Abstract:
A constant on-time control switching converter with DC calibration is disclosed. A current flowing into a capacitor of a DC calibration circuit is reduced by introducing a transconductance amplifier and a resistor. Thus, the equivalent capacitance of the capacitor is magnified, which allows the user to integrate a capacitor with smaller capacitance to realize DC calibration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of CN application 201310401805.2, filed on Sep. 6, 2013, and incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates generally to electronic circuits, and more particularly but not exclusively to switching converters and control circuits and methods thereof. 
     BACKGROUND 
     In switching converters with COT (constant on-time) control, DC calibration circuits are usually used to improve the line regulation and the load regulation. 
     The prior DC calibration circuits usually comprise a transconductance amplifier and a capacitor. The transconductance amplifier receives a reference voltage and a sample signal and generates an error current based on the reference voltage and the sample signal. The error current flows through the capacitor and generates a calibrated reference voltage across the capacitor. In the prior DC calibration circuits, the capacitance of the capacitor should be large enough to prevent the DC calibration circuit from saturating at light load. For example, the capacitance may be 22 nF. However, a capacitor with such large capacitance is hard to fabricate in an integrated circuit. Therefore, a dedicated pin is usually needed for connection to an external capacitor. However, the more pins the integrated circuit has, the higher the cost is. 
     SUMMARY 
     present invention are directed to a control circuit of a switching converter. The switching converter has a switching circuit providing an output voltage. The control circuit has a DC calibration circuit, a comparison circuit, an on-time control circuit and a logic circuit. The DC calibration circuit has a first input terminal, a second input terminal and an output terminal. The first input terminal and the second input terminal are configured to respectively receive a reference voltage and a sample signal, and the DC calibration circuit generates a calibrated reference voltage at the output terminal based on the reference voltage and the sample signal representative of the output voltage. The DC calibration circuit comprises a first amplifier, a second amplifier, a first resistor and a capacitor. The first amplifier has a first input terminal, a second input terminal and an output terminal. The first input terminal and the second input terminal are configured to respectively receive the reference voltage and the sample signal, and the first amplifier amplifies the difference between the reference voltage and the sample signal and generates an error current at the output terminal. The second amplifier has a first input terminal, a second input terminal and an output terminal. The first input terminal is electrically coupled to the output terminal of the first amplifier to receive the error current, and the output terminal is electrically coupled to the first input terminal of the second amplifier and the output terminal of the DC calibration circuit. The first resistor is electrically coupled between the first input terminal and the second input terminal of the second amplifier. The capacitor is electrically coupled between the second input terminal of the second amplifier and a reference ground. The comparison circuit is electrically coupled to the DC calibration circuit. The comparison circuit compares the sample signal with the calibrated reference voltage and generates a comparison signal. The on-time control circuit is configured to generate an on-time control signal. The logic circuit is electrically coupled to the comparison circuit and the on-time control circuit. Based on the on-time control signal and the comparison signal, the logic circuit generates a control signal to control the switching circuit. 
     Embodiments of the present invention are also directed to a switching converter. The switching converter has a DC calibration circuit, a comparison circuit, an on-time control circuit, a logic circuit and a switching circuit providing an output voltage. The DC calibration circuit has a first input terminal, a second input terminal and an output terminal. The first input terminal and the second input terminal are configured to respectively receive a reference voltage and a sample signal representative of the output voltage, and the DC calibration circuit generates a calibrated reference voltage at the output terminal based on the reference voltage and the sample signal. The DC calibration circuit has a first amplifier, a second amplifier, a first resistor and a capacitor. The first amplifier has a first input terminal, a second input terminal and an output terminal. The first input terminal and the second input terminal are configured to respectively receive the reference voltage and the sample signal, and the first amplifier amplifies the difference between the reference voltage and the sample signal and generates an error current at the output terminal. The second amplifier has a first input terminal, a second input terminal and an output terminal. The first input terminal is electrically coupled to the output terminal of the first amplifier to receive the error current, and the output terminal is electrically coupled to the first input terminal of the second amplifier and the output terminal of the DC calibration circuit. The first resistor is electrically coupled between the first input terminal and the second input terminal of the second amplifier. The capacitor is electrically coupled between the second input terminal of the second amplifier and a reference ground. The comparison circuit is electrically coupled to the DC calibration circuit. The comparison circuit compares the sample signal with the calibrated reference voltage and generates a comparison signal. The on-time control circuit is configured to generate an on-time control signal. The logic circuit is electrically coupled to the comparison circuit and the on-time control circuit. Based on the on-time control signal and the comparison signal, the logic circuit generates a control signal to control the switching circuit. The switching circuit is configured to receive an input voltage and the control signal. The switching circuit converts the input voltage into an output voltage based on the control signal. 
     Embodiments of the present invention are further directed to a control method for controlling a switching converter. The switching converter has a switch and is configured to provide an output voltage. The control method has the following steps: amplifying the difference between a reference voltage and a sample signal representative of the output voltage and generating an error current based thereupon; providing a portion of the error current to a capacitor to generate a voltage across the capacitor; comparing the voltage across the capacitor with the sample signal and generating a comparison signal; providing an on-time control signal to control the on-time of a switch; and generating a control signal based on the on-time control signal and the comparison signal to control the switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be further understood with reference to the following detailed description and the appended drawings, wherein like elements are provided with like reference numerals. 
         FIG. 1  schematically illustrates a COT converter  100  in accordance with an embodiment of the present invention; 
         FIG. 2  illustrates a small signal equivalent circuit  106 ′ of the DC calibration circuit  106  shown in  FIG. 1 ; 
         FIG. 3  illustrates a DC calibration circuit  206  in accordance with another embodiment of the present invention; 
         FIG. 4  illustrates a small signal equivalent circuit  206 ′ of the DC calibration circuit  206  shown in  FIG. 3 ; 
         FIG. 5  illustrates a control method  500  for controlling a switching converter in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is now described. While it is disclosed in its preferred form, the specific embodiments of the invention as disclosed herein and illustrated in the drawings are not to be considered in a limiting sense. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Indeed, it should be readily apparent in view of the present description that the invention may be modified in numerous ways. Among other things, the present invention may be embodied as devices, methods, software, and so on. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Throughout the specification, the meaning of “a,” “an,” and “the” may also include plural references. 
       FIG. 1  schematically illustrates a COT converter  100  in accordance with an embodiment of the present invention. As shown in  FIG. 1 , the COT converter  100  comprises a control circuit, a switching circuit  104  and a feedback circuit  105 . The switching circuit  104  employs synchronous buck topology and comprises switches M 1  and M 2 , an inductor L and an output capacitor C. 
     The switching circuit  104  is configured to receive an input voltage VIN and to further convert the input voltage VIN into an output voltage VOUT. The switch M 1  has a first terminal, a second terminal and a control terminal, wherein the first terminal is configured to receive the input voltage VIN. The switch M 2  has a first terminal, a second terminal and a control terminal, wherein the first terminal is electrically coupled to the second terminal of the switch M 1 , and the second terminal is grounded. The inductor L has a first terminal and a second terminal, wherein the first terminal is electrically coupled to the second terminal of the switch M 1  and the first terminal of the switch M 2 . The output capacitor C is electrically coupled between the second terminal of the inductor L and a reference ground, and a voltage across the output capacitor C is provided as the output voltage VOUT of the switching circuit  104 . 
     The feedback circuit  105  is electrically coupled to the switching circuit  104  to receive the output voltage VOUT and is configured to generate a feedback signal VFB based thereupon. In an embodiment, the feedback circuit  105  comprises a voltage divider consisting of resistors RF 1  and RF 2  which are connected in series. In another embodiment, the feedback circuit  105  may be omitted, and the output voltage VOUT is fed into the control circuit instead of the feedback signal VFB. The feedback signal VFB and the output voltage VOUT may be referred to as a sample signal in general. 
     The control circuit comprises an on-time control circuit  101 , a comparison circuit  102 , a logic circuit  103  and a DC calibration circuit  106 . The DC calibration circuit  106  has a first input terminal  106 A, a second input terminal  106 B and an output terminal  106 C. The DC calibration circuit  106  comprises transconductance amplifiers GM 1  and GM 2 , a capacitor C 1  and a resistor R 1 . The transconductance amplifier GM 1  has a non-inverting input terminal, an inverting input terminal and an output terminal, wherein the non-inverting input terminal is configured as the first input terminal  106 A of the DC calibration circuit  106  to receive a reference voltage VREF, the inverting input terminal which is configured as the second input terminal  106 B of the DC calibration circuit  106  is electrically coupled to the feedback circuit  105  to receive the feedback signal VFB. The transconductance amplifier GM 1  amplifies the difference between the reference voltage VREF and the feedback signal VFB and generates an error current  11  at the output terminal. The transconductance amplifier GM 2  has a non-inverting input terminal, an inverting input terminal and an output terminal, wherein the inverting input terminal is electrically coupled to the output terminal of the transconductance amplifier GM 1  to receive the error current  11 , the output terminal is electrically coupled to the inverting input terminal of the second transconductance amplifier GM 2  and the output terminal  106 C of the DC calibration circuit  106 . The resistor R 1  is electrically coupled between the non-inverting input terminal and the inverting input terminal of the transconductance amplifier GM 2 . The capacitor C 1  is electrically coupled between the non-inverting input terminal of the transconductance amplifier GM 2  and the reference ground. In the embodiment illustrated in  FIG. 1 , an output voltage VO 2  generated by the transconductance amplifier GM 2  is provided as a calibrated reference voltage VREF&#39; generated by the DC calibration circuit  106 . 
     The comparison circuit  102  is electrically coupled to the DC calibration circuit  106  and the feedback circuit  105  to respectively receive the calibrated reference voltage VREF&#39; and the feedback signal VFB. The comparison circuit  102  compares the calibrated reference voltage VREF&#39; with the feedback signal VFB and generates a comparison signal SET. In an embodiment, the comparison circuit  102  comprises a comparator CMP having a non-inverting input terminal, an inverting input terminal and an output terminal. The non-inverting input terminal is configured to receive the calibrated reference voltage VREF&#39;, the inverting input terminal is electrically coupled to the feedback circuit  105  to receive the feedback signal VFB. The comparator CMP compares the calibrated reference voltage VREF&#39; with the feedback signal VFB and generates the comparison signal SET at the output terminal. 
     The on-time control circuit  101  generates an on-time control signal CO to control the on time of the switch M 1 . In an embodiment, the on-time of the switch M 1  is a constant value. In another embodiment, the on-time of the switch M 1  varies with the input voltage VIN and/or the output voltage VOUT. 
     The logic circuit  103  is electrically coupled the on-time control circuit  101  and the comparison circuit  102  to respectively receive the on-time control signal CO and the comparison signal SET. The logic circuit  103  generates control signals based on the on-time control signal CO and the comparison signal SET to control the switches M 1  and M 2 . In an embodiment, the logic circuit  103  comprises a RS trigger. The RS trigger has a set terminal S, a reset terminal R and output terminals Q and Q′, wherein the set terminal S is electrically coupled to the output terminal of the comparison circuit  102  to receive the comparison signal SET, and the reset terminal R is electrically coupled to the on-time control circuit  101  to receive the on-time control signal CO. Based on the comparison signal SET and the on-time control signal CO, the RS trigger generates control signals at the output terminals Q and Q′ to respectively control the switches M 1  and M 2 . 
     In an embodiment, the control circuit may further comprise a minimum off time control circuit. The minimum off time control circuit provides a minimum off time signal to the logic circuit  103  to ensure a minimum off time of the switch M 1 . 
     By applying small signal analysis, the DC calibration circuit  106  of  FIG. 1  may be simplified into a small signal equivalent circuit  106 ′ as shown in  FIG. 2 . The transconductance amplifier GM 2 , the resistor R 1  and the capacitor C 1  are equivalent to a resistor with a resistance of 1/G 2  and a capacitor with a capacitance of C 1 ×R 1 ×G 2  which are connected in series, wherein G 2  is the gain of the transconductance amplifier GM 2 . 
     Therefore, through the transconductance amplifier GM 2  and the resistor R 1 , the equivalent capacitance of the capacitor C 1  is magnified by R 1 ×G 2  times. For example, when R 1 =500 Kohm, G 2 =0.2 ms, C 1 =30 pF, the equivalent capacitance of the capacitor C 1  is magnified by 100 times and is 3 nF. Thus, with the DC calibration circuit  106  shown in  FIG. 1 , the DC calibration can be achieved by a capacitor having smaller capacitance, and such a capacitor may be easily fabricated in an integrated circuit. 
       FIG. 3  illustrates a DC calibration circuit  206  in accordance with another embodiment of the present invention. As shown in  FIG. 3 , the DC calibration circuit  206  comprises a first input terminal  206 A, a second input terminal  206 B and an output terminal  206 C. Compared with the DC calibration circuit  106  illustrated in  FIG. 1 , the DC calibration circuit  206  further comprises a transconductance amplifier GM 3  and a resistor R 2 . The transconductance amplifier GM 3  has a non-inverting input terminal, an inverting input terminal and an output terminal, wherein the non-inverting input terminal is coupled to the output terminal of the transconductance amplifier GM 2  to receive the output voltage VO 2 , the inverting input terminal is configured to receive a bias voltage VBIAS, and the output terminal is electrically coupled to the output terminal  206 C of the DC calibration circuit  206 . The resistor R 2  is electrically coupled between the output terminal of the transconductance amplifier GM 3  and the non-inverting input terminal of the transconductance amplifier GM 1 . The transconductance amplifier GM 3  generates an output voltage VO 3  at its output terminal based on the output voltage VO 2  of the transconductance amplifier GM 2  and the bias voltage VBIAS. In the embodiment illustrated in  FIG. 3 , the output voltage VO 3  of the transconductance amplifier GM 3  is provided as a calibrated reference voltage VREF&#39; of the DC calibration circuit  206 . 
     Under small signal analysis, when VBIAS=VREF/(R 2 ×G 3 ), the DC calibration circuit  206  illustrated in  FIG. 3  may be simplified into a small signal equivalent circuit  206 ′ illustrated in  FIG. 4 . As shown in  FIG. 4 , the transconductance amplifiers GM 2  and GM 3 , the resistors R 1  and R 2  and the capacitor C 1  are equivalent to an equivalent resistor with a resistance of G 3 ×R 2 /G 2  and an equivalent capacitor with a capacitance of C 1 ×R 1 ×G 2 /(G 3 ×R 2 ), wherein G 3  represents the gain of the transconductance amplifier GM 3 . 
     Thus, through the transconductance amplifiers GM 2  and GM 3  and the resistors R 1  and R 2 , the equivalent capacitance of the capacitor C 1  is magnified by R 1 ×G 2 /(G 3 &#39;R 2 ) times. For example, when R 1 =500 Kohm, G 2 =0.2 ms, C 1 =30 pF, R 2 =50 ohm, G 3 =0.2 ms, the equivalent capacitance of the capacitor C 1  is magnified by 10000 times and is 300 nF. Thus, with the DC calibration circuit  206  shown in  FIG. 3 , the DC calibration can be achieved by a capacitor having smaller capacitance, and such a capacitor may be easily fabricated in an integrated circuit. 
     Besides, the use of the transconductance amplifier GM 3  and the resistor R 2  may reduce the fluctuation of the output voltage VO 2  of the transconductance amplifier GM 2 . 
       FIG. 5  illustrates a control method  500  for controlling a switching converter in accordance with an embodiment of the present invention. The switching converter comprises a switch and is configured to provide an output voltage. As shown in  FIG. 5 , the control method  500  comprises steps  501 - 505 . In step  501 , the difference between a reference voltage and a sample signal representative of the output voltage is amplified to generate an error current. The sample signal may be the output voltage of the switching converter, or a feedback signal indicative of the output voltage. In step  502 , a portion of the error current is provided to a capacitor to generate a voltage across the capacitor. In step  503 , the voltage across the capacitor is compared with the sample signal and a comparison signal is generated accordingly. In step  504 , an on-time control signal is provided to control an on time of the switch. In step  505 , a control signal is generated based on the on-time control signal and the comparison signal, so as to control the switch. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not in the prior art.