threshold voltage Vthn of an NMOS transistor MN22, and a second threshold value Vth2 which the voltage dividing circuit 30 activates, in the power-on reset circuit 1 according to the embodiment.



DESCRIPTION OF EMBODIMENTS
Hereinafter, with reference to the accompanying drawings, an embodiment of the invention is described in detail. Note that, in the drawings for explaining the embodiment, the same reference numeral is attached to the same element, and the repeated explanation thereof is omitted.
FIG. 1 is a circuit diagram of a power-on reset circuit 1 according to the embodiment. The power-on reset circuit 1 shown in the diagram includes a bandgap reference circuit 10, a first start-up circuit 20, a voltage dividing circuit 30, a second start-up circuit 40, and a voltage comparison circuit 50. A common power supply voltage VDD is provided to these circuits.
The bandgap reference circuit 10 is supplied with a power supply voltage VDD and outputs a predetermined first voltage value VA, and includes PMOS transistors MP11-MP13, resistors R11-R17, diodes D10-DIN, and an amplifier A1.
The respective source terminals of the PMOS transistors MP11-MP13 are connected to a power supply voltage terminal to which the power supply voltage VDD is supplied. The respective gate terminals of the PMOS transistors MP11-MP13 are connected to an output terminal of the amplifier A1.
The drain terminal of the PMOS transistor MP11 is connected to an inverting input terminal of the amplifier A1 via the resistor R11, and connected to a ground terminal via the resistor R11 and resistor R12 connected in series, and also connected to the ground terminal via the diode D10.
The drain terminal of the PMOS transistor MP12 is connected to a non-inverting input terminal of the amplifier A1 via the resistor R13, and connected to the ground terminal via the resistor R13 and resistor R14 connected in series, and also connected to one end of the resistor R15. The other end of the resistor R15 is connected to the ground terminal via N diodes (N is an integer of 2 or more) D11-DIN connected in parallel.
The respective resistance values of the resistor R11 and resistor R13 are equal to each other. The respective resistance values of the resistor R12 and resistor R14 are equal to each other. The respective forward voltages of the diodes D10-DIN vary with the magnitude of a current.
The drain terminal of the PMOS transistor MP13 is connected to the ground terminal via the resistor R16 and resistor R17 connected in series. The bandgap reference circuit 10 outputs a voltage value of the drain terminal of the PMOS transistor MP13 as the first voltage value VA.
The first start-up circuit 20 causes the bandgap reference circuit 10 to start a stable operation when the value of the power supply voltage VDD becomes the first threshold value Vth1 or higher. The first start-up circuit 20 includes the PMOS transistors MP21, MP22, the NMOS transistors MN21, MN22, and the inverters INV21, INV22.
The respective source terminals of the PMOS transistors MP21, MP22 are connected to the power supply voltage terminal to which the power supply voltage VDD is supplied. The drain terminal of the PMOS transistor MP21 is connected to the inverting input terminal of the amplifier A1 in the bandgap reference circuit 10. The drain terminal of the PMOS transistor MP22 is connected to the drain terminal of the NMOS transistor MN22. The drain terminal of the NMOS transistor MN21 is connected to the non-inverting input terminal of the amplifier A1 in the bandgap reference circuit 10. The respective source terminals of the NMOS transistors MN21, MP22 are connected to the ground terminal.
The drain terminal of the PMOS transistor MP22 is connected to the gate terminal of the MMOS transistor MN21 via the inverter INV21, and also connected to the gate terminal of the PMOS transistor MP21 via the inverters INV21, INV22 connected in series. The gate terminal of the PMOS transistor MP22 is connected to the output terminal of the amplifier A1 in the bandgap reference circuit 10. The gate terminal of the NMOS transistor MN22 is connected to the power supply voltage terminal.
The on-resistance value of the NMOS transistor MN22 is greater than the on-resistance value of the PMOS transistor MP22. In the NMOS transistor MN22, in order to increase the on-resistance value, a gate length L is long relative to a gate width W, for example, the gate length L is 100 times the gate width W.
The voltage dividing circuit 30 includes: the PMOS transistor MP3 and the first resistor R31 provided in series between the power supply voltage terminal to which the power supply voltage VDD is supplied and the output terminal: and the second resistor R32 provided between the output terminal and the ground terminal, and outputs a second voltage value VB from the output terminal.
The source terminal of the PMOS transistor MP3 is connected to the power supply voltage terminal to which the power supply voltage VDD is supplied, and the drain terminal of the PMOS transistor MP3 is connected to one end of the resistor R31. The other end of the resistor R31 is connected to the ground terminal via the resistor R32. The connection point between the resistor R31 and the resistor R32 serves as the output terminal of the voltage dividing circuit 30.
The PMOS transistor MP3 acts as a switch. That is, when the PMOS transistor MP3 is in an off state, the switch is opened and the second voltage value VB output from the output terminal becomes the ground level. When the PMOS transistor MP3 is in an on state, the switch is closed and the second voltage value VB output from the output terminal becomes a voltage value, which is derived by dividing the value of the power supply voltage VDD according to a ratio of the respective resistance values of the resistor R31 and resistor R32.
The second start-up circuit 40 causes the switch of the voltage dividing circuit 30 to close when the value of the power supply voltage VDD becomes the second threshold value Vth2 or higher which is greater than the first threshold value Vth1. The second start-up circuit 40 includes the PMOS transistor MP4, the resistors R41-R43, and the inverter INV4.
The source terminal of the PMOS transistor MP4 is connected to the power supply voltage terminal to which the power supply voltage VDD is supplied. The gate terminal of the PMOS transistor MP4 is connected to the power supply voltage terminal via the resistor R41 and also connected to the ground terminal via the resistor R42. The drain terminal of the PMOS transistor MP4 is connected to the ground terminal via the resistor R43 and also connected to the gate terminal of the PMOS transistor MP3 of the voltage dividing circuit 30 via the inverter INV4.
The voltage comparison circuit 50 receives the first voltage value VA output from the bandgap reference circuit 10 and the second voltage value VB output from the voltage dividing circuit 30, and outputs the reset level voltage value when the second voltage value VB is smaller than the first voltage value VA, and outputs the power-supply voltage level voltage value if the second voltage value VB becomes the first voltage value VA or higher. The voltage comparison circuit 50 includes the amplifier A5 and the inverter INV5.
The first voltage value VA output from the bandgap reference circuit 10 is input to the non-inverting input terminal of the amplifier A5. The second voltage value VB output from the voltage dividing circuit 30 is input to the inverting input terminal of the amplifier A5. The voltage comparison circuit 50 outputs a signal passing from the output terminal of the amplifier A5 through the inverter INV5, as a power-on reset signal POR.
Next, the operation of the power-on reset circuit 1 according to the embodiment is described. FIG. 2 is a graph showing the temporal variation of each voltage value in the power-on reset circuit 1 according to the embodiment. This graph shows the temporal variation of each of the power supply voltage VDD supplied to the power-on reset circuit 1, the first voltage value VA output from the bandgap reference circuit 10, the second voltage value VB output from the voltage dividing circuit 30, and the power-on reset signal POR output from the voltage comparison circuit 50.
The value of the power supply voltage VDD supplied to the power-on reset circuit 1 is the ground potential level at the start of supplying the power supply voltage, and thereafter will gradually increase.
In the bandgap reference circuit 10, for a certain period immediately after starting to supply the power supply voltage, the operation is not stable and the output voltage value VA takes uncertain values between the ground potential level and the power supply voltage level.
When the value of the power supply voltage VDD reaches the first threshold value Vth1, then in the first start-up circuit 20, the NMOS transistor MN22 is turned on and a connection point P2 between the PMOS transistor MP22 and the NMOS transistor MN22 becomes the ground potential level. The first threshold value Vth1 is equal to the threshold voltage Vthn of the NMOS transistor MN22. If the connection point P2 becomes the ground potential level, the output terminal of the inverter INV21 becomes the power supply voltage level and the output terminal of the inverter INV22 becomes the ground potential level.
Thus, the PMOS transistor MP21 is turned on and the power supply voltage level is input to the inverting input terminal of the amplifier A1 in the bandgap reference circuit 10. Moreover, the NMOS transistor MN21 is turned on and the ground potential level is input to the non-inverting input terminal of the amplifier A1 in the bandgap reference circuit 10.
In the amplifier A1, if the power supply voltage level is input to the inverting input terminal and the ground potential level is input to the non-inverting input terminal, then the ground potential level is output from the output terminal. The ground potential level output from the output terminal of the amplifier A1 is applied to the respective gate terminals of the PMOS transistors MP11-MP13, MP22. This turns on each of the PMOS transistors MP11-MP13, MP22.
If the PMOS transistor MP11 is turned on, a current I11 flows from the power supply voltage terminal through the source terminal and drain terminal of the PMOS transistor MP11. The current I11 is divided into two, and one current flows through the resistor R11 and the resistor R12 and the other current flows though the diode D10.
If the PMOS transistor MP12 is turned on, then a current I12 flows from the power supply voltage terminal though the source terminal and drain terminal of the PMOS transistor MP12. The current I12 is divided into two, and one current flows through the resistor R13 and the resistor R14 and the other current flows through the resistor R15 and N diodes D11-DIN.
If the PMOS transistor MP13 is turned on, a current I13 flows from the power supply voltage terminal through the source terminal and drain terminal of the PMOS transistor MP13. The current I13 further flows through the resistor R16 and the resistor R17.
Moreover, the PMOS transistor MP22 of the first start-up circuit 20 is turned on. Since the on-resistance value of the NMOS transistor MN22 is greater than the on-resistance value of the PMOS transistor MP22, the connection point P2 becomes the power supply voltage level. As a result, the respective PMOS transistor MP21 and NMOS transistor MN21 are turned off.
The action of the first start-up circuit 20 as described above allows the bandgap reference circuit 10 to start a stable operation. In the bandgap reference circuit 10 during the stable operation, a potential applied from the amplifier A1 to the respective gate terminals of the PMOS transistors MP11-MP13 is set so that the connection point P11 between the resistor R11 and the resistor R12 and the connection point P12 between the resistor R13 and the resistor R14 become a potential equal to each other. Thus, even if the value of the power supply voltage VDD varies, a stable first voltage value VA may be output from the output terminal. Moreover, because the temperature dependences of the respective resistance values of the resistor and the diode cancel out with each other, the stable first voltage value VA may be output from the output terminal even if the temperature varies.
Accordingly, the first voltage value VA output from the bandgap reference circuit 10 takes uncertain values between the ground potential level and the power supply voltage level until the value of the power supply voltage VDD reaches the first threshold value Vth1. However, the first voltage value VA becomes a stable constant value if the value of the power supply voltage VDD reaches the first threshold value Vth1.
On the other hand, in the voltage dividing circuit 30 and the second start-up circuit 40, for a certain period immediately after starting to supply the power supply voltage, a connection point P41 between the resistor R41 and the resistor R42 is close to the ground potential level and the PMOS transistor MP4 is in an off state. Moreover, a connection point P42 between the PMOS transistor MP4 and the resistor R43 is also close to the ground potential level and the PMOS transistor MP3 is in an off state. Accordingly, the second voltage value VB output from a connection point P31 between the resistor R31 and the resistor R32 is at the ground potential level.
If the value of the power supply voltage VDD reaches the second threshold value Vth2, then, in the second start-up circuit 40, the potential difference between the source terminal and gate terminal of the PMOS transistor MP4 becomes the threshold voltage Vthp or higher and the PMOS transistor MP4 is turned on. The second threshold value Vth2 is expressed by a formula “Vth2=Vthp(R41+R42)R42”. If the PMOS transistor MP4 is turned on, the potential difference between the source terminal and gate terminal of the PMOS transistor MP3 becomes the threshold value or higher and the PMOS transistor MP3 is turned on.
Then, if the PMOS transistor MP3 is turned on, the second voltage value VB output from the voltage dividing circuit 30 becomes a voltage value, which is derived by dividing the value of the power supply voltage VDD according to a ratio of the respective resistance values of the resistor R31 and the resistor R32, and gradually increases proportional to the power supply voltage VDD. Neglecting the on-resistance value of the PMOS transistor MP3, the second voltage value VB is expressed by a formula “VB=VDD·R32/(R31+R32)”.
As shown in FIG. 2, the first voltage value VA output from the bandgap reference circuit 10 becomes a stable constant value if the value of the power supply voltage VDD reaches the first threshold value Vth1. On the other hand, the second voltage value VB output from the voltage dividing circuit 30 is at the ground potential level until the value of the power supply voltage VDD reaches the second threshold value Vth2, and becomes a value proportional to the power supply voltage VDD when the value of the power supply voltage VDD is the second threshold value Vth2 or higher. Provided that the second threshold value Vth2 is greater than the first threshold value Vth1.
Accordingly, the magnitude relation between the first voltage value VA and the second voltage value VB is reversed at a certain time instance t1 after starting to supply the power supply voltage VDD until the power supply voltage VDD becomes stable, as a boundary. That is, before the time instance t1, because the second voltage value VB is smaller than the first voltage value VA, the power-on reset signal POR output from the voltage comparison circuit 50 is at the ground potential level. After the time instance t1, because the second voltage value VB is greater than the first voltage value VA, the power-on reset signal POR output from the voltage comparison circuit 50 is at the power supply voltage level. In. this manner, the power-on reset circuit 1 according to the embodiment can stably provide a sufficient length of reset period.
Note that it is important that the respective resistors, diodes, PMOS transistors, and NMOS transistors constituting each circuit are designed and manufactured so that the desired characteristics as described above may be obtained. Particularly with regard to the PMOS transistors and NMOS transistors, their characteristics vary if the manufacturing conditions vary. It is therefore important that these transistors have some design margin so that the desired characteristics may be obtained even if the manufacturing conditions vary.
FIG. 3 is a graph schematically showing the process condition dependence of each of the threshold voltage Vthp of the PMOS transistor MP22, the threshold voltage Vthn of the NMOS transistor MN22, and the second threshold value Vth2 which the voltage dividing circuit 30 activates, in the power-on reset circuit 1 according to the embodiment. In order for the voltage dividing circuit 30 to be activated after the bandgap reference circuit 10 is activated, it is important that the PMOS transistors and the NMOS transistors are designed so as to satisfy a condition “Vth2>Vthn” in a total range of the process condition variations, as shown in this graph, even if the manufacturing conditions vary.
The invention is not limited to the above-described embodiment, but various modifications are possible. For example, various configurations are possible for each circuit constituting the power-on reset circuit 1.
INDUSTRIAL APPLICABILITY
The invention can be applied to the power-on reset circuits for stably providing a sufficient length of reset period.
REFERENCE SIGNS LIST


    1 power-on reset circuit
    10 bandgap reference circuit
    20 first start-up circuit
    30 voltage dividing circuit
    40 second start-up circuit
    50 voltage comparison circuit

## Claims

1. A power-on reset circuit, comprising:
a bandgap reference circuit to which a power supply voltage is supplied, and which outputs a predetermined first voltage value;
a first start-up circuit, causing the bandgap reference circuit to start a stable operation when a value of the power supply voltage becomes a first threshold value or higher, wherein:
at least one input control terminal of the first start-up circuit is tied to the power supply voltage, and
a first output terminal of the first start-up circuit is tied to a first input terminal of the bandgap reference circuit and a second output terminal of the first start-up circuit is tied to a second input terminal of the bandgap reference circuit;
a voltage dividing circuit including: a switch and a first resistor provided in series between a power supply voltage terminal to which the power supply voltage is supplied and an output terminal of the voltage dividing circuit; and a second resistor provided between the output terminal of the voltage dividing circuit and a ground terminal, the voltage dividing circuit outputting a second voltage value from the output terminal of the voltage dividing circuit;
a second start-up circuit which causes the switch of the voltage dividing circuit to close when the value of the power supply voltage becomes a second threshold value or higher which is greater than the first threshold value; and
a voltage comparison circuit receiving the predetermined first voltage value output from the bandgap reference circuit and the second voltage value output from the voltage dividing circuit, and outputting a reset level voltage value when the second voltage value is smaller than the predetermined first voltage value and outputting a power-supply voltage level voltage value if the second voltage value becomes the predetermined first voltage value or higher.

2. The power-on reset circuit according to claim 1, wherein the first input terminal of the bandgap reference circuit and the second input terminal of the bandgrap reference circuit are, respectively, the positive terminal and the negative terminal of a differential circuit element.

3. A power-on reset circuit, comprising:
a bandgap reference circuit to which a power supply voltage is supplied, and which outputs a predetermined first voltage value;
a first start-up circuit which is directly connected to the power supply voltage and causes the bandgap reference circuit to start a stable operation when a value of the power supply voltage becomes a first threshold value or higher;
a voltage dividing circuit including: a switch, having an input control terminal, and a first resistor, which are provided in series between a power supply voltage terminal to which the power supply voltage is supplied and an output terminal of the voltage dividing circuit; and
a second resistor provided between the output terminal of the voltage dividing circuit and a ground terminal, the voltage dividing circuit outputting a second voltage value from the output terminal of the voltage dividing circuit;
a second start-up circuit which is directly connected to the power supply voltage, and which has an output terminal connected to the input control terminal of the switch included in the voltage dividing circuit, and which outputs a voltage which causes the switch of the voltage dividing circuit to close when the value of the power supply voltage becomes a second threshold value or higher which is greater than the first threshold value; and
a voltage comparison circuit receiving the predetermined first voltage value output from the bandgap reference circuit and the second voltage value output from the voltage dividing circuit, and outputting a reset level voltage value when the second voltage value is smaller than the predetermined first voltage value and outputting a power-supply voltage level voltage value if the second voltage value becomes the predetermined first voltage value or higher.