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.
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:
Typically, the ratio of the resistors R12 and R11 is equal to about 10 to nullify the temperature coefficient.
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
Similarly, when the non-zero offset voltage Vos is considered, a bandgap voltage Vbg′ generated by the generating circuit 2 of
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.
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
An exemplary embodiment of a method for calibrating a bandgap voltage generated by a bandgap unit is provided. The bandgap unit comprises an operational amplifier. The method comprises switching a first input with a second input of the operational amplifier to measure a first bandgap voltage and a second bandgap voltage respectively, averaging the first bandgap voltage and the second bandgap voltage to generate a calibration voltage, and modifying the bandgap voltage so that a resulting bandgap voltage is equal to the calibration voltage.
An exemplary embodiment of a calibration circuit for calibrating an input offset of an operational amplifier is provided. The operational amplifier is used in a bandgap unit. The calibration circuit comprises a comparator, a calibration controller, and a voltage DAC. The comparator receives a bandgap voltage and a calibration voltage. The bandgap voltage is generated by the bandgap unit and the comparator generates a comparison result signal. The calibration controller receives the comparison result signal to generate a digital code and generates a control code to adjust the bandgap voltage. The voltage DAC receives the digital code to generate the calibration voltage.
An exemplary embodiment of a reference voltage generating method for a reference voltage device is provided. The reference voltage comprises a bandgap unit which alternately operates in two modes. The reference voltage generating method comprises, for each mode of the bandgap unit, setting an operation mode of the bandgap unit to one of the two modes, generating a calibration voltage based a first control code, comparing the calibration voltage with a bandgap voltage output by the bandgap unit to generate a first comparison result signal to indicate whether or not the calibration voltage is greater than the bandgap voltage, adjusting the first control code to modify the calibration voltage based on the first comparison result signal, and recoding the first control code when the calibration voltage is substantially equal to the bandgap voltage. The reference voltage generating method further comprises generating a second control code by averaging the first control codes, generating the calibration voltage to form a final calibration voltage based on the second control code, and generating the bandgap voltage of the bandgap unit based on the final calibration voltage to serve as the reference voltage.
An exemplary embodiment of a reference voltage device is provided. The reference voltage device comprises a bandgap unit, a voltage generating unit, a comparator, and a calibration controller. The bandgap unit alternately operates in two modes according to a selected signal and generates a bandgap voltage. The voltage generating unit receives a first control code and generates a calibrating voltage according to the first control code in each mode of the bandgap unit. The comparator receives and compares the bandgap voltage and the calibrating voltage, and generates a first comparison result signal according to the comparison result in each mode of the bandgap unit. The first comparison result signal indicates whether or not the calibrating voltage is greater than the bandgap voltage. The calibration controller receives the first comparison result signal, adjusts the first control code to modify the calibrating voltage based on the first comparison result signal, and recodes the first control code when the calibrating voltage is substantially equal to the bandgap voltage in each mode of the bandgap unit.
The calibration controller generates a second control code by averaging the recoded first control codes in the first and second modes. The voltage generating unit generates the calibration voltage to form a final calibrating voltage based on the second control code. The bandgap unit generates the bandgap voltage based on the final calibrating voltage to serve as the reference voltage.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
Reference voltage devices are provided. In the exemplary embodiment of a reference voltage device of
First, the operation mode of the bandgap unit 60 is set to the first mode according to a selecting code MODE provided by the calibration controller 63. The calibration controller 63 provides a first control code CODE_C1 to the voltage generating unit 61. The voltage generating unit 61 generates a calibration voltage Vref based the first control code CODE_C1. The bandgap voltage Vbg has a positive offset voltage in the first mode and serves as a bandgap voltage Vbg_high, and the bandgap voltage Vbg_high is expressed as:
The comparator 62 compares the calibration voltage Vref with the bandgap voltage Vbg_high to generate a first comparison result signal S_1 to indicate the calibration voltage Vref is greater than the bandgap voltage Vbg_high or not. The calibration controller 63 adjusts the first control code CODE_C1 to modify the calibration voltage Vref based on the first comparison result signal S_1. When the calibration voltage Vref is substantially equal to the bandgap voltage Vbg_high, the calibration controller 63 recodes the first control code CODE_C1 to be a code CODE_HIGH.
Then, the operation mode of the bandgap unit is set to the second mode according to the selecting code MODE. The voltage generating unit 61 generates the calibration voltage Vref based the first control code CODE_C1. The bandgap voltage Vbg has a negative offset voltage in the second mode and serves as a bandgap voltage Vbg_low, and the bandgap voltage Vbg_low is expressed as:
The comparator 62 compares the calibration voltage Vref with the bandgap voltage Vbg_low to generate the first comparison result signal S_1 to indicate the calibration voltage Vref is greater than the bandgap voltage Vbg_low or not. The calibration controller 63 adjusts the first control code CODE_C1 to modify the calibration voltage Vref based on the first comparison result signal S_1. When the calibration voltage Vref is substantially equal to the bandgap voltage Vbg_low, the calibration controller 63 recodes the first control code CODE_C1 to be a code CODE_LOW.
After the first and second modes, the calibration controller 63 generates a second control code CODE_C2 by averaging the codes CODE_HIGH and CODE_LOW.
The voltage generating unit 61 generates the calibration voltage Vref to yield a final calibration voltage Vref_F based on the second control code CODE_C2. The final calibration voltage Vref_F is expressed as:
According to the above equation, the variation due to OPAMP offset voltage of the bandgap unit 60 is removed. The comparator 62 compares the bandgap voltage Vbg with the final calibration voltage Vref_F to generate a second comparison result signal S_2 to indicate the bandgap voltage Vbg is greater than the final calibration voltage Vref_F or not. The calibration controller 63 adjusts a third control code CODE_C3 to modify the bandgap voltage Vbg based on the second comparison result signal S_2. When the bandgap voltage Vbg is substantially equal to the final calibration voltage Vref_F, the bandgap unit 60 outputs the bandgap voltage Vbg to serve as the reference voltage. After a final version of the third control code CODE_C3 is sent to adjust the bandgap voltage Vbg, the voltage generating unit 61, the comparator 62, and the calibration controller 63 can be powered down to save power. The voltage generating unit 61 can also be powered down after the third control code CODE_C3 is sent to adjust the bandgap voltage Vbg.
In this embodiment, the first control code CODE_C1 and the second control code CODE_C2 can be digital signals, and the voltage generating unit 61 can be a voltage digital/analog converter (DAC), as shown in
A comparator compares the calibration voltage Vref with the bandgap voltage Vbg_high to generate a first comparison result signal S_1 (step S103), and the first comparison result signal S_1 indicates whether or not the calibration voltage Vref is greater than the bandgap voltage Vbg_high. The calibration controller adjusts the first control code CODE_C1 to modify the calibration voltage Vref based on the first comparison result signal S_1 (step S104). When the calibration voltage Vref is substantially equal to the bandgap voltage Vbg_high, the calibration controller recodes the first control code CODE_C1 to be a code CODE_HIGH (step S105).
The operation mode of the bandgap unit is then set to the second mode, and the steps S102 to S104 are repeated. The calibration controller provides the first control code CODE_C1 to the voltage generating unit. The voltage generating unit generates the calibration voltage Vref (step S102) based the first control code CODE_C1. In the second mode, the bandgap voltage Vbg has a negative offset voltage and serves as a bandgap voltage Vbg_low. The comparator compares the calibration voltage Vref with the bandgap voltage Vbg_low to generate the first comparison result signal S_1 (step S103), and the first comparison result signal S_1 indicates whether or not the calibration voltage Vref is greater than the bandgap voltage Vbg_low. The calibration controller adjusts the first control code CODE_C1 to modify the calibration voltage Vref based on the first comparison result signal S_1 (step S104). When the calibration voltage Vref is substantially equal to the bandgap voltage Vbg_low, the calibration controller recodes the first control code CODE_C1 to be a code CODE_LOW (step S105).
After the first and second modes, the calibration controller generates a second control code CODE_C2 by averaging the codes CODE_HIGH and CODE_LOW (step S106). The voltage generating unit generates the calibration voltage Vref to yield a final calibration voltage Vref_F based on the second control code CODE_C2 (step S107). The comparator compares the bandgap voltage Vbg with the final calibration voltage Verf_F to generate a second comparison result signal S_2 (step S108). The second comparison result signal S_2 indicates whether or not the bandgap voltage Vbg is greater than the final calibration voltage Vref_F. The calibration controller adjusts a third control code CODE_C3 to modify the bandgap voltage based Vbg on the second comparison result signal S_2 (step 109). When the bandgap voltage Vbg is substantially equal to the final calibration voltage Vref_F. the bandgap unit outputs the bandgap voltage Vbg to serve as the reference voltage (step S110).
According to the invention, a reference voltage is generated without a non-zero offset voltage. The invention does not require an area consuming low pass filter. The low pass filter is replaced by digital averaging. Moreover, no high frequency modulation is performed, thus, noise is reduced.
While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
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