1. Field of the Invention
The present invention relates to a reference voltage generating method, and in particular relates to a reference voltage device.
2. Description of the Related Art
Integrated circuits often require a reference voltage which remains stable under PVT (process, voltage, temperature) variations. FIG. 1 depicts a conventional generating circuit of a bandgap voltage serving as a reference voltage for integrated circuits. Referring to FIG. 1, the generating circuit 1 comprises an operational amplifier OPAMP, bipolar devices B11 and B12, and resistors R11 to R13. The values of the resistors R11 and R12 are the same. The operational amplifier OPAMP of a high gain is used, and base-emitter voltage Vbe1 and Vbe1′ are almost equal due the closed loop. A bandgap voltage Vbg is determined according to the base-emitter voltage Vbe1 and Vbe2 (negative temperature coefficient) and thermal voltage VT (positive temperature coefficient) of the bipolar devices B11 and B12. With a proper ratio of the base-emitter voltage and the thermal voltage VT for each bipolar, the temperature dependence of the bandgap voltage Vbg is almost eliminated.
However, because the area of the bipolar devices B11 and B12 is different, the current density in the bipolar devices B11 and B12 is also different, thus, the base-emitter voltage Vbe1 and Vbe2 of the bipolar devices B11 and B12 becomes unequal. Accordingly, the same current density in different areas of the bipolar devices B11 and B12 results in a current magnitude which is equal to ΔVbe/R13 and provides a positive temperature coefficient. The bandgap voltage Vbg is then determined according to the base-emitter voltage Vbe1 and a scale of ΔVbe and expressed as:
  Vbg  =            Vbe      ⁢                          ⁢      1        +                            R          ⁢                                          ⁢          12                          R          ⁢                                          ⁢          13                    ×      Δ      ⁢                          ⁢      Vbe      
Typically, the ratio of the resistors R12 and R11 is equal to about 10 to nullify the temperature coefficient.
FIG. 2 depicts another conventional generating circuit of a bandgap voltage operating at low voltage, serving as a reference voltage for integrated circuits. Referring to FIG. 2, the generating circuit 2 mainly comprises an operational amplifier OPAMP, bipolar devices B21 and B22, and resistors R21 to R24. The values of the resistor R21 and R22 are the same. A bandgap voltage can be scaled freely by adjusting the ratio of the resistors and expressed as:
  Vbg  =                    R        ⁢                                  ⁢        24                    R        ⁢                                  ⁢        22              =          ×              (                              Vbe            ⁢                                                  ⁢            1                    +                                                    R                ⁢                                                                  ⁢                22                                            R                ⁢                                                                  ⁢                23                                      ×            Δ            ⁢                                                  ⁢            Vbe                          )            
According to the two generating circuits 1 and 2, the bandgap voltage of the generating circuit 2 is equal to R24/R22 of that of the generating circuit 1.
In above generating circuits 1 and 2, it is assumed that the operational amplifier OPAMP is ideal, meaning that the voltage of the two input terminals of the operational amplifier OPAMP is ideally equal. In practice, however, the voltage of two input terminals of the operational amplifier OPAMP is not equal, and there is a non-zero offset voltage Vos between the two input terminals. When the non-zero offset voltage Vos is considered, the generating circuit 1 of FIG. 1 is redrawn as in FIG. 3, and a bandgap voltage Vbg′ is expressed as:
                              Vbg          ′                =                              Vbe            ⁢                                                  ⁢            1                    +                                                    R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                13                                      ×            Δ            ⁢                                                  ⁢            Vbe                    +                                    (                              1                +                                                      R                    ⁢                                                                                  ⁢                    12                                                        R                    ⁢                                                                                  ⁢                    13                                                              )                        ×            Vos                                              (                  Equation          ⁢                                          ⁢          1                )            
Similarly, when the non-zero offset voltage Vos is considered, a bandgap voltage Vbg′ generated by the generating circuit 2 of FIG. 2 is expressed as:
                              Vbg          ′                =                                  ⁢                                            R              ⁢                                                          ⁢              24                                      R              ⁢                                                          ⁢              22                                ×                      [                          (                                                Vbe                  ⁢                                                                          ⁢                  1                                +                                                                            R                      ⁢                                                                                          ⁢                      22                                                              R                      ⁢                                                                                          ⁢                      23                                                        ×                  Δ                  ⁢                                                                          ⁢                  Vbe                                +                                                      (                                          1                      +                                                                        R                          ⁢                                                                                                          ⁢                          22                                                                          R                          ⁢                                                                                                          ⁢                          23                                                                                      )                                    ×                  Vos                                            ]                                                          (                  Equation          ⁢                                          ⁢          2                )            
According to Equation 1 and Equation 2, the non-zero offset voltage Vos of the operational amplifier OPAMP is amplified by (1+R22/R23). As described previously, in order to nullify temperature dependence of the bandgap voltage, the ratio of the resistors R2 and R1 is set to about 10. Thus, when the non-zero offset voltage Vos is considered, the bandgap voltage Vbg may drift from its ideal value by 10 times the non-zero offset voltage Vos. The amplified non-zero offset voltage Vos causes the reference voltage to vary greatly from chip to chip.
In order to reduce the variation of the reference voltage from chip to chip, several techniques are introduced. One method uses a trimming circuit. FIG. 4 depicts a trimming circuit. A bandgap voltage Vbg′ with a non-zero offset voltage Vos is amplified by an amplifier 40, and a trimmed output voltage Vref is selected from a resistor string 41. At the beginning of the trimming process, an ideal target voltage value is applied to an external tester and compared with an output voltage Vref of the trimming circuit. The output voltage Vref is selected by varying a final control code CODE40 which is determined according to the comparison result signal. When the output voltage Vref is equal to the ideal target voltage value, the final control code CODE40 is obtained. According to the final control code CODE40, the trimmed output voltage Vref is determined. The ideal target voltage value and the trimmed output voltage Vref can be fixed and remembered in an IC by fuses which can be programmed by laser or electronically. The trimming process, however, requires more time in the tester for calibration and fuses trimming.
Chopper stabilization is another method to reduce the variation in the reference voltage. U.S. Pat. No. 6,462,612 discloses a chopper stabilized bandgap reference circuit to cancel offset variation. Referring to FIG. 5, an input signal of an amplifier 51 is modulated by a high frequency modulator (MOD) 50. The modulated signal is amplified by the amplifier 51 and demodulated by a demodulator (DEMOD) 52. Since an offset voltage of the amplifier 51 is not modulated, the offset voltage is modulated to a high frequency in the demodulation process. The high frequency noise can be filtered by a low pass filter (LPF) 53. The chopper stabilized bandgap reference circuit is useful to reduce the variation caused by the offset voltage of the amplifier 51. The circuit requires a high frequency clock for modulation, however, which produces noise. Moreover, the low pass filter 53 occupies a large area.