Current limiting circuit with fixed conduction time structure

Abstract

The invention provides a current limiting circuit with a fixed on-time structure, and is applied to the technical field of switching power supplies. According to the invention, a power tube current acquisition module detects a current flowing through a power tube, and when the current on the power tube reaches a current limiting point, a power tube current limiting control module outputs an overcurrent protection signal to control the power tube to be turned off; under the control of the power tube current-limiting control module, sampling the initial current of the power tube at the moment when the power tube is conducted next time, and taking the initial current as the minimum current of the inductor after the power tube is turned off this time; and then the power tube turn-off time control module controls the turn-off time when the power tube is in the turn-off state next time by using the difference value between the minimum current and a preset inductive current minimum value, namely, the purpose of adjusting the turn-off time of the power tube and the discharge time of the inductive current is achieved, and the purpose of adjusting the discharge time of the inductive current through negative feedback adjustment after a plurality of cycles is achieved. And the minimum value of the inductive current after the overcurrent protection signal is triggered reaches a set value.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated circuits, and in particular to a current limiting circuit with a fixed on-time structure. Background Art

[0002] Switching power management chips achieve a stable output voltage or current by continuously controlling the on and off cycles of switching transistors. They can be categorized into two main types: AC / DC (alternating current to direct current) and DC / DC (direct current to direct current). Since AC / DC switching power supplies often include a DC / DC converter module, DC / DC is a fundamental component of switching power supplies. DC / DC switching power supplies include three basic topologies: buck, boost, and buck-boost. In step-down DC-DC power supplies, especially for high input voltages such as 80V to 100V, a COT (constant on time) structure is often employed. This structure stabilizes the output voltage by fixing the on-time of the power transistors and dynamically adjusting the off-time based on output voltage feedback.

[0003] A COT (Constant on Time) DC-DC converter maintains a fixed on-time within each cycle. It compares the output feedback voltage VFB with a fixed reference voltage VREF. When VFB falls below VREF, the next on-cycle is immediately triggered. Because the COT structure lacks an internal oscillator, it cannot generate a PWM signal. This makes current limiting difficult, and it cannot achieve current limiting in the same way as PWM mode. Summary of the Invention

[0004] The object of the present invention is to provide a current limiting circuit with a fixed on-time structure, so as to achieve accurate current limiting of the fixed on-time structure without the need for a PWM signal, and ultimately improve the reliability and performance of the fixed on-time structure.

[0005] In order to solve the above technical problems, the present invention proposes a current limiting circuit with a fixed conduction time structure, which is applied to the current limiting control of a voltage-drop switching power supply system. The voltage-drop switching power supply system includes a switching power supply chip, and the switching voltage chip has an output voltage feedback pin FB for feedback of output voltage changes and a power tube for regulating the output voltage.

[0006] The current limiting circuit in the embodiment of the present invention includes: The power tube current acquisition module is connected to the power tube, and is used to collect the current flowing through the power tube in each cycle, and output a first voltage obtained by converting the current flowing through the power tube into a voltage and amplifying it in each cycle.

[0007] The power tube current limiting control module is connected to the power tube current acquisition module and is used to output an overcurrent protection signal and control the power tube to be periodically shut down when the current flowing through the power tube reaches the current limiting threshold in each cycle.

[0008] The power tube shutdown time control module is connected to the power tube current acquisition module and the power tube current limiting control module. It is used to adjust the shutdown time of the power tube after the power tube is shut down in the previous cycle, and adjust the starting current flowing through the power tube in the next cycle until the starting current flowing through the power tube is adjusted to the preset value after multiple cycles.

[0009] Furthermore, the drive signal may include an on-drive signal and an off-drive signal; the power tube current limiting control module includes a driver, which is used to control the power tube to turn on at the beginning of each cycle when the on-drive signal is connected, and when it is detected that the current flowing through the power tube reaches the current limiting threshold within the cycle, the off-drive signal generated by the power tube off-time control module is used to control the power tube to turn off.

[0010] Furthermore, the power tube current limiting control module is connected to the first voltage and is used to output an overcurrent protection signal to the power tube shutdown time control module when it detects that the first voltage is greater than the first reference voltage in each cycle, and control the power tube to be periodically shut down through the shutdown drive signal output by the power tube shutdown time control module.

[0011] Furthermore, the power tube turn-off time control module is specifically used to receive a first voltage corresponding to the starting current of the power tube when it is turned on, which is collected by the power tube current collection module within a cycle, and use the voltage difference between the first voltage corresponding to the starting current and the second reference voltage to adjust the turn-off time of the power tube within the current cycle, and then adjust the starting current flowing through the power tube in the next cycle by adjusting the turn-off time of the power tube; wherein the second reference voltage is the voltage corresponding to the preset value of the starting current.

[0012] Furthermore, the power tube current acquisition module may include: an instrumentation amplifier, a first transmission gate, a first capacitor, a second transmission gate and a second capacitor; wherein the first transmission gate includes an input end, an output end and a control end, and its input end is connected to the output end of the instrumentation amplifier to receive a first voltage, and its output end is connected to the first end of the first capacitor, so as to be turned on at the beginning of the power tube conduction in each cycle and store the first voltage corresponding to the starting current when the power tube is turned on in the first capacitor, and the second end of the first capacitor is connected to the reference ground end.

[0013] The second transmission gate is connected in series with the first transmission gate and includes an input end, an output end and a control end. The input end of the second transmission gate is connected to the output end of the first transmission gate and the first end of the first capacitor. The output end of the second transmission gate is connected to the first end of the second capacitor. After collecting the first voltage corresponding to the starting current when the power tube is turned on, the second transmission gate is turned on and the potential difference of the first voltage is transferred from the first capacitor to the second capacitor. The second end of the second capacitor is connected to the reference ground end.

[0014] Furthermore, the power tube current limiting control module includes a first comparator, a first SR trigger, a pulse generator, a NAND gate and a NOT gate; wherein, the first comparator includes a non-inverting input terminal, an inverting input terminal and an output terminal, the non-inverting input terminal serves as the input terminal of the power tube current limiting control module, is connected to the output terminal of the instrumentation amplifier to access the first voltage, the inverting input terminal is connected to the first reference voltage, the output terminal is connected to the S input terminal of the first SR trigger, and outputs an overcurrent protection signal when the first voltage is greater than the first reference voltage.

[0015] The first SR trigger includes an S input terminal and a Q output terminal. The S input terminal is connected to the output terminal of the first comparator to receive the overcurrent protection signal when the first voltage is greater than the first reference voltage, and the Q output terminal is connected to an input terminal of the NAND gate. The input terminal of the pulse generator is connected to the first input terminal of the driver and is simultaneously connected to the start-up drive signal, and the output terminal of the driver is connected to the power tube. The output terminal of the pulse generator is connected to the other input terminal of the NAND gate, the output terminal of the NAND gate is connected to the input terminal of the NAND gate, and the output terminal of the NAND gate and the output terminal of the NAND gate are respectively connected to the control terminals of the first transmission gate and the second transmission gate to control the conduction and shutdown of the first transmission gate and the second transmission gate.

[0016] Furthermore, the power tube off-time control module may include a first transconductance amplifier, a second transconductance amplifier, a constant current source, a third capacitor and a fourth capacitor; wherein, the non-inverting input terminal of the first transconductance amplifier is connected to the first end of the second capacitor to access the first voltage, its inverting input terminal is connected to the second reference voltage, and its output terminal outputs the first error voltage; the non-inverting input terminal of the second transconductance amplifier is connected to the output voltage feedback pin FB, its inverting input terminal is connected to the third reference voltage, and its output terminal is connected to the constant current source and the first end of the fourth capacitor, so that when the voltage of the voltage feedback pin FB is lower than the third reference voltage, its output terminal draws current from the constant current source, thereby increasing the charging time of the fourth capacitor.

[0017] Furthermore, the power tube off-time control module also includes a second comparator and a second SR trigger; wherein, the second comparator includes a non-inverting input terminal, an inverting input terminal and an output terminal, the inverting input terminal of the second comparator is connected to the output terminal of the first transconductance amplifier and the first terminal of the third capacitor to access the first error voltage, the non-inverting input terminal of the second comparator is connected to the first terminal of the fourth capacitor and the constant current source and the output of the second transconductance amplifier, and when the voltage value of the first terminal of the fourth capacitor rises to the first error voltage, the comparator outputs a high level, otherwise it outputs a low level, and the second terminals of the third capacitor and the fourth capacitor are both connected to the reference ground terminal; the R input terminal of the second SR trigger is connected to the output terminal of the second comparator, the S input terminal of the second SR trigger is connected to the output terminal of the first comparator, and the Q output terminal of the second SR trigger is connected to the second input terminal of the driver.

[0018] Furthermore, the calculation formula for the off-time of the power tube is: T OFF =(C4*V OTA ) / (I1-I OTA2 ); Among them, T OFF is the off time of the power tube, C4 is the capacitance value of the fourth capacitor, V OTA is the output voltage of the first transconductance amplifier, I1 is the current of the constant current source, I OTA2 is the current value drawn by the second transconductance amplifier from the constant current source.

[0019] Furthermore, the power tube includes a PMOS tube or an NMOS tube.

[0020] Compared with the prior art, the present invention has the following beneficial effects: In a current limiting circuit with a fixed on-time structure provided by the present invention, the current (current) on the power tube (upper tube) used to adjust the output voltage is detected in real time by a power tube current acquisition module, and when the current on the power tube reaches the current limit point (or current limit threshold), the power tube current limiting control module outputs an overcurrent protection signal to control the power tube to be turned off; then, under the control of the power tube current limiting control module, the starting current of the power tube is sampled at the moment the power tube is turned on next time, and the starting current is used as the minimum current of the inductor after the power tube is turned off this time; then the power tube off-time control module uses The difference between the minimum current and the preset minimum value of the inductor current is used to control the turn-off time of the power tube when it enters the turn-off state next time, that is, to achieve the purpose of adjusting the turn-off time of the power tube and the discharge time of the inductor current. After several cycles, negative feedback regulation is implemented so that the minimum value of the inductor current after the overcurrent protection signal is triggered reaches the set value. Therefore, in special circumstances such as overcurrent in the upper tube, the upper tube can be quickly turned off for overcurrent protection. In addition, high-efficiency current limiting of the fixed on-time structure can be achieved without the need for a PWM signal, thereby improving the accuracy, timeliness and reliability of the current limiting of the fixed on-time structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings are used to provide a further understanding of the present application and constitute a part of the specification. Together with the following detailed description, they are used to explain the present application but do not constitute a limitation of the present application. In the accompanying drawings: Figure 1 The figure is a circuit diagram of a current limiting circuit with a fixed on-time structure provided in one embodiment of the present invention.

[0022] In the drawings, like components are given like reference numerals, and the drawings are not drawn to scale. DETAILED DESCRIPTION

[0023] To make the objects, advantages, and features of the present invention more clearly apparent, the present invention is further described below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the drawings are all in a very simplified form and are not drawn to scale. They are only used to conveniently and clearly assist in illustrating the purposes of the embodiments of the present invention. In addition, the structures shown in the drawings are often part of the actual structure. In particular, different drawings may need to illustrate different focuses and sometimes use different scales.

[0024] As used in the present invention, the singular forms "a", "an", and "the" include plural referents, the term "or" is generally used to include "and / or", the term "several" is generally used to include "at least one", and the term "at least two" is generally used to include "two or more". In addition, the terms "first", "second", and "third" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features specified as "first", "second", and "third" may explicitly or implicitly include one or at least two of the features, "one end" and "the other end" and "proximal end" and "distal end" generally refer to two corresponding parts, which not only include endpoints, and the terms "mounted", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be a communication between two elements or an interaction relationship between two elements. In addition, as used in the present invention, "one element is disposed on another element" generally only indicates that there is a connection, coupling, cooperation, or transmission relationship between the two elements, and the connection, coupling, cooperation, or transmission between the two elements may be direct or indirect through an intermediate element. It should not be understood as indicating or implying a spatial positional relationship between the two elements. That is, one element can be in any orientation, such as inside, outside, above, below, or to one side of another element, unless the content clearly indicates otherwise. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0025] Please refer to Figure 1 , Figure 1 This is a circuit diagram of a current limiting circuit with a fixed on-time structure according to an embodiment of the present invention. The current limiting circuit with a fixed on-time structure provided in the embodiment of the present invention can be set in a voltage-drop switching power supply system, wherein the voltage-drop switching power supply system may include an inductor (not shown) and a power tube M1 connected in series with the inductor for regulating the output voltage; and the voltage-drop switching power supply system may also have an output voltage feedback pin FB for feedback of output voltage changes, a first input terminal VIN for external connection to other currents or voltages, and a second input terminal SW connected to other circuits or sub-circuits. Figure 1 As shown, in the embodiment provided by the present invention, the current limiting circuit with a fixed on-time structure may include: a power tube current acquisition module 10 , a power tube current limiting control module 20 , and a power tube off-time control module 30 .

[0026] The power tube current acquisition module 10 is connected to the power tube M1 (upper tube) and also to an energy storage element inductor (not shown) to acquire the current flowing through the power tube M1 in each cycle, detect the current flowing between the source and drain of the power tube M1 when the power tube M1 is turned on, and output a first voltage VCS obtained by converting and amplifying the current flowing through the power tube M1 in each cycle.

[0027] The power tube current limiting control module 20 is connected to the power tube current acquisition module 10 and is used to output an overcurrent protection signal OCP and control the power tube M1 to be periodically shut down when the current flowing through the power tube M1 reaches the current limiting threshold (the current corresponding to the current limiting point) in each cycle.

[0028] The power tube shutdown time control module 30 is connected to both the power tube current acquisition module 10 and the power tube current limiting control module 20, and is used to adjust the shutdown time of the power tube M1 after the power tube M1 is turned off in the previous cycle, and adjust the starting current flowing through the power tube M1 in the next cycle, until the starting current flowing through the power tube M1 is adjusted to a preset value after multiple cycles.

[0029] It should be noted that the power tube M1 needs to be cycle-limited, that is, the current flowing through the power tube M1 is monitored in each cycle, and once the current flowing through the power tube M1 reaches the preset current limit threshold, the power tube M1 needs to be immediately turned off to cut off the energy input, and wait for the next cycle to re-judge whether it is allowed to be turned on, so as to achieve continuous current protection through "cycle-by-cycle limitation" and at the same time avoid the complete lock of the entire DC-DC system. According to research, in a step-down switching power supply system, when the power tube M1 is turned on and off in a certain cycle, the current flowing through the power tube M1 first gradually increases linearly and then becomes zero when the power tube M1 is turned off. In this process, the energy generated by the current flowing through the power tube M1 is stored in the inductor, and the inductor begins to discharge after the power tube M1 is turned off. Since the current on the inductor cannot change suddenly, in other words, the current on the inductor is continuous. Therefore, when the current on the power tube M1 reaches the current limit threshold for the Nth time and is turned off and a certain time has passed, it will be in the N+1th cycle. When the power tube M1 is turned on for the N+1th time, the corresponding starting current on the power tube M1 may be approximately equal to the minimum current of the inductor discharged after the power tube M1 is turned off in the Nth cycle. In the embodiment of the present invention, the current flowing through the power tube M1 detected by the power tube current acquisition module 10 at the moment when the power tube M1 is turned on for the N+1th time is used as the minimum current of the inductor discharged after the power tube M1 was turned off for the previous time, that is, the Nth time. This minimum current is compared with a preset minimum current setting value to achieve the control of the turn-off time of the power tube M1 in the N+1th cycle.

[0030] For example, if it is assumed that the current limit threshold (or current) when the current on the power tube M1 reaches the current limit point is I OCP , the minimum current of the inductor is I L , then the average current on the inductor I AVG It can be written as: ; Among them, when the current on the power tube M1 reaches the current limit point, the current limit threshold I OCP It is designed so that as long as the minimum current I L If the design is good, a controllable current limiting average current can be obtained, that is, by adjusting the off time of the power tube M1 and the minimum current I L The two are linked together to achieve overcurrent protection for the power tube M1 while ensuring that the energy stored in the inductor due to overcurrent of the power tube M1 has sufficient time to be released.

[0031] Therefore, a current limiting circuit with a fixed on-time structure is proposed in an embodiment of the present invention, so as to preset a current limiting threshold I OCP and the minimum current I L Then, starting from the first cycle, by adjusting the device parameters of at least some components in the fixed on-time structure, as the number of cycles increases, the starting current flowing through the power tube M1 in the next cycle is gradually reduced, until in a certain cycle, the starting current flowing through the power tube M1 is reduced to the minimum current I L .

[0032] In this embodiment, the power tube current acquisition module 10 is specifically connected to the upper tube in the voltage-drop switching power supply system, namely the power tube M1, and the energy storage element inductor, and is turned on when the driver U1 connected to the gate of the power tube M1 triggers its forward conduction. When the power tube M1 is turned on, the current on the power tube M1 (also the current on the inductor) can be collected and amplified based on the internal components of the power tube current acquisition module 10. That is, the current on the power tube M1 in different cycles is collected and monitored to avoid problems such as overcurrent. When overcurrent occurs, the power tube M1 is promptly shut down to protect the power tube M1 from burning out.

[0033] In one embodiment, the power transistor M1 may be a PMOS transistor or an NMOS transistor, and Figure 1Taking the power tube M1 as an NMOS tube as an example; the power tube current acquisition module 10 may include a driver U1, a sampling resistor R1 and an instrument amplifier INA1; wherein, the driver U1 may include a first input terminal, a second input terminal and an output terminal, and the first input terminal may be connected to the start drive signal DRV-H, so as to give the driver U1 an input by making the start drive signal DRV-H a high level or a low level, and make the output terminal of the driver U1 output a high level or a low level, so as to control the gate voltage of the power tube M1 to be greater than its threshold voltage, so that the power tube M1 is turned on in different cycles; it should be understood that the following examples of the present invention will be based on The subsequent related contents are introduced by taking the example of driving the power tube M1 to be turned on when the driving signal DRV-H is at a high level as an example. In other embodiments, the subsequent related operations can also be carried out by taking the example of driving the power tube M1 to be turned on when the driving signal DRV-H is at a low level as an example, and the invention is not limited thereto. The second input terminal of the driver U1 is connected to the shutdown driving signal OFF, so that when the current on the power tube M1 reaches the current limiting threshold, a high level is input to the driver U1, and then a low level is output to the power tube M1 through the output terminal of the driver U1 connected to the gate of the power tube M1, so as to control the power tube M1 to be turned off, that is, to complete the conduction and shutdown of the power tube M1 within one cycle.

[0034] Exemplarily, the sampling resistor R1 is connected in series with the power transistor M1 to collect the voltage difference generated by the current flowing through the power transistor M1. The two ends of the sampling resistor R1 are connected to the inputs of the instrumentation amplifier INA1 to convert the current into a voltage, amplify it, and output it. The output of the instrumentation amplifier INA1 is connected to the input of the power transistor current limiting control module 20. In one embodiment, the instrumentation amplifier INA1 includes a non-inverting input terminal (+), an inverting input terminal (-), and an output terminal. The two ends of the sampling resistor R1 are connected to the non-inverting input terminal and the inverting input terminal of the instrumentation amplifier INA1, respectively. The current is then converted into a voltage and amplified to obtain the first voltage VCS. It should be understood that in other embodiments, the power tube current acquisition module 10 may also include only the instrumentation amplifier INA1 (without the sampling resistor R1); in this case, the two input terminals of the instrumentation amplifier INA1 are respectively connected to the source and drain of the power tube M1 to acquire the voltage difference generated on the power tube M1 by the current flowing through the power tube M1 in each cycle, and then convert the current into a voltage and amplify it to obtain the first voltage VCS.

[0035] Furthermore, the power tube current acquisition module 10 may further include: a first transmission gate TG1, a first capacitor C1, a second transmission gate TG2, and a second capacitor C2, so as to first store the first voltage VCS output by the instrumentation amplifier INA1 in the power tube current acquisition 10 and then transmit it to a subsequent circuit, such as the power tube off-time control module 30; wherein, the first transmission gate TG1 includes an input end, an output end, and a control end, and its input end is connected to the output end of the instrumentation amplifier INA1 to receive the first voltage VCS, and its output end is connected to the first end of the first capacitor C1, so as to be turned on for a short time, such as 100ns, when the power tube M1 is turned on in each cycle, so as to realize the acquisition of the first voltage VCS corresponding to the starting current on the power tube M1 and the first voltage VCS The potential difference of S is stored in the first capacitor C1, the second end of the first capacitor C1 is connected to the reference ground GND, and then the first transmission gate TG1 is turned off; the second transmission gate TG2 is connected in series with the first transmission gate TG1, and includes an input end, an output end, and a control end. The input end of the second transmission gate TG2 is connected to the output end of the first transmission gate TG1 and the first end of the first capacitor C1, and the output end of the second transmission gate TG2 is connected to the first end of the second capacitor C2. In each cycle, after the first transmission gate TG1 is turned off (that is, after the short period of time when the power tube M1 is turned on, such as 100ns), the potential difference of the first voltage VCS is transferred from the first capacitor C1 to the second capacitor C2, and the second end of the second capacitor C2 is connected to the reference ground GND.

[0036] In this embodiment, the first transmission gate TG1 and the second transmission gate TG2 each include two control terminals, and the voltage control signals connected to the two control terminals of the first transmission gate TG1 and the second transmission gate TG2 are opposite to each other, so that only one transmission gate is in the on state within the same time period, and the voltage control signals of the two control terminals of the first transmission gate TG1 and the second transmission gate TG2 are specifically generated by the power tube current limiting control module 20.

[0037] Continue to refer Figure 1The power tube current limiting control module 20 may include a first comparator COMP1, a first SR trigger SRFF1, a pulse generator U2, a NAND gate NAND1, and an NOT gate INV1; wherein the first comparator COMP1 includes a non-inverting input terminal (+), an inverting input terminal (-), and an output terminal, the non-inverting input terminal (+) serving as the input terminal of the power tube current limiting control module 20, connected to the output terminal of the instrumentation amplifier INA1 to access the first voltage VCS, the inverting input terminal (-) is connected to the preset first reference voltage VREF1, and the output terminal is connected to the S input terminal of the first SR trigger SRFF1, and when the first voltage VCS is greater than the first reference voltage VREF1, the output terminal of the first comparator COMP1 outputs an overcurrent protection signal OCP to the Q output terminal of the first SR trigger SRFF1; the first SR trigger SRFF1 includes an S input terminal, an R input terminal, a Q output terminal, and an inverting output terminal of Q ( ), and the Q output terminal is connected to an input terminal of the NAND gate NAND1, and together with the pulse generator U2 forms the voltage control signals of the two control terminals of the first transmission gate TG1 and the second transmission gate TG2.

[0038] Specifically, the input terminal of the pulse generator U2 is connected to the first input terminal of the driver U1 and is also connected to the start-up drive signal DRV-H. That is, the voltage levels of the output terminals of the driver U1 and the pulse generator U2 are controlled by the same start-up drive signal DRV-H. The output terminal of the pulse generator U2 is connected to the other input terminal of the NAND gate NAND1. The output terminal of the NAND gate NAND1 and the output terminal of the IN gate INV1 are respectively connected to the control terminals of the first transmission gate TG1 and the second transmission gate TG2. In this way, within a short period of time, for example, 100 ns, after the power transistor M1 is turned on in each cycle, the first transmission gate TG1 is turned on and then turned off. After the first transmission gate TG1 is turned off, the second transmission gate TG2 is turned on. The first voltage VCS is transmitted to the first terminal of the second capacitor C2 through the first transmission gate TG1 and the second transmission gate TG2. This voltage VCS can also be understood as being transmitted to the input terminal of the power transistor off-time control module 30. The voltage VCS is further compared with the second reference voltage VREF2 to adjust the off-time of the power transistor M1 in the current cycle. In addition, the R input terminal of the first SR flip-flop SRFF1 inputs the RST1 signal (reset signal), and when the RST1 signal is at a high level, the Q output terminal of the first SR flip-flop SRFF1 is controlled to be low.

[0039] Furthermore, the power tube off-time control module 30 may include a first transconductance amplifier OTA1, a second transconductance amplifier OTA2, a constant current source I1, a third capacitor C3, a fourth capacitor C4, a second comparator COMP2 and a second SR trigger SRFF2; wherein, the first transconductance amplifier OTA1 may include a non-inverting input (+), an inverting input terminal (-) and an output terminal, and its non-inverting input terminal (+) is connected to the first terminal of the second capacitor C2 to access the first voltage VCS, its inverting input terminal (-) is connected to the preset second reference voltage VREF2, the output terminal of the first transconductance amplifier OTA1 is connected to the first terminal of the third capacitor C3 and the inverting input terminal (-) of the second comparator COMP2, the non-inverting input terminal of the second comparator COMP2 is connected to the first terminal of the fourth capacitor C4 and the constant current source I1, the output terminal of the second comparator COMP2 is connected to the R input terminal of the second SR trigger SRFF2, and the second SR trigger The S input terminal of SRFF2 is connected to the output terminal of the first comparator COMP1 in the power tube current limiting control module 20. Therefore, when the current of the power tube M1 reaches the current limiting threshold, the first comparator COMP1 outputs a high-level overcurrent protection signal OCP, and the second comparator COMP2 in the power tube shutdown time control module 30 converts the overcurrent protection signal OCP into a shutdown drive signal OFF and transmits it to the second input terminal of the driver U1 to control the power tube M1 to be shut down in time. The second transconductance amplifier OTA2 may also include a non-inverting input (+), an inverting input terminal (-), and an output terminal. The non-inverting input terminal (+) of the second transconductance amplifier OTA2 is connected to the output voltage feedback pin FB of the voltage drop switching power supply chip, the inverting input terminal (-) of the second transconductance amplifier OTA2 is connected to the preset third reference voltage VREF3, and the output terminal is connected to the constant current source I1 and the first terminal of the fourth capacitor C4. The second terminals of the third capacitor C3 and the fourth capacitor C4 are both connected to the reference ground terminal GND.

[0040] In this embodiment, starting from the first cycle, when the start-up drive signal DRV-H is at a high level, the driver U1 and the pulse generator U2 are synchronously driven. The driver U1 outputs a high voltage to the gate of the power transistor M1, turning on the power transistor M1 for the first time, thereby generating current between the source and drain of the power transistor M1. At the same time, when the start-up drive signal DRV-H is at a high level, the pulse generator U2 outputs a narrow pulse signal VP (hereinafter referred to as the VP signal). However, because the VP signal is a narrow pulse, the VP signal disappears shortly after the power transistor M1 is first turned on. When the VP signal disappears, the NAND gate NAND1 and the INV gate INV1 are unable to generate the corresponding VA control signal and VB control signal, that is, the first transmission gate TG1 and the second transmission gate TG2 are both turned off. As the current in the power transistor M1 continues to increase linearly until the current between the source and drain of the power transistor M1 reaches a preset current limit threshold, the current corresponding to the current limit threshold also flows through the sampling resistor R1, thereby generating a voltage difference across the sampling resistor R1. This voltage difference is amplified by the instrumentation amplifier INA1 by a factor of A1 and converted into a voltage. The output is the first voltage VCS. At this time, the first voltage VCS is greater than the voltage value of the first reference voltage VREF1, thereby triggering the output terminal of the first comparator COMP1 to output an overcurrent protection signal OCP (hereinafter referred to as the OCP signal). At this time, the OCP signal is at a high level. The high-level OCP signal triggers the Q output terminal of the first SR flip-flop SRFF1 to output a high level through the S input terminal of the first SR flip-flop SRFF1, and the second SR flip-flop SRFF2 outputs a high-level OFF signal, turning off the power tube M1. As the OCP signal disappears, the inductor begins to discharge until the fourth capacitor C4 is charged to the same voltage as VOTA through the constant current source I1. Then, the voltage at the output terminal of the second comparator COMP2 reverses, that is, the S input terminal of the second SR flip-flop SRFF2 is connected to a high level, the voltage at the Q output terminal of the second SR flip-flop SRFF2 reverses, the OFF signal disappears, and the first cycle ends. At this time, the start-up drive signal DRV-H is high again. Driven by the driver U1, the power tube M1 is turned on for the second time, that is, the second cycle begins.

[0041] In the second cycle, the principle of turning on and off the power tube M1 is the same as that in the first cycle, which will not be repeated here. However, the difference between the two is that in the instantaneous time after the power tube M1 is turned on for the second time (for example, about 100 nm after M1 is turned on), the power tube current acquisition module 10 samples the starting current on the power tube M1 to serve as the basis for adjusting the turn-off time of the power tube M1 in the second cycle (or understood as the minimum current when the inductor is discharged in the first cycle); specifically, in the instantaneous time after the power tube M1 is turned on for the second time, since the Q output terminal of the first SR flip-flop SRFF1 is set high in the first cycle, therefore, at the moment when the start drive signal DRV-H is high for the second time to drive the power tube M1 to turn on, the VP signal output by the pulse generator U2 and the high level of the Q output terminal of the first SR flip-flop SRFF1 can control the first transmission gate TG1 to turn on, and the power tube M1 collected by the power tube current acquisition module 10 is turned on. The first voltage VCS corresponding to the starting current of the second cycle is transmitted to the first capacitor C1 to obtain a second voltage VCS1, and VCS1=VCS. Subsequently, the first transmission gate TG1 is closed (after the instantaneous time) until the current on the power tube M1 reaches the current limit threshold. At this time, the output terminal of the first comparator COMP1 outputs an overcurrent protection signal OCP (high level). Since the Q output terminal of the first SR flip-flop SRFF1 is also connected to the S input terminal of the second SR flip-flop SRFF2 in the power tube off-time control module 30, and the Q output terminal of the second SR flip-flop SRFF2 is connected to the second output terminal of the driver U1 for driving the power tube M1, when the first comparator COMP1 outputs the overcurrent protection signal OCP, a corresponding shutdown drive signal OFF is generated and transmitted to the driver U1 to control the power tube M1 to be turned off. The power tube M1 is turned off for the second time, thereby achieving overcurrent protection for the power tube M1.

[0042] Before the power tube M1 is turned on for the third time, the Q end of the first SR trigger SRFF1 needs to be at a high level and will not flip until the RST1 signal is at a high level. Therefore, in the second cycle, when the OCP disappears, the Q end of the first SR trigger SRFF1 will not flip immediately, but will flip when the RST1 signal is at a high level. Then, based on the principle that the input end of the NAND gate NAND1 has a 0-output-1, the levels of the VA control signal and the VB control signal are flipped, that is, the VA control signal becomes a high level and the VB control signal becomes a low level. The first transmission gate TG1 is closed, and the second transmission gate TG2 is turned on. The first capacitor C1 starts to discharge and the second capacitor C2 starts to charge. The capacitance is much larger than the capacitance of the second capacitor C2. Therefore, after the power tube M1 is turned off in the second cycle, the voltage VCS2 on the second capacitor C2 = VCS1 = VCS, that is, the first voltage is transmitted to the second capacitor C2; since the voltage on the second capacitor C2 increases, it is bound to be larger than the second VREF2, and the output current of the first transconductance amplifier OTA1 gradually increases. Therefore, the voltage at the inverting input terminal (-) of the second comparator COMP2 gradually increases until the fourth capacitor C4 is charged to the same voltage as the inverting input terminal (-) of the second comparator COMP2. At this time, the voltage at the Q output terminal of the second comparator COMP2 flips, and the power tube M1 is turned on for the third time, that is, the third cycle begins.

[0043] Obviously, in the embodiment of the present invention, the current on the power tube M1 is sampled at the moment after the power tube M1 is turned on for the (N+1)th time. The voltage corresponding to the current is then transmitted to the power tube off-time control module 30. The voltage is compared and calculated with a second reference voltage corresponding to a preset target set value of the inductor current (or the set value or the ideal minimum value of the inductor current). The VOTA voltage is then output. The charging time of the fourth capacitor C4 is then adjusted based on the level of the VOTA voltage. The off-time of the power tube M1 when it is turned off for the (N+1)th time is also adjusted based on the level of the VOTA voltage.

[0044] Furthermore, embodiments of the present invention also propose how to utilize the second transconductance amplifier OTA2 in the power tube off-time control module 30 to control the off-time of the power tube M1 when the entire DC-DC system is just started or short-circuited. Specifically, because the voltage at the output voltage feedback pin FB is relatively low when the entire DC-DC system is just started or short-circuited, and the third reference voltage VREF3 at the inverting input terminal (-) of the second transconductance amplifier OTA2 is at a set value, when the entire DC-DC system is just started or short-circuited, the voltage at the non-inverting input terminal (+) of the second transconductance amplifier OTA2 in the power tube off-time control module 30 is less than the voltage at the inverting input terminal (-). Therefore, current can be drawn from the constant current source I1. Therefore, when the power tube M1 is turned off within a certain period and the fourth capacitor C4 begins to charge, the charging current of the fourth capacitor C4 connected to the constant current source I1 can be reduced by drawing current, thereby indirectly achieving the purpose of controlling the off-time of the power tube M1.

[0045] Based on the working principle of the voltage drop switching power supply system, the power tube M1 is first turned on and then turned off in each cycle to achieve voltage conversion. At present, in the existing technology, a fixed off time is usually set, but this fixed off time cannot completely release the energy on the inductor or be understood as the current on the inductor dropping to a set value; in the embodiment of the present invention, after the power tube M1 is turned off by the overcurrent protection signal OCP in the previous cycle, the interval time from the cycle to the next cycle, that is, the off time of the power tube M1, can be controlled by the power tube current limiting control module 20 and the power tube off time control module 30, so that the energy stored in the inductor in the previous cycle is fully released, thereby avoiding increasing the starting current of the power tube M1 at the start of the next cycle, and further the starting current of the power tube M1 at the start of the next cycle collected by the power tube current collection module 10 is compared with the preset minimum current I L The gap between the two can be solved by adjusting the parameters of one or more components in the power tube current acquisition module 10, the power tube current limiting control module 20 or the power tube off-time control module 30, such as the gain parameter of the instrumentation amplifier INA1, so as to dynamically adjust the starting current of the power tube M1 at the start of the next cycle, so that from the first cycle to multiple cycles later, the starting current of the power tube M1 (that is, the lowest current of the inductor after the power tube M1 is turned off) can be reduced to the set value.

[0046] For the convenience of description, the current value corresponding to the VCS2 voltage can be considered as I L , so I L The calculation formula is as follows: ; Among them, VCS2 is the voltage corresponding to the first voltage VCS corresponding to the starting current after the power tube M1 is turned on in a certain cycle when it is transmitted to the second capacitor C2. Since the capacitance of the first capacitor C1 is much larger than the capacitance of the second capacitor C2, VCS2=VCS1=VCS; that is, by controlling the voltage of VCS2 in a negative feedback manner, I L The current, thereby controlling the average inductor current I AVG , thereby controlling the energy of the entire system; and the calculation formula for the off time of the power tube M1 is: T OFF =(C4*V OTA ) / (I1-I OTA2 ); Among them, T OFF is the off time of the power tube, C4 is the capacitance value of the fourth capacitor, V OTA is the output voltage of the first transconductance amplifier, I1 is the current of the constant current source, I OTA2 is the current value drawn by the second transconductance amplifier from the constant current source.

[0047] In summary, in a current limiting circuit with a fixed on-time structure provided by the present invention, the current (current) on the power tube (upper tube) used to adjust the output voltage is detected in real time by the power tube current acquisition module, and when the current on the power tube reaches the current limit point (or current limit threshold), the power tube current limiting control module outputs an overcurrent protection signal to control the power tube to be turned off; then, under the control of the power tube current limiting control module, the starting current of the power tube is sampled at the moment when the power tube is turned on next time, and the starting current is used as the minimum current of the inductor after the power tube is turned off this time; then the power tube off-time control module The block uses the difference between the minimum current and the preset minimum value of the inductor current to control the turn-off time of the power tube when it is in the turn-off state next time, that is, to achieve the purpose of adjusting the turn-off time of the power tube and the discharge time of the inductor current, and realizes that after several cycles, through negative feedback regulation, the minimum value of the inductor current after the overcurrent protection signal is triggered reaches the set value. Therefore, in special circumstances such as overcurrent in the upper tube, the upper tube can be quickly turned off for overcurrent protection, and high-efficiency fixed on-time structure current limiting can be achieved without PWM signal, thereby improving the accuracy, timeliness and reliability of the current limiting of the fixed on-time structure.

[0048] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "example," or "specific example" means that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments. Furthermore, those skilled in the art may combine and reconcile different embodiments or examples described in this specification.

[0049] The above description is merely a preferred embodiment of the present invention and does not limit the present invention in any way. Any person skilled in the art who, without departing from the scope of the present invention, makes any equivalent substitution, modification, or other changes to the technical solution and technical content disclosed in the present invention shall be deemed to be within the scope of the present invention and still fall within the scope of protection of the present invention.

Claims

1. A current limiting circuit with a fixed on-time structure, characterized in that: The current limiting control is applied to a voltage drop switching power supply system. The voltage drop switching power supply system includes a switching power supply chip. The switching voltage chip has an output voltage feedback pin FB for feedback of output voltage changes and a power tube for regulating the output voltage. The current limiting circuit includes: A power tube current acquisition module, connected to the power tube, for acquiring the current flowing through the power tube in each cycle, and outputting a first voltage obtained by converting the current flowing through the power tube into a voltage and amplifying it in each cycle; The power tube current limiting control module is connected to the power tube current acquisition module and is used to output an overcurrent protection signal and control the power tube to be periodically shut down when the current flowing through the power tube reaches the current limiting threshold in each cycle; The power tube shutdown time control module is connected to the power tube current acquisition module and the power tube current limiting control module. It is used to adjust the shutdown time of the power tube after the power tube is shut down in the previous cycle, and adjust the starting current flowing through the power tube in the next cycle until the starting current flowing through the power tube is adjusted to the preset value after multiple cycles.

2. The current limiting circuit with a fixed on-time structure according to claim 1, wherein: The driving signal includes an on-driving signal and an off-driving signal; The power tube current limiting control module includes a driver, which is used to control the power tube to turn on at the beginning of each cycle when the start drive signal is connected, and when it is detected that the current flowing through the power tube reaches the current limiting threshold within the cycle, the power tube is controlled to turn off based on the shutdown drive signal generated by the power tube shutdown time control module.

3. The current limiting circuit with a fixed on-time structure according to claim 2, wherein: The power tube current limiting control module is connected to the first voltage and is used to output an overcurrent protection signal to the power tube shutdown time control module when it detects that the first voltage is greater than the first reference voltage in each cycle, and control the power tube to be periodically shut down through the shutdown drive signal output by the power tube shutdown time control module.

4. The current limiting circuit with a fixed on-time structure according to claim 3, wherein: The power tube off-time control module is used to receive a first voltage corresponding to the starting current of the power tube when it is turned on, which is collected by the power tube current collection module within a cycle, and use the voltage difference between the first voltage corresponding to the starting current and a second reference voltage to adjust the off-time of the power tube within the current cycle, thereby adjusting the starting current flowing through the power tube in the next cycle by adjusting the off-time of the power tube; wherein the second reference voltage is the voltage corresponding to the preset value of the starting current.

5. The current limiting circuit with a fixed on-time structure as claimed in claim 2, characterized in that: The power tube current acquisition module includes: an instrumentation amplifier, a first transmission gate, a first capacitor, a second transmission gate, and a second capacitor; wherein the first transmission gate includes an input terminal, an output terminal, and a control terminal, and its input terminal is connected to the output terminal of the instrumentation amplifier to receive a first voltage, and its output terminal is connected to the first terminal of the first capacitor, so as to turn on at the beginning of each cycle when the power tube is turned on and store a first voltage corresponding to the starting current when the power tube is turned on in the first capacitor, and the second terminal of the first capacitor is connected to the reference ground terminal; The second transmission gate is connected in series with the first transmission gate and includes an input end, an output end and a control end. The input end of the second transmission gate is connected to the output end of the first transmission gate and the first end of the first capacitor. The output end of the second transmission gate is connected to the first end of the second capacitor. After collecting the first voltage corresponding to the starting current when the power tube is turned on, the second transmission gate is turned on and the potential difference of the first voltage is transferred from the first capacitor to the second capacitor. The second end of the second capacitor is connected to the reference ground end.

6. The current limiting circuit with a fixed on-time structure as claimed in claim 5, characterized in that: The power tube current limiting control module includes a first comparator, a first SR trigger, a pulse generator, a NAND gate and a NOT gate; The first comparator includes a non-inverting input terminal, an inverting input terminal, and an output terminal. The non-inverting input terminal serves as the input terminal of the power tube current limiting control module and is connected to the output terminal of the instrumentation amplifier to access the first voltage. The inverting input terminal is connected to the first reference voltage. The output terminal is connected to the S input terminal of the first SR trigger and outputs an overcurrent protection signal when the first voltage is greater than the first reference voltage. The first SR trigger includes an S input terminal and a Q output terminal. The S input terminal is connected to the output terminal of the first comparator to receive the overcurrent protection signal when the first voltage is greater than the first reference voltage, and the Q output terminal is connected to an input terminal of the NAND gate. The input terminal of the pulse generator is connected to the first input terminal of the driver and is simultaneously connected to the start-up drive signal, and the output terminal of the driver is connected to the power tube. The output terminal of the pulse generator is connected to the other input terminal of the NAND gate, the output terminal of the NAND gate is connected to the input terminal of the NAND gate, and the output terminal of the NAND gate and the output terminal of the NAND gate are respectively connected to the control terminals of the first transmission gate and the second transmission gate to control the conduction and shutdown of the first transmission gate and the second transmission gate.

7. The current limiting circuit of the fixed on-time structure according to claim 6, characterized in that: The power tube off-time control module includes a first transconductance amplifier, a second transconductance amplifier, a constant current source, a third capacitor and a fourth capacitor; wherein, the non-inverting input terminal of the first transconductance amplifier is connected to the first terminal of the second capacitor to access the first voltage, its inverting input terminal is connected to the second reference voltage, and its output terminal outputs a first error voltage; the non-inverting input terminal of the second transconductance amplifier is connected to the output voltage feedback pin FB, its inverting input terminal is connected to the third reference voltage, and its output terminal is connected to the constant current source and the first terminal of the fourth capacitor, so that when the voltage of the voltage feedback pin FB is lower than the third reference voltage, its output terminal draws current from the constant current source, thereby increasing the charging time of the fourth capacitor.

8. The current limiting circuit with a fixed on-time structure as claimed in claim 7, wherein: The power tube off time control module further includes a second comparator and a second SR trigger; Among them, the second comparator includes a non-inverting input terminal, an inverting input terminal and an output terminal, the inverting input terminal of the second comparator is connected to the output terminal of the first transconductance amplifier and the first terminal of the third capacitor to access the first error voltage, the non-inverting input terminal of the second comparator is connected to the first terminal of the fourth capacitor and the constant current source and the output of the second transconductance amplifier, and when the voltage value of the first terminal of the fourth capacitor rises to the first error voltage, the comparator outputs a high level, otherwise it outputs a low level, and the second terminals of the third capacitor and the fourth capacitor are both connected to the reference ground terminal; the R input terminal of the second SR trigger is connected to the output terminal of the second comparator, the S input terminal of the second SR trigger is connected to the output terminal of the first comparator, and the Q output terminal of the second SR trigger is connected to the second input terminal of the driver.

9. The current limiting circuit with a fixed on-time structure as claimed in claim 8, characterized in that: The calculation formula for the power tube off time is: T OFF =(C4*V) OTA ) / (I1-I OTA2 ); Among them, T OFF is the off time of the power tube, C4 is the capacitance value of the fourth capacitor, V OTA is the output voltage of the first transconductance amplifier, I1 is the current of the constant current source, I OTA2 is the current value drawn by the second transconductance amplifier from the constant current source.

10. The current limiting circuit with a fixed on-time structure as claimed in claim 1, wherein: The power tube includes a PMOS tube or an NMOS tube.

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