Patent Publication Number: US-10763748-B2

Title: Buck-boost DC-DC converter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Under 35 U.S.C. § 120, this continuation application claims benefits of and priority to U.S. patent application Ser. No. 15/672,429 (TI-77412), filed on Aug. 9, 2017, the entirety of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     DC-DC converters are switched mode power supplies (SMPS) used in a variety of circuits to provide a DC output signal by converting a DC input signal. The input and output signals can be of the same or opposite polarities. A variety of different DC-DC converter topologies are available, including isolated and non-isolated supplies. Non-isolated SMPS converters include buck, boost, buck-boost, Cuk and other types. Conventional buck-boost DC-DC converters can operate to deliver output voltages above or below the voltage of the input signal, in contrast to buck converters in which the output voltage is less than or equal to the input voltage, or boost converters where the output voltage is greater than or equal to the input voltage. 
     However, switching transistors in conventional buck-boost converters are exposed to voltages equal to the sum of the input voltage and the absolute value of the output voltage. In particular, the voltage swing on a switching node, and the voltage across the inductor of a standard two transistor inventing buck-boost converter, are higher than the switch and inductor voltages of buck or boost converters. 
     The inductor voltage is approximately equal to the input voltage during the on time of the output transistor (the Ton phase), and is approximately equal to the output voltage during the output transistor off time (the Toff phase). Higher inductor voltages lead to larger ripple currents that cause higher inductive switching losses. At the same time, the buck-boost converter switching transistors must be larger than buck or boost converter transistors to accommodate the higher voltage amplitudes. As a result, the larger switching transistors lead to higher switching losses. Thus, while buck-boost converters generally provide more flexibility with respect to output voltage levels, conventional buck-boost converters suffer from lower output current to average inductor current ratios and higher inductor losses compared to buck or boost converters. Consequently, conventional buck-boost converts have lower efficiency, particularly for extreme duty cycle operation. 
     SUMMARY 
     Disclosed examples include buck-boost DC-DC converter circuits and DC-DC conversion methods. A buck-boost converter (switcher) includes a control circuit and a switching circuit. The buck-boost converter can be coupled to an external buck-boost inductor and an external flying capacitor to for a switched mode power supply (SMPS). In one example, the switching circuit includes five transistors to provide a single-ended inverting output voltage signal VOUT, where only one of the transistors is exposed to voltages equal to the sum of the input voltage and the absolute output voltage value. The transistors form two dual transistor leg circuits joined by the series combination of the external buck-boost inductor and the flying capacitor. A fifth transistor connects the capacitor and inductor to a reference node. A sixth transistor can be included in the switching circuit for a bipolar output implementation providing VOUT+ and VOUT−. The control circuit operates the switches to implement a first mode for buck operation and a second mode for boost operation. In the first mode, the switching circuit alternates between first and second buck mode phases. In the first buck mode phase, the switching circuit connects the inductor and the capacitor in series between an input node and a reference node to charge the inductor and the capacitor. In the second buck mode phase, the switching circuit connects the inductor and the capacitor in parallel between an output node and the reference node to discharge the inductor and the capacitor to deliver power to the output node. 
     For boost operation in the second mode, the switching circuit alternates between first and second boost mode phases. In the first boost mode phase, the switching circuit connects the inductor and the capacitor in series between the input node and the reference node to discharge the inductor and charge the capacitor. In the second boost mode phase, the switching circuit connects the inductor between the input node and the reference node to charge the inductor and connects the capacitor between the first output node and the reference node to discharge the capacitor to deliver power to the output node. In certain examples, the switching circuit operates in a third mode for buck-boost operation by sequentially transitioning through three buck-boost mode phases to change operation of the DC-DC converter circuit between the first and second modes. In certain examples, the control circuit switches operation between the first, second and third modes according to a feedback signal and a clock signal. 
     A method is disclosed for switched mode conversion of a DC input voltage signal from an input node to provide a DC output voltage signal, the method useable in a switched mode power supply that includes a buck-boost inductor and a flying capacitor. The method includes alternating between first and second buck mode phases in a first mode when the input voltage signal is greater than an absolute value of the output voltage signal. The first buck mode phase includes connecting the inductor and the capacitor in series with one another between the input node and a reference node to charge the inductor and the flying capacitor. The second buck mode phase includes connecting the inductor and the capacitor in parallel between the first output node and a reference node to discharge the inductor and the capacitor to deliver power to the first output node. The method further includes alternating between first and second boost mode phases in a second mode when the input voltage signal is less than the absolute value of the output voltage signal. The first boost mode phase includes connecting the inductor and the capacitor in series with one another between the input node and the reference node to discharge the inductor and charge the capacitor. The second boost mode phase includes connecting the inductor between the input node and the reference node to charge the inductor, and connecting the capacitor between the first output node and the reference node to discharge the capacitor to deliver power to the first output node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an example five switch buck-boost DC-DC converter circuit, providing regulated VOUT, the buck-boost converter (including a controller and a switch network) for use in a switched mode power supply (SMPS) including an external buck-boost inductor and an external flying capacitor. 
         FIG. 2  is a schematic diagram of an example six switch bipolar split supply buck-boost DC-DC converter circuit, providing regulated VOUT+ and VOUT−, the bipolar buck-boost converter (including a controller and a switch network) for use in a switched mode power supply (SMPS) including an external buck-boost inductor and an external flying capacitor. 
         FIG. 3  is a schematic diagram of the buck-boost converter of  FIG. 1  showing three different operating phases. 
         FIG. 4  is a schematic diagram of the buck-boost converter of  FIG. 1  alternating between two buck mode phases. 
         FIG. 5  is a graph of inductor current, inductor voltage, and flying capacitor voltage signals over an example converter cycle during buck mode operation of the buck-boost converter of  FIG. 1 . 
         FIG. 6  is a schematic diagram of the buck-boost converter of  FIG. 1  alternating between two boost mode phases. 
         FIG. 7  is a graph of inductor current, inductor voltage, and flying capacitor voltage signals over an example converter cycle during boost mode operation of the buck-boost converter of  FIG. 1 . 
         FIG. 8  is a schematic diagram of the buck-boost converter of  FIG. 1  alternating between three buck-boost mode phases. 
         FIG. 9  is a graph of inductor current, inductor voltage, and flying capacitor voltage signals over an example converter cycle during buck-boost mode operation of the buck-boost converter of  FIG. 1 . 
         FIG. 10  is a state diagram of example operation of the control circuit in the buck-boost converter of  FIG. 1  to implement a method for converting a DC input voltage signal to provide a DC output voltage signal using different buck, boost and buck-boost operating modes. 
         FIG. 11  is a graph of an inductor current waveform and transition from buck mode operation to buck-boost mode operation in the buck-boost converter of  FIG. 1 . 
         FIG. 12  is a graph of inductor current for two example converter cycles in buck-boost mode operation of the converter of  FIG. 1 . 
         FIG. 13  is a graph of inductor current for two further example converter cycles in the buck-boost mode operation of the converter of  FIG. 1 . 
         FIG. 14  is a graph of inductor current for two example converter cycles, including transitioning from buck-boost mode operation to boost mode operation of the converter of  FIG. 1 . 
         FIG. 15  is a graph of duty cycle as a function of output voltage for a standard buck-boost converter and the converter of  FIG. 1 . 
         FIG. 16  is an example graph of inductor current change as a function of output voltage for the converter of  FIG. 1  in comparison to an alternative buck-boost converter. 
         FIG. 17  is an example graph of output current to average inductor current ratio as a function of output voltage for the converter of  FIG. 1  in comparison to an alternative buck-boost converter. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, like reference numerals refer to like elements throughout, and the various features are not necessarily drawn to scale. In the following discussion and in the claims, the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are intended to be inclusive in a manner similar to the term “comprising”, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to include indirect or direct electrical or mechanical connection or combinations thereof. For example, if a first device couples to or is coupled with a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via one or more intervening devices and connections. 
     Referring initially to  FIGS. 1 and 2 ,  FIG. 1  shows a five switch, single-ended buck-boost converter  100  that implements an inverting DC-DC converter circuit to convert an input voltage signal VIN to provide a single-ended output voltage signal VOUT. The buck-boost converter  100  includes an input node (terminal)  102  to receive the input voltage signal VIN and a first output node (terminal)  104  to provide the output voltage signal VOUT. The first output node  104  provides a first output voltage signal VOUT that is negative relative to a reference node  106  at a ground or common reference potential by converting a positive input signal VIN, although input-output polarity inversion is not a strict requirement of all examples. The circuit in  FIG. 1  provides a single first output signal VOUT at the first output node  104 .  FIG. 2  shows a bipolar buck-boost DC-DC converter circuit  100  with two output nodes  104  and  204  to provide a bipolar split supply output VOUT−, VOUT+. 
     The single-ended buck-boost converter  100  of  FIG. 1 , and the bipolar (split supply) buck-boost converter  100  of  FIG. 2 , are for use in a switched mode power supply (SMPS) including an external buck-boost inductor L, and, according to aspects of this disclosure, an external flying capacitor CF, Inductor L and capacitor CF are connected at an intermediate node  110 , and are coupled to the buck-boost converter  100  ( FIGS. 1 and 2 ) through nodes (terminals) SW, CAP 1  and CAP 2 . The buck-boost converters  100  of  FIGS. 1 and 2  can be implemented as an SMPS module that incorporates the inductor L and capacitor CF. 
     The single-ended buck-boost converter  100  in  FIG. 1  includes a switching circuit with transistor switches M 1 -M 5  and the control circuit  120 . Any suitable switches can be used, such as field effect transistors, bipolar transistors or other semiconductor-based switches operable according to corresponding switching control signals from the control circuit  120 . In the example of  FIG. 1 , the switches M 1 -M 5  are NMOS transistors with gate control terminals to receive switching control signals. The control circuit  120  includes outputs  121 ,  122 ,  123 ,  124  and  125  that provide switching control signals to the gates of the transistors M 1 -M 5  according to a state machine  130 . The control circuit  120  can be any suitable logic circuit with suitable driver circuitry to implement the switch control functions described herein. In certain examples, the control circuit  120  may be programmed or programmable, although programmability is not a strict requirement of all possible implementations. The buck-boost converter  100  can be implemented as a controller integrated circuit  120 , providing switch control inputs to an external switch network (e.g., switches M 1 -M 5 ), the switch network coupled to the external buck-boost inductor L and flying capacitor CF. In another non-limiting example, an integrated circuit  101  includes a switching circuit as well as a control circuit  120 , with suitable terminals (e.g., pins or pads) allowing electrical interconnection for input and output voltage signals, ground fault common reference connections, as well as the terminals for connection to an external inductor L and/or an external capacitor CF. In certain examples, moreover, one or both of the inductor L and/or the capacitor CF can be incorporated into an integrated circuit that also includes one or more of the converter switches and/or a control circuit  120 . In further non-limiting examples, one or more of the switches can be external to the IC  101 . 
       FIG. 1  illustrates one example in which an integrated circuit  101  includes the control circuit  120  as well as transistors M 1 -M 5 . This example includes IC terminals for connecting an external inductor L as well as an external capacitor CF. The IC  101  in  FIG. 1  also includes a separate terminal for connection of a sense signal to indicate a current flowing through the inductor. 
     The switching circuit is coupled to the input node  102 , and the first output node  104 , and to the nodes that connect to the external buck-boost inductor L and flying capacitor CF. In the illustrated example, the IC  101  includes a first switching node terminal SW connected to the first switching node  108 , and the first switching node terminal SW allows electrical connection to a first terminal of an external buck-boost inductor L. In addition, the illustrated IC  101  includes a first capacitor terminal CAP 1  connected to the intermediate node  110 , and the terminal CAP 1  allows electrical connection to a second terminal of the inductor L, and to a first terminal of the capacitor CF. The illustrated IC  101  also includes a second capacitor terminal CAP 2  connected to a second switching node  112 , which allows electrical connection to a second terminal of the external capacitor CF. 
     The converter IC  101  includes a pair of transistors M 1  and M 2  connected in series with one another to form a first circuit branch or leg circuit between the input and output nodes  102  and  104 . The first transistor M 1  includes a gate connected to the control circuit output  121 , a drain connected to the input node  102  and a source connected to a first switching node  108  connected to a first switching node (terminal) SW, with a first switching node voltage VSW at the first switching node terminal designated SW. The second transistor M 2  includes a gate connected to the control circuit output  122 , a drain connected to first switching node  108  connected to the first switching node terminal SW, and a source connected to the first output node  104 . A second pair of transistors M 4  and M 5  is connected in series with one another to form a second branch or circuit leg circuit between the reference node  106  and the first output node  104 . The fourth transistor M 4  includes a gate connected to the control circuit output  124 , a drain connected to the reference node  106  and a source connected to a second switching node  112  connected to node (terminal) CAP 2 . The fifth transistor M 5  includes a gate connected to the control circuit output  125 , a drain connected to the second switching node  112  and a source connected to the first output node  104 . For the buck-boost converter  100  of  FIGS. 1 , M 4  and M 5  can be replaced by diodes. 
     The buck-boost converter  100  is coupled to the (external) buck-boost inductor L connected between the first switching node (terminal SW), connected to the first switching node  108  between with a first inductor terminal connected to the first switching node (terminal) SW connected to the first switching node  108  and a second inductor terminal connected through the intermediate node  110  to the (external) flying capacitor CF. In operation, the inductor L conducts an inductor current designated IL in  FIG. 1 , and the inductor L has an inductor voltage designated VL. 
     The control circuit  120  receives from a current sensor  114 , through a sense node (terminal) SENS, an inductor current feedback signal. In certain examples, the inductor current IL is sensed by measuring the voltage drop across one or more of the transistors M 1 -M 5  of the switching circuit. The control circuit  120  in certain examples compares the measured current IL with a threshold to detect whether the current IL has reached a predetermined peak threshold value Ipeak. The control circuit  120  also receives further feedback signals including the input voltage VIN and the output voltage VOUT through appropriate feedback connections (not shown). In certain examples, the control circuit  120  receives a clock signal clk for timing control to implement a continuous series of converter cycles for closed loop control and regulation of the output voltage VOUT according to an internal or external setpoint value. 
     The capacitor CF is connected in series with the inductor L between the intermediate node  110 , connected to the CAP 1  node (terminal), and the second switching node  112  (CAP 2  terminal). The second switching node  112  joins the transistors M 4  and M 5 . The capacitor CF operates as a flying capacitor to control the inductor voltage. The capacitor CF has an associated capacitor voltage VCF, with the voltage at the intermediated node  110  connected to the CAP 1  terminal designated as VCAP 1 , and the voltage at the CAP 2  terminal connected to the second switching node  112  designated as VCAP 2  in  FIG. 1 . The switching circuit also includes a third transistor M 3  with a gate connected to the control circuit output  123 , a drain connected to the CAP 1  terminal, connected to the intermediate node  110 , and a source connected to the reference node  106 . 
     The first output node  104  provides the output voltage signal VOUT to drive an external load, shown as a resistor RL 1  in  FIG. 1 , to conduct an output current IOUT. An output capacitor CO 1  can be coupled in parallel with the load RL. 
     The bipolar (split supplies) example in  FIG. 2  includes a bipolar buck-boost converter  100  coupled to first and second loads RL 1  and RL 2 , as well as first and second output capacitors CO 1  and CO 2 . The bipolar buck-boost converter  100  in  FIG. 2  provides first and second voltage output signals VOUT− and VOUT+ and corresponding output current signals IOUT−, IOUT+ at the corresponding output nodes (terminals)  104  and  204 . 
     For the bipolar buck-boost converter  100  of  FIG. 2 , M 4 , M 5  and M 6  can be replaced by diodes. 
     For bipolar boost converter applications where VIN&lt;|VOUT|, the switches M 1  and M 2  can be removed from the converter  100  of  FIG. 2  and the inductor L can be directly connected to the input node  102  to provide a four switch (M 3 -M 6 ) boost converter topology. 
       FIG. 3  shows three possible operating phases in the buck-boost converter  100 , including dashed-lines showing current flow paths for three example phases PhA, PhB and PhC. The switching circuit operates according to the switching control signals provided at the outputs  121 - 125  of the control circuit  120  to implement two or more distinct phases PhA, PhB and PhC in each of two or three operating modes in each converter cycle. This operation provides a first mode (BUCK) for buck operation when the input voltage signal VIN is greater than the absolute value |VOUT| of the output voltage signal VOUT and a second mode (BOOST) for boost operation when the input voltage signal VIN is less than the absolute value |VOUT|. In certain implementations, the switching circuit operates in a third mode (BUCK-BOOST) for buck-boost operation when VIN is within a non-zero pre-defined range of |VOUT|. In the first mode, the control circuit state machine  130  alternates operation between first and second buck mode phases PhB and PhC, respectively. In the second mode, the state machine  130  alternates operation between first and second boost mode phases PhB and PhA, respectively. In the third (buck-boost) mode, the control circuit state machine  130  sequentially transitions through first, second and third buck-boost mode phases PhB, PhA and PhC. 
     In one example, the switching signals and the corresponding switch states for phase PhA are the same for the second and third modes. During the PhA operation, the control circuit  120  turns the transistors M 1 , M 3  and M 5  on and turns M 2  and M 4  off. In this state, the inductor L is connected between the input node  102  and the reference node  106  and the capacitor CF is connected between the first output node  104  and the reference node  106 . The switching signals and the corresponding switch states for PhB are the same for the first, second and third modes. In this state, the control circuit  120  turns the transistors M 1  and M 4  on and turns M 2 , M 3  and M 5  off. This connects the inductor L and the capacitor CF in series with one another between the input node  102  and the reference node  106 . The switching signals and the corresponding switch states for PhC are the same for the first and third modes. In the phase PhC, the control circuit  120  turns M 2 , M 3  and M 5  on and turns M 1  and M 4  off. This state connects the inductor L and the capacitor CF in parallel between the first output node  104  and the reference node  106 . 
     In operation, the control circuit  120  provides switching control signals of appropriate levels at the outputs  121 - 125  to turn the switch transistors M 1 -M 5  on and off to implement various switching states according to a current operating mode. In one example, the transistors M 1 -M 5  are NMOS transistors and the control circuit  120  provides active high switching control signals to turn individual ones of the transistors M 1 -M 5  on, and low switching control signals to turn the corresponding transistor off. For buck operation in the first mode, the control circuit  120  provides signals at the outputs  121 - 125  to alternate switching circuit operation between the first and second buck mode phases PhB and PhC. In the illustrated example, moreover, each converter cycle begins with operation in the phase PhB, although not a strict requirement of all possible implementations. In the first mode (BUCK), the control circuit  120  implements the individual control cycles from clock to clock based on the clock signal clk. Similarly, in the third mode (BUCK-BOOST), the control cycles extend from clock to clock. The second mode (BOOST) in one example includes control cycles extending from a maximum time Tmax to a subsequent time Tmax, where Tmax in one example is (0.8 or 0.9)*Tsw, and the switching cycle period Tsw starts with a rising edge of the clk signal. 
     Various concepts are described below in the context of the single-ended inverting buck-boost converter circuit  100  of  FIG. 1 . The following description is also applicable to bipolar implementations, for example, as shown in  FIG. 2 . In particular, the bipolar implementations include the sixth transistor switch M 6 , which is turned on in the PhB phase of the first, second and third modes. In certain bipolar converter implementations, moreover, the control circuit  120  turns M 6  off before turning the first and fourth transistors M 1  and M 4  off. 
     Referring now to  FIGS. 4 and 5 ,  FIG. 4  shows operation of the single-ended buck-boost converter  100  in the first mode (BUCK), with dashed lines showing the current flow paths during buck mode phases designated as PhB and PhC. The switching circuit M 1 -M 5 ,  120  operates in a first mode BUCK to alternate between first and second buck mode phases PhB and PhC for buck operation when the input voltage signal VIN is greater than the output voltage absolute value |VOUT|. In the first buck mode phase PhB, the control circuit  120  asserts the control signals on the outputs  121  and  124  (e.g., active high) to turn the transistors M 1  and M 4  on, and provides appropriate control signals (e.g., low) on the outputs  122 ,  123  and  125  to turn the transistors M 2 , M 3  and M 5  off. This connects the inductor L and the capacitor CF in series between the input node  102  and the reference node  106 , providing a charge current flow path designated PhB in  FIG. 4  from the input voltage signal VIN through the inductor L and the flying capacitor CF in order to charge the inductor L and the capacitor CF. In the first buck mode phase PhB, the average capacitor voltage VCF is approximately |VOUT|, and the inductor voltage VL is approximately VIN−|VOUT|. 
     In the second buck mode phase PhC, the control circuit  120  provides switching control signals at the outputs  121 - 125  to turn M 2 , M 3  and M 5  on, and to turn M 1  and M 4  off. The dashed line current flow path designated PhC in  FIG. 4  illustrates operation in this state, where the inductor L and the capacitor CF are connected in parallel between the first output node  104  and the reference node  106 . In this phase, moreover, the inductor L and the capacitor CF are at least partially discharged to deliver power to the first output node  104 . In the second buck mode phase PhC, the capacitor voltage VCF is approximately equal to |VOUT|, and the inductor voltage is approximately |VOUT|. The control circuit  120  alternates between the first and second buck mode phases PhB and PhC in this manner, and controls the duty cycle in closed loop fashion according to one or more of the feedback signals or values in order to regulate the output voltage signal VOUT when VIN&gt;|VOUT. In this example, the ratio of the output voltage to the input voltage in buck mode operation is given by the following equation VOUT/VIN=−Db, where Db is the on-time duty cycle ratio of the operation in the first buck mode phase PhB during each converter cycle, and the converter cycles have a temporal duration of Tsw. 
       FIG. 5  illustrates a graph  500  including an inductor current curve  502  (IL), an inductor voltage curve  504  (VL), and a capacitor voltage curve  506  over an example converter cycle  501  (BUCK MODE CYCLE) during the first mode (BUCK) in the buck-boost converter  100 . The illustrated cycle  502  begins with PhB and concludes with operation in the phase PhC. As seen in  FIG. 5 , the inductor current IL (curve  502 ) increases and the capacitor voltage VCF (curve  506 ) increases during the first phase PhB, and the inductor voltage (curve  504 ) is approximately VIN−|VOUT|. During the second buck mode phase PhC, the inductor current IL and the capacitor voltage VCF decrease as the load is provided with power, and the inductor voltage is approximately equal to VOUT. The control circuit  120  implements a series of these alternating phases in a generally continuous sequence of converter cycles for operation in the first mode. The control circuit  120  also implements mode switching between the first mode and the second mode, and certain implementations include operation in a third mode for buck-boost control using three phases as discussed further below. 
     Referring now to  FIGS. 6 and 7 , the control circuit  120  implements operation in the second mode (BOOST) when VIN&lt;|VOUT|.  FIG. 6  shows the buck-boost converter  100  alternating between first and second boost mode phases PhB and PhA, where the switch states of the first boost mode phase PhB are the same as are used in the first buck mode phase described above. In particular, the control circuit  120  provides the control signals at the outputs  121 - 125  in the first boost mode phase PhB to turn M 1  and M 4  on and to turn M 2 , M 3  and M 5  off. This connects the inductor L and the capacitor CF in series between the input node  102  and the reference node  106 , where the current flow path for this phase is designated PhB in  FIG. 6 . This switching circuit state discharges the inductor L and charges the capacitor CF. 
       FIG. 7  shows a graph  700  including an inductor current curve (IL)  502 , an inductor voltage curve (VL)  504 , and a capacitor voltage curve (VCF)  506  over an example boost mode converter cycle  702 . In the boost mode, the capacitor voltage is increased in the first phase PhB by charging using current flow IL from the inductor L. During this phase, moreover, the inductor voltage VL (curve  504  in  FIG. 7 ) is approximately VIN 
     In the second boost mode phase PhA, the control circuit  120  provides the switching control signals to turn M 1 , M 3  and M 5  on, and to turn M 2  and M 4  off. Turning on M 1  and M 3  connects the inductor L between the input node  102  and the reference node  106  to charge the inductor L. In addition, turning on M 3  and M 5  connects the capacitor CF between the first output node  104  and the reference node  106  to discharge the capacitor CF and deliver power to the first output node  104 . The current flow paths for the second boost mode phase are designated as PhA in  FIG. 6 . As seen in the graph  700  of  FIG. 7 , the inductor current IL increases during the second boost mode phase PhA (curve  502 ). In addition, the capacitor voltage VCF decreases as the capacitor CF delivers the output current IOUT to the output node  104  and the second boost mode phase. As shown in the curve  506  of  FIG. 7 , moreover, the average capacitor voltage VCF during the boost mode operation is approximately equal to −VOUT. During this phase, moreover, the inductor voltage VL (curve  504 ) is approximately equal to VIN. The ratio of the output voltage to the input voltage in the boost mode operation is given by the following equation VOUT/VIN=−1/(1−Da), where Da is the on-time duty cycle ratio of the operation in the second boost mode phase PhA during each converter cycle. In this example, moreover, M 1  is permanently on and M 2  is permanently off in phase PhA and phase PhB in the boost. This further reduces the switching losses. 
       FIGS. 8 and 9  illustrate a third operating mode (BUCK-BOOST) implemented by the control circuit  120  when the input voltage VIN is approximately equal to −VOUT. In the third mode, the control circuit  120  generates the switching control signals at the outputs  121 - 125  to sequentially transition through first, second and third buck-boost mode phases PhB, PhA and PhC for buck-boost operation to change operation of the DC-DC converter circuit  100  between the first and second modes. The buck-boost operating mode, moreover, may include a single converter cycle or any number of converter cycles during which the three phase sequence is repeated. The control circuit  120  in one example implements the same switch states corresponding to the above-described phases PhB, PhA and PhC, although not a requirement of all possible implementations. 
       FIG. 8  illustrates the buck-boost converter circuit  100 , and illustrates the corresponding current flow paths designated PhB, PhA and PhC corresponding to the first, second and third buck-boost mode phases implemented by the control circuit  120 .  FIG. 9  shows a graph  900  illustrating the inductor current and voltage curves  502  and  504 , as well as the capacitor voltage curve  506  over an example converter cycle  902  during buck-boost mode operation of the circuit  100 . 
     The control circuit  120  provides the switching control signals at the outputs  121 - 125  in the first buck-boost mode phase PhB to turn M 1  and M 4  on and to M 2 , M 3  and M 5  off. This connects the inductor L and the capacitor CF in series with one another between the input node  102  and the reference node  106 . As seen in  FIG. 9 , the inductor L is charged and the capacitor CF is charged in the first buck-boost mode phase PhB, and the inductor voltage VL is approximately VIN−|VOUT|. As shown in the curve  506  of  FIG. 9 , the average capacitor voltage VCF is approximately equal to −VOUT throughout the buck-boost mode cycle  902 . In the second buck-boost mode phase PhA, the control circuit  120  turns M 1 , M 3  and M 5  on and turns M 2  and M 4  off. This connects the inductor L between the input node  102  and the reference node  106  to charge the inductor L, and also connects the capacitor CF between the first output node  104  and the reference node  106  to discharge the capacitor CF to deliver power to the first output node  104 . In the third buck-boost mode phase PhC, the control circuit  120  turns M 2 , M 3  and M 5  von and turns M 1  and M 4  off. This connects the inductor L and the capacitor CF in parallel between the first output node  104  and the reference node  106  to discharge the inductor L and the capacitor CF to deliver power to the first output node  104 . 
     As described above, the buck-boost converter circuit  100  advantageously provides the ability to generate output voltages VOUT that are less than, approximately equal to, or greater than the input voltage VIN. The converter circuit  100 , moreover, advantageously reduces the voltage stress on certain of the switching transistors M 1 -M 5 . As mentioned above, for example, both switching transistors in conventional dual-switch buck-boost converters experience voltages of approximately VIN+|VOUT|. In contrast, example voltage ratings for the illustrated buck-boost converter circuit  100  are as follows: 
     M 1 :2.1*VIN voltage rating 
     M 2 :VIN+|VOUT| voltage rating 
     M 3 , M 4  and M 5 :&gt;|VOUT| voltage rating 
     Thus, only M 2  needs to be sized according to the voltage amount VIN+|VOUT|. The reduced switching transistor sizes, in turn, reduces switching losses in the converter  100  compared with conventional designs. Furthermore, the converter inductor L is exposed to lower maximum voltages than was the case for conventional designs. The illustrated circuit  100  advantageously reduces the voltage stress on the inductor L, and also reduces the inductor ripple current levels compared with conventional buck-boost converters. These advantages lead to significantly improved converter efficiency, making buck-boost converters applicable to a larger number of potential applications. 
     Referring again to  FIG. 2 , the bipolar implementation achieves these same advantages, and provides a differential or bipolar output VOUT−, VOUT+ at the first and second output terminals  104 ,  204 . In this example, the first output node  104  provides a first output voltage signal VOUT− that is negative relative to the voltage of the reference node  106 , and the second output node  204  provides the second output voltage signal VOUT+ that is positive relative to the voltage of the reference node  106 . The sixth transistor M 6  is connected between the intermediate node  110  and the second output node  204 , and is turned on in phase PhB and turned off in the phases PhA and PhC. In one example, the control circuit  120  provides a control signal to the gate of the transistor M 6  via a corresponding output (not shown) to turn M 6  on when the third transistor M 3  is turned off, and to turn M 6  off when M 3  is turned on. In another possible example, the control circuit  120  turns M 6  off before turning M 1  and M 4  off to accommodate situations where the energy consumption in VOUT+ is lower than in the negative output rail VOUT−. For the bipolar example of  FIG. 2 , example transistor voltage ratings are as follows: 
     M 1 :2.1*VIN voltage rating 
     M 2 :VIN+|VOUT| voltage rating 
     M 3 , M 4 , M 5  and M 6 :|VOUT| voltage rating 
     As seen above, the single-ended and bipolar implementations of the circuit  100  provide a 5 or 6 switch topology which can advantageously employ power switches with smaller voltage ratings. In addition, the circuit  100  operates the inductor L with reduced ripple current and higher ratio ILaverage/Iout of the average inductor current to the output current. In addition, the problem of high voltages across the power switches and the inductor is most severe with large magnitudes of the output voltage VOUT. As opposed to conventional designs, the disclosed buck-boost converter  100  advantageously operates at lower duty cycles for the same VIN/VOUT ratio compared to the standard topologies through intelligent mode selection by the control circuit  120 . In particular, the converter  100  addresses the high-voltage problems with two different operational modes, including the first buck mode (when VIN&gt;|VOUT|) and the second boost mode (when VIN&lt;|VOUT|). As seen above, the duty cycles in the first and second modes are 50% or less. In addition, the control circuit  120  in certain examples advantageously provides for operation of the third (BUCK-BOOST) mode for changing between the first and second modes. This advantageously provides seamless transition between the inverting buck and inverting boost operation modes. 
     Referring now to  FIGS. 10-17 , the control circuit  120  in certain implementations includes a state machine  130 , which can be implemented in any suitable circuitry, whether programmable or not.  FIG. 10  provides a state diagram  1000  that illustrates example operation of the state machine  130  of the control circuit  120 . The state diagram  1000  is used in certain examples to implement a method for converting a DC input voltage signal to provide a DC output voltage signal using different buck, boost and buck-boost operating modes as previously described. In one example, the state machine  130  switches between the first, second and third modes according to a feedback signal Ipeak and the clock signal clk. In one example, the inductor current feedback signal IL is received by the control circuit  120  and is provided as an input to a comparator (not shown). The comparator compares the inductor current signal IL with a threshold signal representing a predetermined peak value for the inductor current (Ipeak). The output of the comparator is used by the state machine  130  as a variable, where Ipeak can be =0 or =1 depending on the inductor current value measured at any given time. The state machine also receives an enable input “en”, where an enable variable value of 0 indicates that the circuit is disabled, and an enable variable value of “1” indicates that the circuit is enabled. The state machine  130  also operates according to the clock signal clk, and a signal Tmax. In the illustrated implementation, the Tmax has a value of 0 before a predetermined time has elapsed within a given converter cycle, and once the predetermined time has elapsed within the converter cycle, the variable Tmax has a value of 1. In one example, Tmax is (0.8 or 0.9)*Tsw, and the switching cycle period Tsw starts with a rising edge of the clk signal. 
     The state machine operation begins in  FIG. 10  at state  1002  (St 0 ) when the circuit is off (en=0). Once the circuit  100  is enabled, the state machine  130  moves to the state  1004  (SU). The state transitions thereafter follow different paths depending on the appropriate operating mode of the buck-boost converter  100 . In particular, buck mode operation is designated by a dashed line  1021 , boost mode operation is designated by a dashed line  1022 , and buck-boost mode operation is designated by a dashed line  1023  in  FIG. 10 . As seen in the above description of the first, second and third modes, each mode begins with the phase PhB, shown as state  1004  in  FIG. 10 . Operation proceeds to the left to state  1006  (PhC) if the variable Ipeak=1 before the variable Tmax transitions to “1”. This means that the inductor current reaches the predefined threshold peak value before the predefined amount of time since the beginning of the current converter cycle. If the converse is true (Tmax is set to “1” prior to Ipeak being set to “1”), control transfers from the state  1004  to the right to state  1010  (PhA) for boost mode operation. In the buck mode control path  1021 , the state machine  130  transitions from the PhC state  1006  back to the PhB phase at state  1008 , and thereafter returns for the next cycle when the clock signal clk=0 and Ipeak=0 back to the state  1004 . 
     For boost mode operation, the cycle begins at the state  1004 , and transitions when Tmax=1 to the state  1010  for operation in the boost mode phase PhA. From the state  1010 , control proceeds along the boost transition path  1022  if the next clock signal (clk=1) is received prior to the circuit reaching the peak inductor current (Ipeak=1). In this case, control is transferred to continue the PhA phase operation at state  1014 . In this case, once the Ipeak variable then reaches “1”, the state machine  130  transitions to the PhB boost mode phase at state  1016 . The control circuit  120  then returns to the state  1004  when clk=0, Tmax=0 and Ipeak=0. 
     The state machine  130  can enter the buck-boost mode path  1023  from the PhA state  1010  if the peak current is reached (Ipeak=1) before the next clock signal (e.g., before clk=1). In this case, the state machine transitions to the state  1012  to begin the phase PhC operation. The phase PhC in this case continues with transition to the state  1006  when Tmax=0, and the control circuit transitions to the phase PhB operation at the state  1008  and then the state  1004  as described above. As seen in the state diagram  1000  of  FIG. 10 , the control circuit  120  implements the state machine  130  in order to selectively operate in one of the three modes according to the feedback signal IL and the clock signal clk. 
       FIGS. 11-14  illustrate inductor current signal waveforms as the control circuit  120  transitions through the various operational modes in the circuit  100 .  FIG. 11  provides a diagram  1100  that shows the inductor current curve  502  for an example buck mode cycle on the left (path  1021  in  FIG. 10 ), followed by a subsequent transition to the three phase buck-boost mode cycle on the right (path  1023  in  FIG. 10 ).  FIG. 12  shows a diagram  1200  indicating continued buck-boost mode operation for two example converter cycles, and a diagram  1300  in  FIG. 13  illustrates additional example buck-boost mode converter cycles in the circuit  100 .  FIG. 14  shows another buck-boost mode cycle (on the left), and subsequent transition to the boost mode (path  1022  in the state diagram of  FIG. 10 ). As seen in  FIGS. 11 through 14 , the relative timing of the clock signal and the point at which the inductor current IL reaches the peak value Ipeak determines whether the state machine receives the Ipeak=1 transition before or after the Tmax signal transitions to “1” within a given converter cycle. In the illustrated example of  FIGS. 11-14 , the relative timing between Ipeak=1 and Tmax gradually increases until the peak current has not been reached by the time the next converter cycle begins (e.g., on the right in  FIG. 14 ). At this point, the control circuit  120  transitions out of the buck-boost mode and into the boost mode operation (path  1022  in  FIG. 10 ). 
       FIGS. 15-17  show comparative simulation results for a conventional two switch buck-boost converter and the buck-boost converter  100  of  FIG. 1  for an input voltage VIN of 3.75 volts. A graph  1500  in  FIG. 15  illustrates a graph  1500  showing duty cycle as a function of the output voltage VOUT, including a curve  1502  for a conventional two switch buck-boost converter and a curve  1504  for the buck-boost converter  100  of  FIG. 1 . The duty cycle performance of the converter  100  is significantly lower for boost operation above approximately −4.00 V and for buck-boost operation between −4.0 V and −4.2 V. For output voltages VOUT below approximately 3.6 V, the duty cycle drops from approximately 0.93 to less than 0.20 as the VOUT approaches −0.5V.  FIG. 16  shows a graph of ΔIL as a function of the VOUT, including a curve  1602  for a conventional two switch buck-boost converter and a curve  1604  for the buck-boost converter  100 . The curve  1604  demonstrates a significant reduction in the inductor current change ΔIL compared with the conventional converter.  FIG. 17  shows an IOUT/ILAVG vs. VOUT graph  1700  including a curve  1702  for a conventional two switch buck-boost converter and a curve  1704  for the buck-boost converter  100  of  FIG. 1 . The new design  100  provides improved efficiency with a significantly higher output current to inductor current ratio IOUT/ILAVG as shown in the curve  1704 . 
     Disclosed examples provide many advantages over conventional buck-boost converters, including reduced voltage ratings of at least some of the power switches and reduced voltage swing on the inductor L compared to a standard inverting buck boost topology. In addition, the converter  100  provides automatic reset of the CF bias voltage to the output signal VOUT, and the novel use of the flying capacitor CF provides a voltage source with the average VCF approximately equal to |VOUT|. The negative voltage capacitor node  112  node sees only CF and not the inductor L, which leads to little or no overshoots or undershoots on the voltage at the node  112 , thereby reducing the risk of circuit latchup. The converter design  100  also facilitates the use of a smaller inductor size for the same VIN/VOUT ratio, as well as higher efficiency, particularly for large |VOUT|/VIN ratios. Moreover, the converter  100  can be expanded with a sixth switch M 6  as shown in  FIG. 2  for a bipolar split supply topology to generate symmetrical positive and negative output voltages, which can be beneficial for amplitudes modulated organic LED (OLED) power supplies. In addition, the converter  100  significantly reduces the inductor voltage VL and facilitates reduced peak inductor currents, the use of a smaller inductor L and lowered inductor resistive losses to support of higher load currents and the use of peak current control. These benefits lead to improved peak efficiency, particularly in boost mode operation. Moreover, the converter  100  provides better bandwidth, transient response and stability along with easier compensation by moving the right half plane zero to a slightly higher frequency compared to conventional buck-boost converters. 
     The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.