Voltage generating device

Abstract: A voltage generating device is capable of generating a voltage which does not depend upon temperature even if a power source voltage is not higher than 1.25 V. The device comprises an output terminal (1) thereof, current sources (11 and 15), resistors (13 and 14) and a diode like connected transistor (12). A voltage on the output terminal is obtained by causing a current to flow through series-connected resistors (13 and 14). The current sources are band gap current sources. The current source (15) is assumed as opened. A part of the diode like connected transistor (12) is represented by an equivalent circuit (120) including a voltage source (121) and a resistor (122). The equivalent circuit (120) and the resistors (13 and 14) are represented by an equivalent circuit (130) by using the Thevenin's theorem.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a voltage generating device which generates a voltage which does not depend upon temperature. Description of the Prior Art Such type of prior art voltage generating device comprises a voltage source including a semiconductor PN junction for generating a voltage which negatively changes with temperature and a voltage source for generating a thermal voltage (kT/q) which positively changes with temperature, both voltage sources being series-connected for cancelling the changes in voltage with temperature with each other. The structure of a prior art voltage generating device is shown in Fig. 1. In Fig. 1, a reference numeral 1 denotes an output terminal of a voltage generating device; 21 denotes a current source; 22 a resistor; 23 a diode like connected transistor. A voltage on the output terminal 1 is obtained by causing a current from the current source 21 to flow through the series-connected resistors 22 and 23. The current source 21 is a band gap current source as is disclosed in JP-A-60-191508. The current value Ics is determined by equation (1). Ics = [(k x T/q) x ln(N)] / Rcs wherein k denotes the Boltzmann's constant; T denotes an absolute temperature; q denotes the charge of electrons; N denotes a constant; Rcs denotes a current presetting resistance. The voltage Vo on the output terminal 1 can be expressed by equation (2). Vo = Vf23 + (R22 x Ics) wherein Vf23 and R22 denote the forward voltage of the transistor 23 and the resistance of the resistor 22, respectively. The first clause in equation (2) denotes the forward voltage of the diode like connected transistor. It is generally well known that this voltage changes at -2 mV/deg with temperature when it is about 650 mV. Therefore, a change in voltage with temperature in the second clause is preset to a value which has the opposite sign, and is equal to the absolute value of that in the first clause, the changes in voltage with temperature in the first and second clauses can be cancelled with each other. Briefly, in order to make Vo a temperature independent voltage, equation (2) is put into the second clause to provide equation (3). R22 x Ics = R22 x (k x T/q) x ln(N)/Rcs = (k x T/q) x ln(N) x R22/Rcs A change in voltage with temperature is obtained by differentiating the equation (3) with respect to the absolute temperature T. If the change is represented by +2 mV, equation (4) is obtained. d(R22 x Ics) / dT = (k/q)ln(N) x R22/Rcs = +2 mV By putting equation (4) into equation 3 and by making the thermal coefficients of R22 and Rcs equal to each other and T = 300 K, equation (5) is obtained. R22 x Ics = d(R22 x Ics) / dT x T = +2 mV x 300° K = 600 mV Accordingly, if R22 or Ics is preset in such a manner that R22 x Ics = 600 mV, Vo is determined as about 1.25 V in accordance with equation (2). Vo is independent of temperature. This approach has been widely adopted since the thermal coefficients of R22 and Rcs can be easily made equal if these components are formed on a single semiconductor chip. In such a manner, even the prior art voltage generating device is capable of generating a voltage which is independent of temperature. However, the prior art voltage generating device cannot be used for a circuit which requires a power source voltage which is lower than 1.25 V since the voltage which is independent of temperature is as low as 1.25 V. In other words, since the first clause in equation (2) is fixed as 650 mV, the second clause should be equal or lower than 600 mV. Resultingly, Vo is dependent upon temperature. JP-A-63177214 describes a voltage generator as defined in the preamble of claim 1. SUMMARY OF THE INVENTION The present invention aims at solving the above mentioned problems of the prior art. It is therefore an object of the present invention to provide an excellent voltage generating device which is capable of providing a voltage which is independent of temperature even if a power source voltage is not higher than 1.25 V. According to the present invention there is provided a voltage generating device as defined in claim 1. Accordingly the forward voltage which negatively changes with the temperature which is obtained by causing a forward current to flow through the diode and the voltage dividing means from the current generating means is divided by the voltage dividing means and a voltage which positively changes with temperature is properly superposed upon the divided forward voltage by the current generating means and the voltage dividing means. Thus, a voltage which is independent of temperature can be obtained even if the power source voltage is equal to or less than 1.25 V. If the current generating means is formed of a low voltage operating type source as is disclosed in JP-A-60-191508, the power source voltage can be lowered to the output voltage Vo + about 0.2 V and the device can be easily formed of a semiconductor integrated circuit. In a development of the present invention, a voltage generating device further comprises biasing means connected directly to said diode for generating a forward voltage across the diode when a current from said biasing means flows in a forward direction of the diode. Accordingly, in accordance with this development, the forward voltage which negatively changes with the temperature which is obtained by causing the forward current to flow through the diode from the biasing means is divided by the voltage dividing means and a voltage which positively changes with temperature is properly superposed upon the divided forward voltage by the current generating means and the voltage dividing means. Thus, a voltage which is independent of temperature can be obtained even if the power source voltage is equal to or less than 1.25 V. If the output voltage Vo is preset equal to or less than 0.7 V and the current generating means is formed of a low voltage operating type source as is disclosed in JP-A-60-191508, the power source voltage can be lowered 0.9 V and the device can be easily formed of a semiconductor integrated circuit. The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Fig. 1 is a circuit diagram showing a prior art voltage generating device; Fig. 2A is a circuit diagram showing a first embodiment of the voltage generating device of the present invention; Fig. 2B is an equivalent circuit diagram showing a part of the device of Fig. 2A including a current source and a transistor; Fig. 2C is an equivalent circuit diagram showing a part of the device of Fig. 2A including the current sources, the transistor and resistors; and Fig. 3 is a circuit diagram showing a second embodiment of a voltage generating device of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figs. 2A to 2C, there is shown the structure of a first embodiment of the present invention. In Fig. 2A, a reference numeral 1 denotes an output terminal of a voltage generating device; 11 and 15 denote current sources; 13 and 14 denote resistors; 12 a diode like connected transistor. A voltage on the output terminal 1 is obtained by causing a current to flow through series-connected resistors 13 and 14. The current sources 11 and 15 are formed of current Miller circuits and the like using a band gap current source disclosed in JP-A-60-191508. The operation of the embodiment of Fig. 2A will be described with reference to Figs. 2B and 2C. Since there are two signal sources in the embodiment of Fig. 2A, the operation will be described by using the principle of superposition. The current source 15 is assumed as opened. In Fig. 2B, the diode like connected transistor 12 is represented by an equivalent circuit 120 including a voltage source 121 and a resistor 122. The value V121 of the voltage sources 121 and the value R122 of the resistor 122 are expressed by equations (6) and (7), respectively. V121 = Vf12 R122 = (k x T/q)|n(N) / I12 wherein Vf12 and I12 denote the forward voltage of the transistor 12 and the collector current of the transistor 12, respectively. In Fig. 2C, the equivalent circuit 120 and the resistors 13 and 14 are represented by an equivalent circuit 130 by using Thevenin's theorem. The value of V131 of the voltage source 131 and the value R132 of the resistor 132 are represented by equations (8) and (9). V131 = Vf12 x R14 / (R13 + R122 + R14) R132 = (R13 + R122) x R14/(R13 + R122 + R14) wherein R13 and R14 denote the resistances of the resistors 13 and 14, respectively. The current source 15 will be considered. A current Ics from the current source 15 is also defined by the equation (1). Since the current I15 from the current source 15 flows into the voltage source 131 through the resistor 132, an output voltage Vo on the output terminal 1 can be expressed by equation (10). Vo = V131 + (R132 x I15) Vo = M x {Vf12 + [(k x T/q) x ln (N) x (R13 + R122) / Rcs]} wherein M = R14 / (R13 + R122 + R14). Equation (10) resembles to equation 2 of the prior art. The output voltage Vo which is independent of the temperature can be generated by an approach similar to the prior art. In other words, the first clause in the parenthesis { } in equation (10) denotes the forward voltage of the diode like connected transistor and is about 650 mV. Since this forward voltage changes at -2 mV / degree with respect to temperature, the changes in voltage with the temperatures in the first and second clauses are cancelled with each other if the R13 and Rcs are preset so that the change in voltage relative to the temperature in the second clause in the parenthesis { ) is +2 mV/deg. This value is the same as the value of equation (5). Accordingly, the output voltage Vo can be finally made independent of temperature and the level of the voltage Vo can be desiredly preset by presetting M. If the output voltage is preset to, for example, 0.5 V, M is preset to 0.5 V / 1.25 V, the values R13, R14, I11 and I15 of the resistors 13 and 14 and the current sources 11 and 15 can be determined in accordance with equations 6 to 10. If R122 is sufficiently lower than R13, Vo is represented by the ratio of R13, R14 and the resistor Rcs which determines the current from the current source 15, so that designing of the circuit can be made easier. Since a voltage having a level which is a product of an absolute temperature T which is obtained from the resistors 13 and 14 and the current source 15 and a coefficient such as resistance ratio which is independent of temperature is superposed upon the forward voltage which is obtained by the diode like connected transistor 12 and the current source 11 in equation (10) in accordance with the first embodiment of the present invention, the output voltage can be preset for cancelling the changes in the output voltage with temperature as similarly to prior art and the level of the output voltage can be easily preset with a constant M. The voltage on the output terminal of the current source 11 will not become equal or higher than the forward voltage of the diode. If the voltage Vo is preset equal to or lower than the forward voltage of the diode and a low voltage operative current source which is disclosed in JP-A-60-191508 is used, a power source, the voltage of which is lowered to about 0.9 V can be used. Since the values of the resistors 13 and 14 which are related with the output voltage define a ratio, the present device can be easily formed of an semiconductor integrated circuit independently of the accuracy of the absolute values of the resistors. The characteristics relative to temperature can be determined by (R13 + R122) / Rcs in accordance with equation (10) and thus does not depend upon R14. There is an advantage that the voltage Vo can be desiredly determined. While the forward voltage which is obtained from the current source 11 and the diode like connected transistor 12 is applied to a voltage divider including the resistors 13 and 14 without passing through other components, it may be applied to the voltage divider via a buffer amplifier (not shown). In this case, designing of device is made easier since R122 becomes sufficiently lower. While components are preset in the first embodiment so that the output voltage Vo does not depend upon temperature, they may be preset to provide the device with a desired temperature characteristic. Referring now to Fig. 3, there is shown the structure of a second embodiment of the present invention, a reference numeral 1 denotes an output terminal of a voltage generating device; 15 denotes a current source; 13 and 14 denote resistors; and 12 denotes a diode like connected transistor. A voltage on the output terminal 1 is obtained by causing a current to flow from the current source 15 through the series-connected resistors 13 and 14. The current source 15 is made of a Miller circuit and the like using a band gap current source as is disclosed in JP-A-60-191508. The second embodiment of the present invention is substantially identical with the first embodiment except that the current source 11 in the first embodiment is omitted. The second embodiment is effective in case where the voltage Vo on the output terminal 1 is higher than the forward voltage of the transistor 12. In this case, the current I13 flowing through the resistor 13 will flow in an opposite direction so that a bias current can be caused to flow through the transistor 12 even if no current Ill flows from the current source 11. Since a voltage having a level which is a product of an absolute temperature T which is obtained from the resistors 13 and 14 and the current source 15 and a coefficient such as resistance ratio which is independent of temperature is superposed upon the forward voltage which is obtained by the diode like connected transistor 12 and the current source 15 in equation 10 in accordance with the second embodiment of the present invention, the output voltage can be preset for cancelling the changes in the output voltage with temperature as similarly to the prior art and the level of the output voltage can be easily preset with a constant M. If a low voltage operative current source as is disclosed in JP-A-60-191508 is used, a power source, the voltage Vo of which is lowered to about +0.2 V can be used. Since the values of the resistors 13 and 14 which are related with the output voltage define a ratio, the present device can be easily formed of an semiconductor integrated circuit independently of the accuracy of the absolute values of the resistors. The characteristics of the device with respect to temperature can be determined by (R13 + R122)/Rcs in accordance with equation (10) and thus does not depend upon R14. There is an advantage that the value of the voltage Vo can be desiredly determined. While components are preset in the second embodiment so that the output voltage Vo does not depend upon temperature, they may be preset to provide the device with a desired temperature characteristic. As is apparent from the foregoing, the first embodiment of the present invention is formed so that a voltage having a level which is proportional to an absolute temperature obtained from the voltage dividing means including a plurality of resistors and current sources is superposed upon the forward voltage which is obtained by a current source for biasing a diode like connected transistor in a forward direction. The superposed voltage can be preset for cancelling the changes in voltage with temperature. Resultingly, a voltage output which does not depend upon temperature can be obtained. The level of the output voltage can be easily preset by a voltage dividing ratio of the voltage dividing means. Since the voltage on the terminal of the current source will not become equal to or higher than the forward voltage of the diode if the output voltage Vo is preset not higher than the forward voltage of the diode, the power source voltage which is lowered to about 0.9 V can be used. Since the values of the resistors which determine the output voltage are represented by a ratio, the device can be easily formed of a semiconductor integrated circuit independently of the accuracy of the absolute values. As is apparent from the foregoing, the second embodiment is formed so that a voltage having a level which is proportional to an absolute temperature T obtained from voltage dividing means including a plurality of resistors; and a current source is superposed upon the forward voltage which negatively changes with temperature obtained by causing a forward current through a diode via a voltage dividing means from current generating means, the superposed voltage is preset for cancelling changes in voltage with temperature. Resultingly, a voltage output which does not depend upon temperature can be obtained. The level of the output voltage can be easily preset by a voltage dividing ratio of voltage dividing means. The power source voltage can be used until the output voltage Vo is lowered to about +0.2 V. Since the values of the resistors which determine the output voltage can be represented by a ratio, the device can be easily formed of a semiconductor integrated circuit independently of the accuracy of the absolute values.

Claims: A voltage generating device comprising: a diode (12); voltage dividing means including first and second resistors (13, 14), being connected to the diode, and for generating an output voltage at a junction point between the first and second resistors (13, 14); current generating means (15) connected to the voltage dividing means (13, 14) for generating a current having a positive temperature coefficient; and biasing means (11,15) for generating a forward voltage across the diode (12) when a current from said biasing means flows in a forward direction of the diode, said forward voltage having a negative temperature coefficient; said voltage dividing means (13, 14) being connected across said diode (12) to divide the forward voltage of the diode, the divided forward voltage of the diode (12) appearing as a component of the output of said voltage dividing means; said current generating means (15) being connected directly to the output (1) of the voltage dividing means at the junction point between the first and second resistors (13, 14); and in use, said current generating means (15) causes a current to flow into the output (1) of the voltage dividing means (13, 14) at the junction point between the first and second resistors (13, 14) characterised in that, the voltage dividing means consists only of two resistors and the values of these are chosen so that a temperature independent output voltage is produced at said junction point due to the contributions of the voltage component due to the divided forward voltage and a voltage component having a positive temperature coefficient that is produced across an output resistor (14) of the voltage dividing means (13, 14) due to the current of said current generating means (15), the negative temperature dependancy of the diode voltage being cancelled by a corresponding positive temperature dependancy of the current flowing into the output (1). A voltage generating device as claimed in claim 1, in which said biasing means is said current generating means (15) which, in use, supplies a current to said diode (12) through the voltage dividing means (13,14). A voltage generating device as claimed in claim 1, characterized in that said biasing means (11) is connected directly to said diode (12). A voltage generating device as claimed in claim 1 or claim 2, in which the current generating means (15) has a current presetting resistance, and the current generated by the current generating means is proportional to absolute temperature and is inversely proportional to the current presetting resistance.

Inventors: IKEDA MASAHARU, IKEDA, MASAHARU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1992
Main classification: G05F3
IPC codes: G05F3