CN214045442U - Control circuit and switching converter using same - Google Patents
Control circuit and switching converter using same Download PDFInfo
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- CN214045442U CN214045442U CN202022662989.6U CN202022662989U CN214045442U CN 214045442 U CN214045442 U CN 214045442U CN 202022662989 U CN202022662989 U CN 202022662989U CN 214045442 U CN214045442 U CN 214045442U
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Abstract
The application discloses a control circuit and a switching converter applying the same. The utility model discloses a switch converter acquires output current feedback signal and inductive current sampling signal through sharing a sampling resistor and according to in order to generate on-off control signal PWM to make negative feedback system's direct current gain and phase allowance not receive the influence of output current variation range broad. And further, by adopting a quasi-fixed frequency control mode, the switching period of the switching converter can be maintained to be basically constant under a normal conversion ratio.
Description
Technical Field
The utility model relates to a power electronics technique, concretely relates to switch converter and control circuit thereof.
Background
A switching converter is a power conversion circuit capable of converting an input voltage into another fixed output signal or an adjustable output signal through different forms of architectures, and thus is widely used in electronic products such as mobile devices. Constant off time (CFT) control is a Pulse Frequency Modulation (PFM) control technique that regulates the output voltage by controlling the power transistors of the switching converter to turn off for a fixed time in each switching cycle.
In the prior art, an IR LED driver for security monitoring generally has a wide input voltage range and a wide output voltage range, so a reliable control scheme for a wide conversion ratio of a switching converter has a very strong application requirement.
SUMMERY OF THE UTILITY MODEL
In view of the above, embodiments of the present invention provide a switching converter and a control circuit thereof, so that the dc gain and the phase margin of the negative feedback system are not affected by the wide variation range of the output current.
According to the utility model discloses an aspect of the embodiment provides a control circuit for switching converter, its characterized in that, control circuit acquires output current feedback signal and inductive current sampling signal and according to the generation switch control signal through sharing a sampling resistance, in order to strengthen switching converter's negative feedback system is in the stability under the wide range output current change.
Preferably, the output current feedback signal and the inductor current sampling signal are obtained by sharing the sampling resistor, so that the direct current gain of the negative feedback system is independent of the change of the output current, and the stability of the negative feedback system of the switching converter under the wide-range output current change is enhanced.
Preferably, the phase margin of the negative feedback system is not affected by the output current by making the dc gain of the negative feedback system independent of the change of the output current.
Preferably, when the output current changes, the ratio of the output current feedback signal to the inductor current sampling signal does not change.
Preferably, the resistance values of the sampling resistors correspond to the magnitude of the expected output current in a one-to-one manner.
Preferably, the sampling resistor is connected in series with an inductance of the switching converter.
Preferably, the output capacitor is connected in parallel with the load and then connected in series with the sampling resistor.
Preferably, the sampling resistor is connected between the inductor and a common terminal of the output capacitor and an anode of the LED load.
Preferably, the sampling resistor is connected between the common terminal of the output capacitor and the cathode of the LED load and a reference ground.
Preferably, the control circuit is configured to generate a switch control signal to control a switching state of a main power transistor of the switching converter in dependence on the output current feedback signal and the inductor current sampling signal.
Preferably, the control circuit obtains an error amplification signal between the output current feedback signal and a current reference value of the output current by using a transconductance amplifier to generate a compensation signal, and changes an effective state of the switch control signal when the inductor current sampling signal rises to the compensation signal.
Preferably, the control circuit is configured to control the switching state of a main power transistor of the switching converter in a quasi-fixed frequency control manner.
Preferably, the control circuit generates a timing signal according to a duty cycle of the switching converter and generates a timing reference signal inversely related to the duty cycle of the switching converter, and the switching state of the main power transistor is changed when the timing signal reaches the timing reference signal.
According to a second aspect of an embodiment of the present invention, there is provided a switching converter, comprising;
a power stage circuit comprising a power transistor coupled to an inductor element, wherein the inductor element couples an input; and the number of the first and second groups,
the control circuit is used for controlling the power transistor to generate an output signal at an output end to drive a load.
The embodiment of the utility model provides a switch converter acquires output current feedback signal and inductive current sampling signal through sharing a sampling resistance and according to in order to generate on-off control signal PWM to make negative feedback system's direct current gain and phase allowance not receive the influence of output current variation range broad. And further, by adopting a quasi-fixed frequency control mode, the switching period of the switching converter can be maintained to be basically constant under a normal conversion ratio.
Drawings
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:
fig. 1 is a circuit diagram of a first comparative switching converter of the present invention;
fig. 2 is a circuit diagram of a second comparative switching converter of the present invention;
fig. 3 is a circuit diagram of a switching converter according to an embodiment of the present invention;
fig. 4 is a small signal model equivalent circuit of the control loop of the present invention;
FIG. 5 is a comparison of a bode plot of a transfer function of the present invention;
fig. 6 is a comparison of bode plots of another transfer function of the present invention;
fig. 7 is a circuit diagram of a first control circuit of the present invention.
Detailed Description
The present invention will be described below based on examples, but the present invention is not limited to only these examples. In the following detailed description of the present invention, certain specific details are set forth in detail. It will be apparent to those skilled in the art that the present invention may be practiced without these specific details. Well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
Further, those of ordinary skill in the art will appreciate that the drawings provided herein are for illustrative purposes and are not necessarily drawn to scale.
Meanwhile, it should be understood that, in the following description, a "circuit" refers to a conductive loop constituted by at least one element or sub-circuit through electrical or electromagnetic connection. When an element or circuit is referred to as being "connected to" another element or element/circuit is referred to as being "connected between" two nodes, it may be directly coupled or connected to the other element or intervening elements may be present, and the connection between the elements may be physical, logical, or a combination thereof. In contrast, when an element is referred to as being "directly coupled" or "directly connected" to another element, it is intended that there are no intervening elements present.
Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is, what is meant is "including, but not limited to".
In the description of the present invention, it is to be understood that the terms "first," "second," and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance. In addition, in the description of the present invention, "a plurality" means two or more unless otherwise specified.
Fig. 1 is a circuit diagram of a first comparative switching converter of the present invention. As shown in fig. 1, the switching converter of the present comparative example includes a power stage circuit. The power stage circuit is a BUCK topology and specifically comprises a main power transistor Q1, wherein a first end of the main power transistor Q1 receives an input voltage Vin; an inductor L having a first terminal connected to the second terminal of the main power transistor Q1 and a second terminal connected to the output terminal of the switching converter; a power transistor Q2 having one end connected to the second end of the main power transistor Q1 and the other end coupled to the reference ground of the switching converter. An output capacitor Cout is connected to the output of the power converter to produce an output voltage Vout across it and is connected in parallel with the LED load to provide energy storage. In the present embodiment, the LED load is a plurality of series-connected light emitting diodes. In dimming applications, the current I through the light emitting diode can be variedLEDTo change the brightness.
The control circuit 10 is based on an output current feedback signal Vfb representing the output current Iout of the switching converter and a representation of the inductor current ILThe switch control signal PWM of the main power transistor Q1 of the switching converter is adjusted in a closed loop control mode to control the switching state of the main power transistor Q1 and to adjust the output current Iout of the switching converter to power the load. In the embodiment of the present invention, in particular, the control circuit 10 is configured to generate the second control signal Vres according to the output current feedback signal Vfb and the inductor current sampling signal Vsen to control the turn-off time of the main power transistor Q1 of the switching converter. In particular, according toThe compensation signal Vcomp is generated by an error amplification signal between an output current feedback signal Vfb representing the output current Iout and a reference value Vref representing the desired output current Iout, and is taken as a current reference value. The control circuit 10 adjusts the on-time of the power transistor Q1 according to the inductor current sampling signal Vsen of the switching converter and the current reference value. Here, the inductor current sampling signal Vsen may select a sampled peak current or a sampled valley current as needed. In the present comparative example, a fixed-frequency clock signal CLK, or a fixed off-time control signal Voff may be selected as the first control signal Vset to control the turn-on timing of the main power transistor Q1 of the switching converter.
In another embodiment, when the control circuit 10 adopts a fixed on-time control scheme (not shown), the fixed on-time control signal Von is adaptively used as the second control signal Vres to control the turn-off time of the main power transistor Q1 of the switching converter, and the current comparison signal between the compensation signal Vcomp and the inductor current sampling signal Vsen is used as the first control signal Vset to control the turn-on time of the main power transistor Q1 of the switching converter.
With continued reference to fig. 1, the inductor current sampling resistor Rsen for inductor current sampling may discharge the main power transistor Q1 as peak current sampling or the discharge power transistor Q2 as valley current sampling, which is exemplified in fig. 1. The output current sampling resistor Rfb is directly connected in series with the light emitting diode. In the control mode, the output current sampling resistor Rfb is connected in parallel with the output capacitor Cout after being connected in series with the load, so that the main power transfer function of the system is the alternating current resistor R of the output capacitor Cout and the LED loadLED_ACForming a first-order system. The main power transfer function therefore behaves differently for different output currents, different output capacitances or different light emitting diodes. The disadvantage of this control method is that the control loop is unstable under the application of small output current, resulting in large output current ripple, and the control loop bandwidth is low under the application of large output capacitance, resulting in slow response speed and risk of system instability.
Fig. 2 is a circuit diagram of a second comparative switching converter. As shown in fig. 2, the structure and operation of the main power circuit and the control circuit 20 are the same as those of the first comparative switching converter, and are not described herein again.
Compared with the first comparative technical solution, the present comparative example is different in that the output current sampling resistor Rfb is connected in series with the inductor L, and specifically, the output capacitor Cout is directly connected in parallel with the LED load and then connected in series with the output current sampling resistor Rfb. This arrangement is such that the main power transfer function of the system is not affected by the output capacitor Cout, and thus the main power transfer function is a 0 th order system. However, this scheme has a common disadvantage with the first comparative scheme, that is, the inductor current sampling resistor Rsen is usually a fixed value, and under the specification of a wide output current range, for example, 1mA to 1A, since the reference value Vref representing the output current Iout is fixed, the output current sampling resistor Rfb needs to be inversely proportional to the desired output current, and the minimum value and the maximum value of the output current sampling resistor Rfb have a difference of 1000 times, so that the dc gain of the negative feedback system of the control system is greatly influenced by the output current, and it is difficult for the compensation loop to satisfy the output current in such a wide range.
As shown in fig. 3, which is a small signal model equivalent circuit diagram of the control loop of the second comparative switching converter. The effect of the output current on the dc gain of the negative feedback system of the control system is analyzed in connection with the figure below. The transfer function is the ratio of the laplace transform of the linear system response (i.e., output) quantity to the laplace transform of the excitation (i.e., input) quantity under zero initial conditions. As shown in fig. 3, the compensation signal Vcomp is used as the current reference value of the inductor current sampling signal Vsen by the control circuit 20. The variation of the inductor current can be expressed as:
thus, the overall closed-loop control system transfer function is as follows:
when the clock signal CLK is selected as the first control signal Vset, i.e., the set signal, to control the turn-on time of the main power transistor Q1 of the switching converter,
wherein T isswFor the switching period, Sn and Sf are the rising and falling slopes of the inductor current, respectively, and Se is a slope compensation value (0 when not compensated).
From the above expression, it is obvious that in this control method, the loop is affected by the input voltage Vin, the output voltage Vout, and the inductance values L and Se, and is not the optimal solution. And when the output current sampling resistor Rfb varies with the desired output current, the dc gain of the negative feedback system of the control system varies greatly, so that it is difficult for the compensation loop to satisfy such a wide range of output current.
When the fixed off-time control signal Voff is used as the first control signal Vset, i.e., the set signal, to control the on-time of the main power transistor Q1 of the switching converter, or when the fixed on-time control signal Von is used as the second control signal Vres to control the off-time of the main power transistor Q1 of the switching converter,
wherein tV is a fixed on-time value tONOr a fixed off-time value tOFFAnd the control system only needs to consider one second-order pole, so that the design difficulty of the loop is greatly simplified. This solution is in addition to the above, the DC gain of the negative feedback system of the control system is influenced by the output currentIn addition to the very large variation in noise, there is a disadvantage in that the switching frequency variation is relatively large for applications with a relatively wide conversion ratio.
Based on this, the utility model provides a constant current control method suitable for wide output current aims at making the main power transfer function of system not influenced by output capacitance and output current to become 0 rank main power system.
Referring to fig. 4, the only difference between the switching converter of the present invention and the second comparative switching converter is that the control circuit 30 obtains the output current feedback signal Vfb and the inductive current sampling signal Vsen by sharing a sampling resistor Rfb and generates the switching control signal PWM accordingly, so that the dc gain of the negative feedback system is not affected by the output current Iout. The sampling resistor Rfb is connected in series with the inductor L of the switching converter in such a way that the main power transfer function of the system is not affected by the output capacitor Cout. In one embodiment, for example, as shown in fig. 4, the sampling resistor Rfb is connected between the output capacitor Cout and the common terminal of the cathode of the LED load and the ground reference, which is more advantageous for the simplified design of the circuit, the control ground of the current sampling, i.e., the ground reference of the power stage circuit, and the voltage of the non-ground terminal of the sampling resistor Rfb can be used as the output current feedback signal Vfb. In another embodiment, the sampling resistor Rfb is connected between the inductor L and the common terminal of the output capacitor Cout and the cathode of the LED load. Therefore, it can be understood that the connection mode of the sampling resistor Rfb only needs to be connected in series with the inductor L, and other reasonable connection modes are also within the application range of the present invention. Because the resistance of sampling resistance Rfb and the size of the output current of expectation need the one-to-one, adopt the utility model discloses a control circuit can eliminate output current's wide range change to control system's negative feedback system's direct current gain's influence.
Preferably, the output current feedback signal Vfb is proportional to the inductor current sampling signal Vsen. Specifically, the output current feedback signal Vfb is directly generated at the non-grounded terminal of the sampling resistor Rfb, and the voltage at the non-grounded terminal of the sampling resistor Rfb is multiplied by the scaling factor K to be used as the inductor current sampling signal Vsen. In the embodiment shown in fig. 4, the control circuit 30 is configured to generate the second control signal Vres to control the turn-off of the main power transistor Q1 of the switching converter according to the output current feedback signal Vfb and the inductor current sampling signal Vsen. Specifically, the compensation signal Vcomp is generated from an error amplification signal between an output current feedback signal Vfb representing the output current Iout and a reference value Vref representing a desired output current Iout, and is used as a current reference value. The control circuit 30 generates an active second control signal Vres to control the turn-off of the main power transistor Q1 to adjust the on-time of the power transistor Q1 when the inductor current sampling signal Vsen of the switching converter rises to the current reference value. In an example of the present invention, the constant frequency clock signal CLK or the fixed off-time control signal Voff may be selected as the first control signal Vset to control the on-time of the main power transistor Q1 of the switching converter. The control circuit 30 generates a switching control signal PWM based on the second control signal Vres and the first control signal Vset.
In another embodiment, when the control circuit 30 adopts a fixed on-time control scheme (not shown), the fixed on-time control signal Von is adaptively used as the second control signal Vres to control the turn-off time of the main power transistor Q1 of the switching converter, and the current comparison signal between the compensation signal Vcomp and the inductor current sampling signal Vsen is used as the first control signal Vset to control the turn-on time of the main power transistor Q1 of the switching converter.
Under the control of the control circuit 30, the transfer function of the whole closed-loop control system of the switching converter of the present invention is as follows:
from the above expression, by sharing one sampling resistor to sample the inductor current and the output current at the same time, the dc gain of the negative feedback system of the control system is fixed to 1/K, and does not change with the change of the sampling resistor Rfb, that is, does not change with the change of the output current.
Fig. 5 and 6 are comparison bode plots of transfer functions of the closed-loop control system of the switching converter of the present invention. Wherein, a1 and b1 are logarithmic amplitude-frequency curves representing direct current gain and logarithmic phase-frequency curves representing phase allowance of the baud chart of the existing control scheme, and a2 and b2 are logarithmic amplitude-frequency curves representing direct current gain and logarithmic phase-frequency curves representing phase allowance of the baud chart of the control scheme of the utility model. As shown in FIG. 5, at ILEDUnder 1A, the existing control scheme is compared with the bode diagram of the control scheme of the present invention, and as can be seen from fig. 5, the dc gain and the phase margin of the two basically behave in accordance. As shown in FIG. 6, at ILEDAt 50mA, the existing control scheme is compared with the bode plot of the control scheme of the present invention, as can be seen from fig. 6, at ILEDUnder 50mA, the direct current gain under the existing control scheme is increased by 20 times due to the increase of the resistance value of the sampling resistor Rfb, thereby increasing the bandwidth. Resulting in a reduction in the phase margin due to the higher bandwidth. As can be seen from the figure, the frequency of the gain crossing point (corresponding to the log amplitude frequency curve 0 db) of the prior art control scheme is around 150K, so the corresponding phase margin at 150K is about 34.4 degrees; the frequency of the gain crossing point (corresponding to the logarithmic amplitude-frequency curve 0 db) of the control scheme of the present invention is about 7.95K, and therefore the corresponding phase margin at 7.95K is about 87.3 degrees. Therefore, the utility model discloses a control scheme more is applicable to the output current requirement of wide range, can eliminate the influence of the wide range change of output current to control system's negative feedback system's direct current gain and phase allowance.
It should be noted that, similarly, when the frequency-selecting control mode, the fixed on-time control mode, or the fixed off-time control mode is used, the values of ω and Q may be different respectively. However, there are some disadvantages of these control methods, for example, in the case of constant frequency control, the loop is affected by the magnitude of the input voltage Vin, the output voltage Vout, the inductance L and Se, which is not the best solution; when the on-time is controlled fixedly or the off-time is controlled fixedly, the switching frequency change is large for the application of a wide conversion ratio.
Based on this, further, in order to better adapt to the requirement of the output current in a wide range, on the basis of eliminating the influence of the wide range change of the output current on the direct current gain and the phase margin of the negative feedback system of the control system, the purpose that the switching frequency is basically constant is achieved, the control circuit of the utility model further adopts a quasi-fixed frequency control mode to generate the first control signal Vset so as to control the on-state of the main power transistor Q1, thereby adjusting the cut-off time of the power transistor Q1.
Referring to fig. 7, a circuit diagram of the first control circuit is shown. Preferably, in one embodiment, the control circuit 30 includes a first control circuit 31. The first control circuit 31 receives the switching control signal PWM to obtain the off duty of the switching converter. In one implementation, the switching period includes an on-time and an off-time. The first control circuit 31 generates a timing signal according to the switching control signal PWM, and adjusts the length of the off-time of the power transistor Q1 to maintain the length of the switching period substantially constant by comparing the timing signal with a timing reference signal, which is related to the duty ratio of the switching converter, and the timing signal is a ramp signal. When the timing signal reaches the timing reference signal, the first control circuit 31 generates the first control signal Vset to control the power transistor Q1 to be turned on.
With continued reference to fig. 7, the first control circuit 31 includes a timing circuit 311 and a timing reference signal generation circuit 312. The timing circuit 311 includes a current source S1 and a timing capacitor C1 connected in parallel, and the current source S1 generates a timing current I1. Timing current I1 generates timing signal VB by charging timing capacitor C1. The timing circuit further comprises a timing switch K1, wherein the timing switch K1 is connected in parallel across the controlled current source S1 and the timing capacitor C1, so as to control the timing current I1 to charge the timing capacitor C1 at the beginning of the off-time of the switching cycle and reset the timing signal VB to zero when the off-time is switched to the on-time.
The timing reference signal generation circuit 312 includes a current source S2, a switch K2, and a resistor R1. The current source S2 and the switch K2 are connected in parallel, and the resistor R1 is connected in parallel with the current source S2 and the switch K2. The switch K2 is controlled by a switch control signal PWM. During the on-time of the switching cycle, the switch K2 is turned on, the voltage across the resistor R1 is zero, and during the off-time of the switching cycle, the current source S2 generates the current I2 to charge the resistor R1 and generate a voltage across the resistor R1, so that the voltage across the resistor R1 is approximately a square wave. The timing reference signal generation circuit further includes a filter circuit for filtering the voltage across resistor R1 to generate timing reference signal VA, such that timing reference signal VA is inversely proportional to the duty cycle of the switching converter. In the present embodiment, the filter circuit includes a resistor R2 and a capacitor C2 connected in series, and the timing reference signal VA is generated at the common terminal of the resistor R2 and the capacitor C2.
The first control circuit 31 further comprises a comparison circuit CMP1, a first input terminal (e.g. an inverting input terminal) of the comparison circuit CMP1 receiving the timing reference signal VA, a second input terminal (e.g. a non-inverting input terminal) receiving the timing signal VB, and generating the first control signal Vset by comparing the timing reference signal VA and the timing signal VB. In the present embodiment, the timing reference signal VA can be expressed as follows:
VA＝I2×R1×(1-D)
where D is the duty cycle of the switching converter.
The timing signal VB can be expressed as follows:
where Ts is the switching period of the switching converter. When the timing signal VB reaches the timing reference signal VA, the first control circuit 101 generates the active first control signal Vset, and the power transistor Q1 is turned on again. The switching period of a switching converter can therefore be expressed as follows:
therefore, by adopting the control circuit of the embodiment of the present invention, the switching period Ts of the switching converter is a constant value, so that the system operates in the quasi-constant frequency mode.
To sum up, the utility model discloses a switching converter acquires output current feedback signal and inductive current sampling signal and according to in order to generate on-off control signal PWM through sharing a sampling resistor to make negative feedback system's direct current gain and phase margin not receive the influence of output current variation range broad. And further, by adopting a quasi-fixed frequency control mode, the switching period of the switching converter can be maintained to be basically constant under a normal conversion ratio.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included within the protection scope of the present invention.
Claims (14)
1. A control circuit is used for a switching converter and is characterized in that the control circuit acquires an output current feedback signal and an inductive current sampling signal by sharing a sampling resistor and generates a switching control signal according to the output current feedback signal and the inductive current sampling signal so as to enhance the stability of a negative feedback system of the switching converter under the condition of wide-range output current change.
2. The control circuit of claim 1, wherein the output current feedback signal and the inductor current sampling signal are obtained by sharing the sampling resistor, so that the dc gain of the negative feedback system is independent of the output current, thereby enhancing the stability of the negative feedback system of the switching converter under a wide range of output current variations.
3. The control circuit of claim 1, wherein the phase margin of the negative feedback system is not affected by the output current by making the dc gain of the negative feedback system independent of the change in the output current.
4. The control circuit of claim 1, wherein a ratio of the output current feedback signal to the inductor current sampling signal is constant when the output current varies.
5. The control circuit of claim 1, wherein the resistance of the sampling resistor corresponds to the desired output current.
6. The control circuit of claim 1, wherein the sampling resistor is connected in series with an inductance of the switching converter.
7. The control circuit of claim 6, wherein the output capacitor is connected in parallel with the load and then connected in series with the sampling resistor.
8. The control circuit of claim 7, wherein the sampling resistor is connected between the inductor and a common terminal of the output capacitor and an anode of the LED load.
9. The control circuit of claim 7, wherein the sampling resistor is connected between the common of the output capacitor and the cathode of the LED load and a reference ground.
10. The control circuit of claim 1, wherein the control circuit is configured to generate a switch control signal to control a switching state of a main power transistor of the switching converter as a function of the output current feedback signal and the inductor current sampling signal.
11. The control circuit of claim 10, wherein the control circuit utilizes a transconductance amplifier to obtain the error amplified signal between the output current feedback signal and the current reference value of the output current to generate the compensation signal, and changes the active state of the switch control signal when the inductor current sampling signal rises to the compensation signal.
12. The control circuit of claim 1, wherein the control circuit is configured to control the switching state of a main power transistor of the switching converter in a quasi-constant frequency control manner.
13. The control circuit of claim 12, wherein the control circuit generates a timing signal based on a duty cycle of a switching converter and generates a timing reference signal inversely related to the duty cycle of the switching converter, wherein the timing signal changes the switching state of the main power transistor when the timing signal reaches the timing reference signal.
14. A switching converter, comprising;
a power stage circuit comprising a power transistor coupled to an inductor element, wherein the inductor element couples an input; and the number of the first and second groups,
a control circuit according to any of claims 1 to 13 for controlling the power transistor to produce an output signal at the output terminal to drive a load.
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