Abstract:
A regulated voltage generator provides different regulated voltages to an integrated circuit. The regulated voltage generator includes a bandgap reference circuit and at least one gain stage connected to an output thereof. The output voltage of the bandgap reference circuit varies as a function of temperature to compensate for variations in the gain stage made up of first and second transistors. A regulated voltage output by the regulated voltage generator is independent of temperature and of the supply voltage. The value of the regulated voltage is adjusted via a load resistor and via the first and second transistors along with an output transistor of the bandgap reference circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to integrated circuits, and more particularly, to voltage generators which provide different reference voltages required for supplying integrated circuits. 
     BACKGROUND OF THE INVENTION 
     External power supplies for integrated circuits now vary between three volts and ten volts, whereas the voltages required by the internal power supplies for the electrical circuits within the integrated circuits are, depending on the application, 2.5 volts, 3 volts, 5 volts and 7 volts. These voltages are within ±10%. It is therefore imperative that an integrated circuit itself generate these different voltages in order that they be independent of the power supply voltage and of temperature. For instance, the temperature may vary between −40° C. and 125° C. 
     To this end, there has been proposed a regulated voltage generator which exploits the properties of a reference voltage given by a circuit described in an article by E. Vittoz and J. Fellrath, entitled “CMOS Analog Integrated Circuits Based on Weak Inversion Operation”, published in IEEE Journal of Solid State Circuits, Vol. SC-12, no. 3, 1997, pages 224-231. This voltage reference circuit is generally known as a bandgap voltage reference circuit. 
     This prior art circuit supplies a reference voltage of 1.28 volts, known as the bandgap voltage, which is constant over a wide range of supply voltages and temperatures. To obtain the different required voltages, the circuit&#39;s output voltage is applied to gain stages, with each gain stage producing one of the required voltages. 
     However, these gain stages are sensitive to the supply voltage and to temperature, and the same holds for the power stage that follows them for supplying the required power. As a result, the voltages supplied vary significantly as a function of power supply voltage and of the temperature. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a generator for at least one regulated voltage that is not very sensitive to variations over a wide range of power supply voltages and temperatures. 
     This object is achieved by using a potential barrier reference voltage circuit, known as a bandgap type of circuit, and at least one gain stage. To provide a regulated voltage generator that is not sensitive to variations in the power supply voltage and temperature, the characteristics of the reference voltage are degraded to compensate for the variations due to the gain stage. The reference voltage then delivers a voltage which is a function of temperature variations opposite to that of the gain stage. 
     Another object of the present invention is to provide a generator producing a plurality of regulated voltages by implementing several gain stages. 
     The invention thus relates to a regulated voltage generator for supplying at least one regulated voltage to an integrated circuit comprising a bandgap type of reference voltage circuit and at least one gain stage. The bandgap type of reference voltage circuit comprises a current generator which supplies a bipolar transistor configured as a diode via a load resistor connected to the emitter of the bipolar transistor. 
     The gain stage comprises two MOS transistors in series between the supply voltage and a ground potential. The gate of a first transistor is connected to the gate of the output transistor of the current generator, and the gate of the second transistor is connected to the output of the bandgap type reference voltage circuit. 
     The characteristics of the first and second transistors are chosen to obtain the regulated voltage. The value of the load resistor is chosen such that the emitter-base voltage of the bipolar transistor varies with temperature in a manner to compensate for the variation of the gate-source voltage of the second transistor as a function of temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the present invention shall become more apparent from reading the following description of the preferred embodiments, given with reference to the appended drawings in which: 
     FIG. 1 is a schematic circuit diagram of a regulated voltage generator in accordance with the present invention; and 
     FIG. 2 is a block diagram of a device which delivers a regulated voltage among several available voltages in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The regulated voltage generator  10  in accordance with the invention comprises (FIG. 1) a bandgap (potential barrier) reference voltage circuit  12  and at least one gain stage  14 . The circuit  12  comprises four transistors M 1 , M 2 , M 3  and M 4  which are connected in a closed loop. 
     Transistors M 1  and M 2  are N-type MOS transistors whose sources are connected to a terminal at ground potential GND, either directly for transistor M 2 , or via a resistor R for transistor M 1 . The gates of transistors M 1  and M 2  are connected to one another and to the drain of transistor M 2 , which is connected to the drain of MOS transistor M 4 . Transistor M 4  is a P-type transistor, and its source is connected to the supply voltage V PS . The gate of transistor M 4  is connected to the gate and to the drain of MOS transistor M 3 , which is a P-type transistor, and is connected to the drain of transistor M 1 . The source of transistor M 3  is connected to the supply voltage V PS . 
     The gates of transistors M 3  and M 4  are connected to the gate of a P-type MOS transistor M 5  whose source is connected to the supply voltage V PS . The drain of transistor M5 is connected to the ground potential GND via a resistor R 2 , and a PNP type bipolar transistor Q 1  is connected as a diode. Bipolar transistor Q 1  has its emitter connected to a terminal of resistor R 2  while its other two electrodes are connected to the ground potential GND so that it functions as a diode. 
     The bandgap type reference voltage circuit  12  has two output terminals  16  and  18 . One output terminal  16  corresponds to the common node of the gates of transistors M 3 , M 4  and M 5 , and the other output terminal  18  corresponds to the drain of transistor M 5 . 
     The gain stage  14  comprises two P-type MOS transistors M 6  and M 7 . The gate of transistor M 6  is connected to output terminal  16 , while the gate of transistor M 7  is connected to output terminal  18 . The source of transistor M 6  is connected to the supply voltage V PS , while its drain is connected to the source of transistor M 7 . The drain of transistor M 7  is connected to the ground potential GND. The regulated output voltage VG 2  is taken from the terminals of transistor M 7 , i.e., between the ground potential GND and the source of transistor M 7 . 
     Transistors M 1  to M 5  and resistor R form a current source producing a current I GT . This current is supplied by transistor M 5 , and flows through resistor R 2  and bipolar transistor Q 1 . Transistor Q 1  is connected as a PN diode, and the current IGT varies proportionally with temperature. 
     In a prior art bandgap type of reference voltage circuit, the value of R 2  is chosen to produce a voltage V GAP ≈1.28 volts at the terminals of Q 1  and R 2 , which is not sensitive to temperature. This voltage V GAP  is used in the gain stage  14  to obtain the required voltage VG G2 , which is greater than V GAP . 
     In this gain stage, since the output voltage VG G2  is the sum of V GAP  and the voltage V SG7  between the gate and the source of transistor M 7 , with V SG7  varying with temperature, V G2  also varies with temperature. 
     The invention includes making V GAP  vary, so that it becomes V* GAP , as a function of temperature in order to compensate for the variation of V SG7  as a function of temperature. This is obtained by adjusting the value of resistor R 2  and the sizes of transistors M 5 , M 6  and M 7 . 
     To this end, a first equation defines the current I GT : 
     
       
           I   GT ( T )≈ I   GT ( T   0 )×( T/T   0 )  (1) 
       
     
     with the temperature T being expressed as an absolute value, and the temperature To being the reference temperature of 27° C. 
     A second equation defines the output voltage V G2  such that: 
     
       
           V   G2   =V * GAP   +V   SG7   ≈V   EB   +R   2   I   GT   +V   T7 η 2   I   GT   (2) 
       
     
     where 
     V EB  is the emitter-base voltage of transistor Q 1 , 
     η 2  is a term which depends on the W/L coefficients of transistors M 5 , M 6  and M 7 , 
     V T7  is an intrinsic characteristic voltage of transistor M 7 , referred to as the threshold voltage, and 
     V* GAP  is the variable voltage which depends on the temperature at the terminals of resistor R 2  and of bipolar transistor Q 1 . This is the output voltage of the bandgap reference voltage stage. 
     A third equation defines the variation of  72   2  as a function of temperature: 
     
       
         η 2 ( T )≈η 2 ( T   0 )( T   0   /T ) m   (3) 
       
     
     with m being in the region of two. 
     These three equations (1), (2) and (3) make it possible to determine the values of η 2  and R 2  by the following formulas: 
     
       
         η 2 ≈0.4[( V   G2   −V   EB   +V   V7 )− T   0 (δ V   EB   /δT )]/[ I   GT ( T   0 )]  (4a) 
       
     
     
       
           R   2 =0.2[3( V   G2   −V   EB   +V   T7 )+2 T   0 ( δV   EB   /δT ) ]/[I   GT ( T   0 )]  (4b) 
       
     
     with δV EB /δT being in the region of 1.8 mV/°C. 
     These two formulas lead to values of R 2 =550 kΩ and η 2 =493 to obtain a value V G2 =2.94 volts, which varies by 300 μV/° C., that is 49.5 mV in the temperature range of −40° C. to +125° C. for V PS =10 volts. 
     The voltage V* GAP  can be used to obtain other voltages V G1  and V G3  by applying that voltage to two gain stages  14 ′ and  14 ″ in which the transistors M′ 6 , M′ 7  and M″ 6 , M″ 7  are determined by the coefficients η 1  and η 3  calculated using formula (4a). Calculated values of η 2 =493 for V G1 =2.46 volts and η 3 =635 for VG G3 =3.43 volts are provided, for example. 
     However, these voltages V G1  and V G3  are sensitive to temperature variations, on the order of a millivolt per degree Celsius. To obtain a voltage V G1  or V G3  that would not be sensitive to temperature, it would be necessary to modify R 2  according to formula (4b) to obtain R 1 , for the case of voltage V G1,  and R 3  for the case of voltage VG 3 . 
     Moreover, coefficient η 2  not only determines the characteristics of transistors M 6  and M 7 , but also transistor of M 5  according to the formula:          η   2     =         [     W6   ·     L5   /   W5     ·   L6     ]       1   /   2           [     μ                   7   ·     Cox        (     W7   /   L7     )           ]       1   /   2                                
     where: 
     W and L are respectively the width W and the length L of the drain-source channel of transistors M 5  (W 5  and L 5 ), M 6  (W 6 , L 6 ) and M 7  (W 7 , L 7 ), μ 7  is the mobility of transistor M 7 , and Cox is the oxide capacitance. 
     FIG. 2 is a functional block diagram of a device which supplies one of the three voltages V G1 , V G2  or V G3  on demand. This device comprises the bandgap type reference voltage circuit  12  of the diagram in FIG. 1, and supplies on output terminal  18  the voltage V* GAP  as well as the voltage V GT  of transistor M 5  on output terminal  16 . Output terminals  16  and  18  are connected to the input terminals of the gain stages  14 ′,  14  and  14 ″, which respectively supply the voltages V G1 , V G2  and V G3 . 
     Only the voltage V G2  which corresponds to the value R 2  calculated from formula (4b) is in fact regulated, and hence substantially independent of temperature variations. The output terminals of gain stages  14 ′,  14  and  14 ″ are each connected to one of three input terminals  22 ,  24 ,  26  of a multiplexing circuit  30  which produces the connection between one of the three input terminals  22 ,  24 ,  26  and its output terminal  28 . Selection of the connection is obtained by a control circuit  32  using appropriate signals. 
     The output terminal  28  of the multiplexing circuit  30  is connected to the input terminal of a power amplifier  34 , whose output terminal  36  is connected to an electronic circuit to be supplied, such as a microprocessor  38 , for example.