A soft start circuit and switching converter

Abstract

The application discloses a soft start circuit for a switching converter, which is used to generate a first reference voltage gradually increasing over time, and an output voltage of the switching converter follows the first reference voltage to work. The soft start circuit comprises a slope voltage generation module, which comprises a current source, a first transistor and a capacitor connected in sequence between a power supply voltage and the ground, a shorting point of a control end and a first end of the first transistor being used to output a slope voltage, and an initial voltage of the slope voltage being greater than 0; and a voltage following module, an input end of which is used to input the slope voltage and generate the first reference voltage by following the slope voltage. The application clamps the initial voltage of the slope voltage by adding a MOS diode, so that the time when the first reference voltage is 0V can be effectively shortened after the chip is powered on, and the problem of output voltage oscillation in the initial stage of the soft start of the switching converter is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of integrated circuit technology, and in particular, to a soft start circuit and a switching converter. BACKGROUND

[0002] With the development of power electronic products and semiconductor technology, power management chips are more and more widely used in portable computers, mobile phones, personal digital assistants and other portable or non-portable electronic devices. A switching converter uses a power switch to control the transmission of electric energy from an input terminal to an output terminal, so as to provide a constant output voltage and / or output current at the output terminal. During the start-up process of the switching converter, a large current from the power supply to the output terminal is generated due to the load and the output capacitor, and thus a soft start circuit is needed to control the start-up process of the switching converter.

[0003] Figure 1 A circuit schematic diagram of a switching converter according to the prior art is shown. The switching converter 100 includes a power circuit and a control circuit. The power circuit includes a power switch Mx and a diode D1 connected in series between an input terminal and a ground terminal, an inductor Lx connected between a middle node of the power switch Mx and the diode D1 and an output terminal, and an output capacitor Cout connected between the output terminal and the ground terminal. The input terminal of the switching converter 100 receives an input voltage Vin, and the output terminal provides an output voltage Vout. The control circuit of the switching converter 100 is configured to provide a switching control signal to the power switch Mx.

[0004] In the control circuit of the switching converter 100, a feedback circuit 130 generates a feedback voltage FB based on the output voltage Vout, an error amplifier EA generates an error amplified signal Vea based on the feedback voltage FB and a reference voltage Vref provided by a soft start circuit 140, and a PWM comparator 120 generates a pulse width modulation signal PWM based on the error amplified signal Vea and a ripple signal Vramp to control the conduction state of the power switch Mx.

[0005] Figure 2 A circuit schematic diagram of a soft start circuit according to the prior art is shown. One implementation of the existing soft start technology is to use a current source Iss to charge a capacitor Css, generate a ramp voltage Vss at one end of the capacitor Css, and then use a source follower to follow the ramp voltage Vss to obtain a reference voltage Vref gradually rising over time. As shown in the figure, the reference voltage Vref is gradually rising over time, and the output voltage Vout is gradually rising over time. Figure 2As shown, the source follower comprises an NMOS transistor M1, resistors R1 and R2, the gate of the NMOS transistor M1 is connected to point A with one end of a capacitor Css, the drain B of the NMOS transistor M1 is connected with a fixed potential reference voltage Vref1, the resistors R1 and R2 are connected in sequence between the source D of the NMOS transistor M1 and the ground, and point E between the resistors R1 and R2 is a reference potential output end of an error amplifier EA.

[0006] In the existing soft start circuit 140, only when the potential of point A is higher than the opening voltage Vthn1 of the NMOS transistor M1, the potential of point E starts to gradually increase from 0V, which leads to that the reference potential input end of the error amplifier is 0V for a long time after the DC / DC switching power supply chip is powered on, but there are switching pulses in the circuit during the time, thus leading to that the output voltage oscillates in the initial stage of the soft start. SUMMARY

[0007] In view of the above problems, the purpose of the present application is to provide a soft start circuit and a switching converter, which solve the problem of output voltage oscillation in the soft start stage of the switching converter.

[0008] According to an aspect of an embodiment of the present application, a soft start circuit for a switching converter is provided, the soft start circuit is used to generate a first reference voltage gradually increasing with time, and the output voltage of the switching converter follows the first reference voltage to work, wherein the soft start circuit comprises: a slope voltage generation module comprising a current source, a first transistor and a capacitor connected in sequence between a power supply voltage and the ground, the short connection point of the control end and the first end of the first transistor is used to output a slope voltage, and the initial voltage of the slope voltage is greater than 0; and a voltage follower module, the input end of which is used to input the slope voltage, and the voltage follower module is used to generate the first reference voltage by following the slope voltage.

[0009] Optionally, the initial voltage of the slope voltage is equal to the threshold voltage of the first transistor.

[0010] Optionally, the voltage follower module is implemented by a source follower.

[0011] Optionally, the voltage follower module comprises a second transistor, a first resistor and a second resistor connected in sequence between a second reference voltage with a fixed potential and the ground, the control end of the second transistor is connected with the slope voltage, and the intermediate node of the first resistor and the second resistor is used to output the first reference voltage.

[0012] Optionally, the first transistor and the second transistor have the same doping type.

[0013] Optionally, the first transistor and the second transistor are NMOS transistors.

[0014] According to another aspect of the embodiments of the present application, there is provided a switching converter, comprising: a power circuit for converting an input voltage to a stable continuous output voltage by controlling the transmission of power from an input terminal to an output terminal using at least one power switch; the soft-start circuit as described above for generating a reference voltage that gradually increases over time; an error amplifier for comparing a feedback voltage of the output voltage with the reference voltage and generating an error amplified signal according to the difference between the two; and a PWM comparator for comparing the error amplified signal with a ripple signal to generate a switching control signal to control the on-off state of the at least one power switch.

[0015] The soft-start circuit of the present application clamps the initial voltage of the ramp voltage by adding a MOS diode, so that the time for the reference voltage to be 0V can be effectively shortened after the chip is powered on, and the problem of output voltage oscillation in the initial stage of soft-start of the switching converter is solved. In addition, the initial value of the reference voltage will not be too high due to process deviation, so that the problem of the output voltage being too high in the initial stage of soft-start will not occur. Furthermore, the soft-start circuit of the present application adds an NMOS device without adding other current branches, so that very low power consumption can be achieved, and can be applied to low-power consumption application scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0016] The above and other objects, features and advantages of the present application will become more apparent from the following description when taken in conjunction with the accompanying drawings, in which:

[0017] Figure 1 a circuit schematic diagram of a switching converter according to the prior art is shown;

[0018] Figure 2 a circuit schematic diagram of a soft-start circuit according to the prior art is shown;

[0019] Figure 3 a circuit schematic diagram of another soft-start circuit according to the prior art is shown;

[0020] Figure 4 a circuit schematic diagram of a switching converter according to an embodiment of the present application is shown;

[0021] Figure 5 a circuit schematic diagram of a soft-start circuit according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0022] The present application will be described in more detail with reference to the accompanying drawings. In the various drawings, like elements are designated with like reference numerals. Each part in the drawings is not drawn to scale for the sake of clarity. Also, some parts that are well known can not be shown in the drawings.

[0023] Many specific details of the application are described below in order to provide a thorough understanding of the present application. However, as will be understood by one skilled in the art, the present application can be practiced without

[0024] It should be understood that, in the following description, "circuitry" can include a single or multiple components or hardware circuits, programmable circuitry, state machine circuitry, and / or elements storing instructions to be executed by the programmable circuitry. When an element or circuitry is referred to as being "connected to" another element or "connected between" two nodes, it can be directly coupled or connected to the other element or there can be intervening elements between the elements, and the connection between the elements can be physical, logical, or a combination thereof. In contrast, when an element is referred to as being "directly coupled to" or "directly connected to" another element, it implies that there are no intervening elements between the two.

[0025] Figure 3 A circuit diagram of another soft start circuit according to the prior art is shown. As shown in Figure 3 The soft start circuit 240 includes current sources Iss and I1, a capacitor Css, transistors M1 and M2, and resistors R1 and R2.

[0026] The current source Iss and the capacitor Css are connected in sequence between a power supply voltage Vdd and ground, and the current source Iss is used to charge the capacitor Css, thereby generating a gradually rising voltage at point A over time. The transistor M2 is a PMOS transistor, for example, with its gate connected to point A, its source connected to the current source I1, and its drain connected to ground. The source of the PMOS transistor M2 is used to output a ramp voltage Vss. The transistor M1 is an NMOS transistor with its gate connected to the source of the PMOS transistor M2, and its drain B connected to a fixed potential reference voltage Vref1. The resistors R1 and R2 are connected between the source D of the NMOS transistor M1 and ground, and the point E between the two is the parameter potential output terminal of an error amplifier.

[0027] With Figure 2Compared with the soft start circuit 140, the soft start circuit 240 adds a PMOS branch, so that the potential of the ramp voltage Vss is first raised by Vthp, and then the ramp voltage Vss is connected to the gate of the source follower composed of the NMOS transistor M1, so that the initial voltage VE of point E in the soft start circuit 240 is Vthp2-Vthn1. Although this method can effectively shorten the time when the reference potential Vref is initially 0V, due to the process deviation in integrated circuit manufacturing, and the deviation directions of PMOS process and NMOS process are different, the difference between the voltage Vthp and the voltage Vthn will be large, resulting in that the initial value of the reference potential VE is high, and then the potential of the output voltage of the switching converter in the initial stage is also high, which cannot meet the requirement of slow rising of the output voltage in the soft start control. In addition, the added PMOS branch also increases the power consumption of the circuit, which cannot be applied to some occasions with high power consumption requirements.

[0028] The present application can be presented in various forms, some examples of which will be described below.

[0029] Figure 4 A circuit schematic diagram of a switching converter according to an embodiment of the present application is shown. As shown, the switching converter 200 of the present embodiment includes a power circuit and a control circuit. The power circuit includes one or more switching elements and filter elements (for example, inductors and / or capacitors, etc.), which are configured to adjust the transmission of electrical energy from the input end to the output end of the switching converter in response to the switching drive signal, so as to convert the input voltage Vin into a stable continuous output voltage Vout. Figure 4

[0030] In some embodiments, according to the topology classification of the power circuit, it can be divided into Buck converter, Boost converter, Flyback converter and Buck-Boost converter.

[0031] ​In the embodiment, the power circuit is implemented by, for example, a step-down topology, and includes a power switch Mx, and a peripheral inductive element Lx and a rectifier diode Dl. The first terminal of the power switch Mx is connected with the input voltage Vin, the second terminal is connected with the anode of the rectifier diode Dl, the cathode of the rectifier diode Dl is grounded, the common terminal of the power switch Mx and the rectifier diode Dl forms a switching node SW, the first terminal of the inductive element Lx is connected with the switching node SW, the second terminal is connected with the output voltage Vout, and an output capacitor Cout is connected between the output terminal and the ground terminal. The power switch Mx can be any controllable semiconductor switching device, such as a metal oxide semiconductor field effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT), etc.

[0032] It should be noted that although a MOSFET is used as the switching element in the embodiment, any other type of suitable switching element can be used without departing from the principles of the present application. In addition, although the embodiment is described with respect to an asynchronous step-down converter, the present application is not limited thereto, and is equally applicable to a synchronous step-down converter, and a synchronous rectification switching element can be used instead of the rectifier diode Dl in the above embodiment.

[0033] The control circuit of the switching converter 200 is configured to provide a switching control signal to the power switch Mx to control the on and off states of the power switch Mx.

[0034] Specifically, the feedback circuit 230 is configured to generate a feedback voltage FB based on the output voltage Vout. In some embodiments, the feedback circuit 230 can be implemented by a voltage dividing resistor network to generate the feedback voltage FB by dividing the output voltage Vout.

[0035] The error amplifier EA compares the feedback voltage FB with a reference voltage Vref provided by the soft start circuit 340, and generates an error amplified signal Vea based on the difference between the two.

[0036] The PWM comparator 220 compares the error amplified signal Vea with a ripple signal Vramp, and generates a switching control signal PWM to control the on and off states of the power switch Mx.

[0037] In the embodiment, the soft start circuit 340 is configured to generate a reference voltage Vref that gradually increases over time, so as to control the output voltage Vout to gradually increase in the initial stage of the power-on of the switching converter, and avoid the generation of inrush current from the power supply to the output in the start-up process.

[0038] Figure 5 A circuit schematic diagram of a soft start circuit according to an embodiment of the present application is shown. As shown in FIG. 4, the soft start circuit 340 includes a reference voltage generator 342 and a voltage divider 344. Figure 5As shown, the soft start circuit 340 of the embodiment includes a slope voltage generating module 341 and a voltage follower module 342. The slope voltage generating module 341 is configured to generate a slope voltage Vss gradually rising over time, and the voltage follower module 342 is configured to generate a reference voltage Vref varying at the same slope as the slope voltage Vss according to the slope voltage Vss.

[0039] Further, the slope voltage generating module 341 includes a current source Iss, an NMOS transistor M3 and a capacitor Css. The current source Iss, the NMOS transistor M3 and the capacitor Css are connected in sequence between a power supply voltage Vdd and ground, and the NMOS transistor M3 is connected in a MOS diode structure. In the embodiment, a shorting point of the gate and the drain of the NMOS transistor M3 is an output end of the slope voltage Vss, so that the initial voltage of the slope voltage Vss can be clamped at a voltage Vthn3, which is a threshold voltage of the NMOS transistor M3. Compared with the prior art, the embodiment can raise the slope voltage Vss, so that the time for the reference voltage Vref to be initially 0V can be effectively shortened, and the oscillation of the output voltage of the switching converter in the soft start stage can be greatly reduced.

[0040] The voltage follower module 342 can be implemented by a source follower and includes an NMOS transistor M1, resistors R1 and R2. The gate of the NMOS transistor M1 is connected with the slope voltage Vss, the drain is connected with B and a reference voltage Vref1 of a fixed potential, and the resistors R1 and R2 are connected between the source D of the NMOS transistor M1 and ground, and the E point between the resistors R1 and R2 is an error amplifier reference potential output end.

[0041] After the chip is powered on, the initial voltage of the slope voltage Vss is Vthn3 due to the clamping effect of the NMOS transistor M3, and the potential VD of the D point is Vss-Vthn1=Vthn3-Vthn1. Therefore, the voltage VD of the D point can gradually rise following the capacitor voltage when the chip is powered on, instead of being at 0V for a long time. In addition, under the influence of process deviation, when the threshold voltage Vthn3 of the NMOS transistor M3 is high, the threshold voltage Vthn1 of the NMOS transistor M1 will also be high, and when the threshold voltage Vthn3 of the NMOS transistor M3 is low, the threshold voltage Vthn1 of the NMOS transistor M1 will also be low. Therefore, under the influence of the process deviation voltage, it can be said that the threshold voltage Vthn3 of the NMOS transistor M3 is equal to the threshold voltage Vthn1 of the NMOS transistor M1, so that the voltage VD of the D point can start from 0V and gradually rise following the capacitor voltage, that is, the initial voltage of the reference voltage Vref will not be high due to the process deviation, and the problem of the high potential of the output voltage in the initial stage of the soft start is solved.

[0042] With the charging of the capacitor Css by the current source Iss, the potential of the upper plate of the capacitor Css is VF, then the slope voltage Vss = VF + Vgs3, where Vgs3 is the gate-source voltage of the transistor M3, the voltage VD of the D point is Vss - Vgs1 = VF + Vgs3 - Vgs1, then the reference voltage Vref = K x VD = K x (VF + Vgs3 - Vgs1), where K is the resistance proportionality coefficient, K = R2 / (R1 + R2). Therefore, the reference voltage Vref of the embodiment can gradually rise along with the voltage of the capacitor.

[0043] In summary, the soft start circuit of the application can effectively shorten the time of the reference voltage being 0V after the chip is powered on by clamping the initial voltage of the slope voltage with a MOS diode, and solve the problem of output voltage oscillation in the initial stage of soft start of the switching converter. In addition, the initial value of the reference voltage will not be too high due to process deviation, so that the problem of the output voltage being too high in the initial stage of soft start will not occur. Furthermore, the soft start circuit of the application adds an NMOS device without adding other current branches, so that very low power consumption can be achieved and can be applied to low-power application scenarios.

[0044] It should be noted that although the devices are described as certain N-channel or P-channel devices, or certain N-type or P-type doped regions in this document, those skilled in the art can understand that complementary devices can also be implemented according to the application. Those skilled in the art can understand that the conductivity type is the mechanism of the conduction of electricity, such as conduction by holes or electrons, so the conductivity type is not related to the doping concentration but to the doping type, such as P-type or N-type. Those skilled in the art can understand that the words "during", "when" and "when" used in this document in relation to the operation of the circuit are not strict terms for actions that occur immediately at the start of the start action, but there can be some small but reasonable delay or delays between them and the reaction initiated by the start action, such as various transmission delays, etc. The use of the words "about" or "substantially" in this document means that the value of the element has a parameter close to the declared value or position. However, as is well known in the art, there is always a small deviation so that the value or position is difficult to be strictly the declared value. It has been appropriately determined in the art that a deviation of at least ten percent (10%) (at least twenty percent (20%) for semiconductor doping concentration) is a reasonable deviation from the described accurate ideal target. When used in conjunction with signal states, the actual voltage value or logic state (e.g. "1" or "0") of the signal depends on whether positive logic or negative logic is used.

[0045] It is also to be noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Furthermore, to the extent that the terms "including," "includes," "having," "has," "with," or "contains" are used in either the detailed description and the claims section below, such terms are intended to be inclusive in a manner similar to the term "comprising" as an open transition term without precluding any additional or other elements.

[0046] In accordance with the practices of the present application, these embodiments have been described in relation to the above-described embodiments with reference to the drawings. Modification and alterations will occur to others upon reading and understanding the above description. It is intended to include all such modifications and alterations insofar as they come within the scope of the above described and following claims and their equivalents.

Claims

1. A soft-start circuit for a switching converter, the soft-start circuit generating a first reference voltage that gradually increases over time, the output voltage of the switching converter operating in accordance with the first reference voltage, wherein, The soft-start circuit includes: A ramp voltage generation module includes a current source, a first transistor, and a capacitor connected in sequence between a power supply voltage and ground. The control terminal and the shorting point of the first terminal of the first transistor are used to output a ramp voltage. A voltage follower module, whose input terminal is used to input the ramp voltage, is used to generate the first reference voltage by following the ramp voltage. The voltage follower module includes: A second transistor, a first resistor, and a second resistor are sequentially connected between a second reference voltage with a fixed potential and ground. The control terminal of the second transistor is connected to the ramp voltage. The intermediate node between the first and second resistors is used to output the first reference voltage. The threshold voltage of the second transistor is equal to the threshold voltage of the first transistor, and the initial voltage of the ramp voltage is equal to the threshold voltage of the first transistor.

2. The soft-start circuit according to claim 1, wherein, The first transistor and the second transistor have the same doping type.

3. The soft-start circuit according to claim 2, wherein, The first transistor and the second transistor are NMOS transistors.

4. A switching converter, comprising: The power circuit uses at least one power switch to control the transfer of electrical energy from the input terminal to the output terminal, so as to convert the input voltage into a stable and continuous output voltage. The soft-start circuit according to any one of claims 1-3 is used to generate a reference voltage that gradually increases over time; An error amplifier is used to compare the feedback voltage of the output voltage with the reference voltage and generate an error amplification signal based on the difference between the two. as well as A PWM comparator is used to compare the error amplification signal with a ripple signal to generate a switch control signal to control the on state of the at least one power switch.

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