A current limiting circuit with fixed on-time configuration

Abstract

The application provides a fixed on-time structure current limiting circuit applied to the technical field of switching power supply. In the application, a power tube current collection module detects the current flowing through the power tube, and outputs an overcurrent protection signal through a power tube current limiting control module to control the power tube to be turned off when the current on the power tube reaches a current limiting point; then, under the control of the power tube current limiting control module, the starting current of the power tube is sampled at the moment of the next turn-on of the power tube, and the starting current is taken as the minimum current of the inductor after the power tube is turned off this time; then, a power tube turn-off time control module controls the turn-off time of the power tube in the next turn-off state by using the difference between the minimum current and a preset minimum inductor current, so as to achieve the purpose of adjusting the turn-off time of the power tube and the discharge time of the inductor current, and to realize that, after several cycles, the minimum value of the inductor current after the overcurrent protection signal is triggered reaches a set value through negative feedback adjustment.

Description

TECHNICAL FIELD

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

[0002] Switching power management chips obtain stable output voltage or current by constantly controlling the conduction and turn-off of switching tubes, which can be divided into AC / DC (alternating current to direct current) and DC / DC (direct current to direct current) two categories. Since the AC / DC switching power supply often includes a DC / DC conversion module, DC / DC is a basic component of the switching power supply, and the DC / DC switching power supply includes three basic topological structures, namely, a step-down type, a step-up type, and a step-up / down type. In a step-down DC-DC power supply, especially when the input voltage is high, for example, 80V~100V, a COT structure (Constant on time, fixed on-time structure) is usually used, which stabilizes the output voltage by fixing the on-time of the power tube and dynamically adjusting the off-time according to the output voltage feedback.

[0003] The DC-DC converter of the COT structure (Constant on time, fixed on-time structure) has the characteristic of maintaining a fixed on-time in each cycle, and comparing the feedback voltage VFB of the output with the fixed reference voltage VREF, when VFB is lower than VREF, the next on-time period is triggered immediately. Since the COT structure does not have an oscillator inside, it cannot generate a PWM signal, which leads to the problem of difficulty in current limiting design or inability to limit current in the PWM mode. SUMMARY

[0004] The purpose of the present application is to provide a current limiting circuit of a fixed on-time structure, which can accurately limit the current of the fixed on-time structure without 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 application provides a current limiting circuit of a fixed on-time structure, which is applied to current limiting control of a step-down switching power supply system, the step-down switching power supply system includes a switching power supply chip, and the switching voltage chip has an output voltage feedback pin FB for feeding back the change of the output voltage and a power tube for adjusting the output voltage.

[0006] The current limiting circuit in the embodiment of the present application includes:

[0007] A power tube current collection module connected to the power tube, for collecting the current flowing through the power tube in each cycle, and outputting a first voltage converted from the current flowing through the power tube in each cycle to a voltage and amplified.

[0008] The power tube current collection module is connected with the power tube current limiting control module, and is configured to output an overcurrent protection signal and control the power tube to be periodically turned off when the current flowing through the power tube reaches the current limiting threshold in each cycle.

[0009] The power tube current collection module is connected with the power tube current limiting control module, and is configured to output an overcurrent protection signal and control the power tube to be periodically turned off when the current flowing through the power tube reaches the current limiting threshold in each cycle.

[0010] Further, the driving signal can include an on driving signal and an off driving signal; the power tube current limiting control module includes a driver, which is configured to control the power tube to be turned on at the beginning of each cycle when the on driving signal is accessed, and control the power tube to be turned off based on the off driving signal generated by the power tube turn-off time control module when the current flowing through the power tube reaches the current limiting threshold in the cycle.

[0011] Further, the power tube current limiting control module accesses the first voltage, and is configured to output an overcurrent protection signal to the power tube turn-off time control module when the first voltage is greater than the first reference voltage in each cycle, and control the power tube to be periodically turned off through the off driving signal output by the power tube turn-off time control module.

[0012] Further, the power tube turn-off time control module is specifically configured to receive the first voltage corresponding to the starting current when the power tube is turned on, which is collected by the power tube current collection module in a cycle, and adjust the turn-off time of the power tube in the cycle by using the voltage difference between the first voltage corresponding to the starting current and a second reference voltage, so as to 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.

[0013] Further, the power tube current collection module can 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 the input end thereof is connected with the output end of the instrumentation amplifier to receive the first voltage, and the output end thereof is connected with the first end of the first capacitor to be turned on at the beginning of the power tube being turned on 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 with a reference ground end.

[0014] The second transmission gate is connected in series with the first transmission gate and comprises an input end, an output end and a control end, the input end of the second transmission gate is connected with the output end of the first transmission gate and the first end of the first capacitor, and the output end of the second transmission gate is connected with the first end of the second capacitor, so as to turn on after the first voltage corresponding to the starting current when the power tube is turned on is collected and transfer the potential difference of the first voltage from the first capacitor to the second capacitor, and the second end of the second capacitor is connected with the reference ground end.

[0015] Further, the power tube current limiting control module comprises a first comparator, a first SR flip-flop, a pulse generator, an NAND gate and a NOT gate; wherein the first comparator comprises a non-inverting input end, an inverting input end and an output end, the non-inverting input end is used as the input end of the power tube current limiting control module and is connected with the output end of the instrument amplifier to access the first voltage, the inverting input end is connected with the first reference voltage, and the output end is connected with the S input end of the first SR flip-flop and outputs an overcurrent protection signal when the first voltage is greater than the first reference voltage.

[0016] The first SR flip-flop comprises an S input end and a Q output end, the S input end is connected with the output end of the first comparator to access the overcurrent protection signal when the first voltage is greater than the first reference voltage, the Q output end is connected with an input end of the NAND gate, the input end of the pulse generator is connected with the first input end of the driver and simultaneously accesses the opening driving signal, the output end of the driver is connected with the power tube, and the output end of the pulse generator is connected with another input end of the NAND gate, the output end of the NAND gate is connected with the input end of the NOT gate, and the output end of the NAND gate is connected with the output end of the NOT gate, respectively, to correspondingly connect the control ends of the first transmission gate and the second transmission gate, so as to control the turn-on and turn-off of the first transmission gate and the second transmission gate.

[0017] Further, the power tube turn-off time control module can comprise a first transconductance amplifier, a second transconductance amplifier, a constant current source, a third capacitor and a fourth capacitor; wherein the non-inverting input end of the first transconductance amplifier is connected with the first end of the second capacitor to access the first voltage, the inverting input end is connected with the second reference voltage, and the output end outputs a first error voltage; the non-inverting input end of the second transconductance amplifier is connected with the voltage feedback pin FB, the inverting input end is connected with the third reference voltage, and the output end is connected with the constant current source and the first end of the fourth capacitor, so that the output end pulls current from the constant current source when the voltage of the voltage feedback pin FB is lower than the third reference voltage, thereby increasing the charging time of the fourth capacitor.

[0018] Further, the power tube turn-off time control module further comprises a second comparator and a second SR flip-flop; wherein the second comparator comprises a non-inverting input end, an inverting input end and an output end, the inverting input end is connected with the output end of the first trans-impedance amplifier and the first end of the third capacitor to access the first error voltage, the non-inverting input end is connected with the first end of the fourth capacitor, a constant current source and the output of the second trans-impedance amplifier, and outputs a high level when the voltage value of the first end of the fourth capacitor rises to the first error voltage, otherwise outputs a low level, the second ends of the third capacitor and the fourth capacitor are connected with a reference ground end; the R input end of the second SR flip-flop is connected with the output end of the second comparator, the S input end is connected with the output end of the first comparator, and the Q output end is connected with the second input end of the driver.

[0019] Further, the calculation formula of the turn-off time of the power tube is:

[0020] T OFF =(C4*V OTA ) / (I1-I OTA2 );

[0021] Wherein, T OFF is the turn-off time of the power tube, C4 is the capacitance value of the fourth capacitor, V OTA is the output voltage of the first trans-impedance amplifier, I1 is the current of the constant current source, and I OTA2 is the current value of the current drawn by the second trans-impedance amplifier from the constant current source.

[0022] Further, the power tube comprises a PMOS tube or an NMOS tube.

[0023] Compared with the prior art, the present application has the following beneficial effects:

[0024] In the fixed on-time structure current limiting circuit provided by the application, the current on the power tube (upper tube) for adjusting the output voltage is detected in real time by a power tube current collection module, and when the current on the power tube reaches a current limiting point (or current limiting threshold), an overcurrent protection signal is output by a power tube current limiting control module 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 of the next turn-on of the power tube, and the starting current is taken as the minimum current of the inductor after the power tube is turned off this time; then the power tube turn-off time control module controls the turn-off time of the next turn-off state of the power tube by using the difference between the minimum current and the preset minimum inductor current, that is, the purpose of adjusting the turn-off time of the power tube and the discharge time of the inductor current is achieved, and 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, so that the upper tube can be turned off quickly and overcurrent protection can be performed under the condition of overcurrent of the upper tube, and high-efficiency fixed on-time structure current limiting can be achieved without PWM signal, and the accuracy, timeliness and reliability of the fixed on-time structure current limiting are improved. BRIEF DESCRIPTION OF DRAWINGS

[0025] The accompanying drawings are included to provide a further understanding of the application, and are incorporated in and constitute a part of this specification, illustrate embodiments of the application, and together with the description serve to explain the principles of the application. In the drawings:

[0026] Figure 1 The circuit schematic diagram of the fixed on-time structure current limiting circuit provided in an embodiment of the application.

[0027] In the drawings, the same components are designated by the same reference numerals, and the drawings are not drawn to scale. DETAILED DESCRIPTION

[0028] To make the objectives, advantages and features of the application more apparent, the application will be further described in detail below with reference to the drawings and specific embodiments. It should be noted that the drawings are very simplified and not drawn to scale, and are only used to facilitate and clearly assist the purpose of explaining the embodiments of the application. In addition, the structures shown in the drawings are often part of the actual structures. In particular, the emphasis of each drawing is different, and sometimes different scales are used.

[0029] As used in this invention, the singular forms “a,” “an,” and “the” include plural objects; the term “or” is generally used to mean “and / or”; the term “a number” is generally used to mean “at least one”; and the term “at least two” is generally used to mean “two or more”. Furthermore, the terms “first,” “second,” and “third” are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as “first,” “second,” or “third” may explicitly or implicitly include one or at least two of that feature. “One end” and “the other end,” as well as “proximal end” and “distal end,” generally refer to two corresponding parts, including not only endpoints. The terms “installed,” “connected,” and “joined” should be interpreted broadly, for example, as a fixed connection, a detachable connection, or an integral part; a mechanical connection or an electrical connection; a direct connection or an indirect connection through an intermediate medium; or a connection within two elements or an interaction between two elements. Furthermore, as used in this invention, the phrase "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 can be direct or indirect through an intermediate element. It should not be construed as indicating or implying a spatial positional relationship between the two elements, i.e., one element can be located arbitrarily inside, outside, above, below, or to one side of the other element, unless otherwise explicitly stated. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0030] 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 this embodiment can be set within a dropout switching power supply system. The dropout switching power supply system may include an inductor (not shown) and a power transistor M1 connected in series with the inductor for adjusting the output voltage. Furthermore, the dropout 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 current or voltage, and a second input terminal SW connected to other circuits or sub-circuits. For example... Figure 1 As shown, in the embodiments provided by the present invention, the current limiting circuit with a fixed conduction time structure may include: a power transistor current acquisition module 10, a power transistor current limiting control module 20, and a power transistor turn-off time control module 30.

[0031] The power tube current collection module 10 is connected with the power tube M1 (upper tube) and an energy storage element inductor (not shown) to collect the current flowing through the power tube M1 in each cycle, detect the current flowing between the source and the drain of the power tube M1 when the power tube M1 is turned on, and output a first voltage VCS obtained by converting the current flowing through the power tube M1 in each cycle into a voltage and amplifying the voltage.

[0032] The power tube current collection module 10 is connected with the power tube M1 (upper tube) and an energy storage element inductor (not shown) to collect the current flowing through the power tube M1 in each cycle, detect the current flowing between the source and the drain of the power tube M1 when the power tube M1 is turned on, and output a first voltage VCS obtained by converting the current flowing through the power tube M1 in each cycle into a voltage and amplifying the voltage.

[0033] The power tube current collection module 10 is connected with the power tube M1 (upper tube) and an energy storage element inductor (not shown) to collect the current flowing through the power tube M1 in each cycle, detect the current flowing between the source and the drain of the power tube M1 when the power tube M1 is turned on, and output a first voltage VCS obtained by converting the current flowing through the power tube M1 in each cycle into a voltage and amplifying the voltage.

[0034] It should be particularly noted that the power tube M1 needs to be periodically limited, that is, the current flowing through the power tube M1 is monitored in each cycle, and the power tube M1 needs to be turned off immediately to cut off the energy input once the current flowing through the power tube M1 reaches a preset current limiting threshold, and the power tube M1 is turned on again in the next cycle to determine whether to be turned on, so as to achieve continuous current protection through "periodic limitation" and avoid complete lock of the DC-DC system. It is found through research that in a step-down switching power supply system, the current flowing through the power tube M1 gradually increases linearly from being turned on to being turned off in a certain cycle, 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 starts to discharge after the power tube M1 is turned off. Since the current of the inductor cannot be suddenly changed, in other words, the current of the inductor is continuous, when the power tube M1 is turned off for the Nth time when the current flowing through the power tube M1 reaches the current limiting threshold, and after a certain period of time, the corresponding starting current of the power tube M1 in the N+1th cycle is approximately equal to the minimum current of the inductor after the power tube M1 is turned off in the Nth cycle. The embodiment of the present application uses the current flowing through the power tube M1 detected by the power tube current collection module 10 at the moment when the power tube M1 is turned on for the N+1th time as the minimum current of the inductor after the power tube M1 is turned off for the Nth time, and compares the minimum current with a preset minimum current to control the turn-off time of the power tube M1 in the N+1th cycle.

[0035] For example, if the current flowing through the power transistor M1 reaches the current limiting threshold (or current) I OCP , the minimum current flowing through the inductor is I L , then the average current flowing through the inductor I AVG may be written as:

[0036]

[0037] wherein the current limiting threshold I OCP of the power transistor M1 when the current flowing through the power transistor M1 reaches the current limiting point is designed, and thus as long as the minimum current I L flowing through the inductor is also designed, a controllable current limiting average current can be obtained, i.e. by associating the off time of the power transistor M1 with the minimum current I L (the set value of the preset minimum current), the overcurrent protection of the power transistor M1 is achieved while ensuring that the energy stored in the inductor due to the overcurrent of the power transistor M1 has sufficient release time.

[0038] Therefore, the fixed on time structure current limiting circuit provided in the embodiments of the present application is proposed to pre-set a current limiting threshold I OCP and a minimum current I L , and then gradually reduce the initial current flowing through the power transistor M1 in the next cycle by adjusting the device parameters of at least part of the components in the fixed on time structure from the first cycle, until the initial current flowing through the power transistor M1 in a certain cycle is reduced to the minimum current I L .

[0039] In the embodiment, the power transistor current collecting module 10 is specifically connected with the upper transistor, i.e. the power transistor M1 and the energy storage element inductor in the voltage drop switching power supply system, and is then opened when the driver U1 connected to the gate of the power transistor M1 triggers the forward conduction of the power transistor M1, and thus the current flowing through the power transistor M1 (also the current flowing through the inductor) can be collected and amplified based on the internal components of the power transistor current collecting module 10 when the power transistor M1 is turned on, i.e. the current flowing through the power transistor M1 in different cycles is collected and monitored to avoid overcurrent and other problems, and the power transistor M1 is turned off in time when overcurrent occurs to protect the power transistor M1 from being burned out.

[0040] In an embodiment, the power transistor M1 can be a PMOS transistor or an NMOS transistor, and Figure 1 ​is an NMOS tube; the power tube current collection module 10 can include a driver U1, a sampling resistor R1 and an instrument amplifier INA1; wherein the driver U1 can include a first input end and a second input end and an output end, the first input end can be connected to the opening driving signal DRV-H, so as to give the driver U1 an input by letting the opening driving signal DRV-H be high or low, and make the output end of the driver U1 output a high or low level, so as to control the gate voltage of the power tube M1 to be greater than the threshold voltage, so that the power tube M1 is turned on in different periods; it should be understood that the following related content will be introduced by taking the example of driving the power tube M1 to be turned on when the opening driving signal DRV-H is high, and in other embodiments, the following related operation can also be taken as an example of driving the power tube M1 to be turned on when the opening driving signal DRV-H is low, and the like; the second input end of the driver U1 is connected to the off driving signal OFF, so as to input a high level to the driver U1 when the current on the power tube M1 reaches the current limiting threshold, and then output a low level to the power tube M1 through the output end of the driver U1 connected with the gate of the power tube M1, to control the power tube M1 to be turned off, that is, to complete the turn-on and turn-off of the power tube M1 in a period.

[0041] For example, the sampling resistor R1 is connected in series with the power tube M1 to collect the voltage difference generated by the current flowing through the power tube M1 on the power tube M1, and the two ends of the sampling resistor R1 are connected with the input end of the instrument amplifier INA1 to convert the current into voltage and output after amplification, and the output end of the instrument amplifier INA1 is connected with the input end of the power tube current limiting control module 20. In an embodiment, the instrument amplifier INA1 includes a non-inverting input end (+) and an inverting input end (-) and an output end, the two ends of the sampling resistor R1 are connected with the non-inverting input end and the inverting input end of the instrument amplifier INA1 respectively, and then the current is converted into voltage and amplified to obtain the first voltage VCS. It should be understood that in other embodiments, the power tube current collection module 10 can also only include an instrument amplifier INA1 (without a sampling resistor R1); at this time, the two input ends of the instrument amplifier INA1 are connected with the source and drain of the power tube M1 respectively, to collect the voltage difference generated by the current flowing through the power tube M1 on the power tube M1 in each period, and then the current is converted into voltage and amplified to obtain the first voltage VCS.

[0042] Further, the power tube current acquisition module 10 can further comprise a first transfer gate TG1, a first capacitor C1, a second transfer gate TG2 and a second capacitor C2, to store and then transfer the first voltage VCS outputted by the instrumentation amplifier INA1 in the power tube current acquisition module 10 to a subsequent circuit, for example, the power tube turn-off time control module 30; wherein the first transfer gate TG1 comprises an input end, an output end and a control end, and the input end of the first transfer gate TG1 is connected with the output end of the instrumentation amplifier INA1 to receive the first voltage VCS, and the output end of the first transfer gate TG1 is connected with the first end of the first capacitor C1, so as to realize the acquisition of the first voltage VCS corresponding to the starting current on the power tube M1 and the storage of the potential difference of the first voltage VCS in the first capacitor C1 in the short time of the conduction of the power tube M1 in each period, for example, 100ns, and the second end of the first capacitor C1 is connected with the reference ground end GND, and then the first transfer gate TG1 is turned off; the second transfer gate TG2 is connected in series with the first transfer gate TG1 and comprises an input end, an output end and a control end, the input end of the second transfer gate TG2 is connected with the output end of the first transfer gate TG1 and the first end of the first capacitor C1, and the output end of the second transfer gate TG2 is connected with the first end of the second capacitor C2, so as to transfer the potential difference of the first voltage VCS from the first capacitor C1 to the second capacitor C2 after the first transfer gate TG1 is turned off (i.e. after the short time of the conduction of the power tube M1 ends, for example, 100ns) in each period, and the second end of the second capacitor C2 is connected with the reference ground end GND.

[0043] In the embodiment, the first transfer gate TG1 and the second transfer gate TG2 each comprise two control ends, and the voltage control signals inputted into the two control ends of the first transfer gate TG1 and the second transfer gate TG2 are opposite to each other, so that only one transfer gate is in the conduction state in the same time period, and the voltage control signals of the two control ends of the first transfer gate TG1 and the second transfer gate TG2 are generated by the power tube current control module 20.

[0044] With reference to the foregoing Figure 1The power tube current limiting control module 20 can include a first comparator COMP1, a first SR flip-flop SRFF1, a pulse generator U2, a NAND gate NAND1 and an inverter 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 (+) is connected with the output terminal of the instrumentation amplifier INA1 to access the first voltage VCS as the input terminal of the power tube current limiting control module 20, the inverting input terminal (-) is connected with a preset first reference voltage VREF1, and the output terminal is connected with the S input terminal of the first SR flip-flop 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 flip-flop SRFF1. The Q output terminal and an input terminal of the NAND gate NAND1 are connected, and the voltage control signal of the two control terminals of the first transmission gate TG1 and the second transmission gate TG2 is formed by the pulse generator U2.

[0045] Specifically, the input terminal of the pulse generator U2 is connected with the first input terminal of the driver U1 and simultaneously accesses the turn-on driving signal DRV-H, that is, the same turn-on driving signal DRV-H is used to control the high and low levels of the output terminals of the driver U1 and the pulse generator U2; the output terminal of the pulse generator U2 is connected with another input terminal of the NAND gate NAND1, and the output terminal of the NAND gate NAND1 and the output terminal of the inverter INV1 are respectively connected with the control terminals of the first transmission gate TG1 and the second transmission gate TG2, so that the first transmission gate TG1 is turned on for a short time, for example, 100 ns, after the power tube M1 is turned on in each period, and then is turned off, and the second transmission gate TG2 is turned on after the first transmission gate TG1 is turned off, so that the first voltage VCS is transmitted to the first end of the second capacitor C2 through the first transmission gate TG1 and the second transmission gate TG2, that is, to the input terminal of the power tube turn-off time control module 30, and is further compared with the second reference voltage VREF2 to adjust the turn-off time of the power tube M1 in the current period. In addition, the R input terminal of the first SR flip-flop SRFF1 inputs an 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 set to low.

[0046] Further, the power tube off-time control module 30 can include a first trans-impedance amplifier OTA1, a second trans-impedance amplifier OTA2, a constant current source I1, a third capacitor C3, a fourth capacitor C4, a second comparator COMP2 and a second SR flip-flop SRFF2; wherein the first trans-impedance amplifier OTA1 can include a non-inverting input (+), an inverting input (-) and an output, and the non-inverting input (+) thereof is connected to the first end of the second capacitor C2 to access the first voltage VCS, and the inverting input (-) thereof is connected to a preset second reference voltage VREF2, the output of the first trans-impedance amplifier OTA1 is connected to the first end of the third capacitor C3 and the inverting input (-) of the second comparator COMP2, the non-inverting input of the second comparator COMP2 is connected to the first end of the fourth capacitor C4 and the constant current source I1, the output of the second comparator COMP2 is connected to the R input of the second SR flip-flop SRFF2, and the S input of the second SR flip-flop SRFF2 is connected to the output of the first comparator COMP1 in the power tube current limiting control module 20, so that 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 overcurrent protection signal OCP is converted into an off driving signal OFF by the second comparator COMP2 in the power tube off-time control module 30 and transmitted to the second input of the driver U1 to control the power tube M1 to shut down in time; the second trans-impedance amplifier OTA2 can also include a non-inverting input (+), an inverting input (-) and an output, and the non-inverting input (+) of the second trans-impedance amplifier OTA2 is connected to the output voltage feedback pin FB of the voltage drop switching power supply chip, the inverting input (-) thereof is connected to a preset third reference voltage VREF3, and the output thereof is connected to the constant current source I1 and the first end of the fourth capacitor C4, and the second ends of the third capacitor C3 and the fourth capacitor C4 are connected to the reference ground terminal GND.

[0047] In the embodiment, from the first cycle, when the start driving signal DRV-H is high, the driver U1 outputs a high voltage to the gate of the power tube M1, the power tube M1 is turned on for the first time, and then the current is generated between the source and the drain of the power tube M1; at the same time, when the start driving signal DRV-H is high, the pulse generator U2 outputs a narrow pulse signal VP (hereinafter referred to as VP signal), but because the VP signal is a narrow pulse, the VP signal disappears soon after the power tube M1 is turned on for the first time, the VP signal disappears, and the NAND gate NAND1 and the inverter INV1 cannot form corresponding VA control signal and VB control signal, that is, the first transmission gate TG1 and the second transmission gate TG2 are turned off, with the continuous linear increase of the current on the power tube M1, until the current between the source and the drain of the power tube M1 reaches the preset current limiting threshold, the current corresponding to the current limiting threshold also flows through the sampling resistor R1, and then a voltage difference is generated across the sampling resistor R1, the voltage difference is amplified by the instrument amplifier INA1 by A1 times, and then converted into a voltage output, that is, the first voltage VCS; at this time, the first voltage VCS is greater than the voltage value of the first reference voltage VREF1, and then the output end of the first comparator COMP1 outputs the overcurrent protection signal OCP (hereinafter referred to as OCP signal), at this time, the OCP signal is high, the high-level OCP signal triggers the Q output end of the first SR flip-flop SRFF1 to output high level through the S input end of the first SR flip-flop SRFF1, and the second SR flip-flop SRFF2 outputs the OFF signal with high level, the power tube M1 is turned off, with the disappearance of the OCP signal, the inductor starts to discharge, until the fourth capacitor C4 is charged to the same voltage as VOTA through the constant current source I1, the output end voltage of the second comparator COMP2 is flipped, that is, the S input end of the second SR flip-flop SRFF2 is connected to high level, the Q output end voltage of the second SR flip-flop SRFF2 is flipped, the OFF signal disappears, and the first cycle ends; at this time, the start driving signal DRV-H is high again, the power tube M1 is turned on for the second time through the driving of the driver U1, that is, the second cycle is entered.

[0048] In the second cycle, the principle of the power tube Ml is turned on and off is the same as the above first cycle, here is not repeated, but the difference is that, in the instantaneous time (for example, about 100 nm after Ml is turned on) after the second time of power tube Ml is turned on, the power tube current acquisition module 10 samples the starting current on the power tube Ml as the basis for adjusting the power tube Ml off time in the second cycle (or understand as the minimum current when the inductor is discharged in the first cycle); Specifically, in the instantaneous time after the second time of power tube Ml is turned on, because the Q output end of the first SR trigger SRFF1 is high in the first cycle, at the moment when the second time of opening the driving signal DRV-H drives the power tube Ml to turn on, the high level of the VP signal output by the pulse generator U2 and the Q output end of the first SR trigger SRFF1 can control the first transmission gate TG1 to turn on, and the first voltage VCS corresponding to the starting current of the power tube Ml in the second cycle collected by the power tube current acquisition module 10 is transmitted to the first capacitor C1, to get the second voltage VCS1, and VCS1=VCS, then the first transmission gate TG1 is closed (after the instantaneous time), until the current on the power tube Ml reaches the current limiting threshold, the overcurrent protection signal OCP (high level) is output from the output end of the first comparator COMP1, because the Q output end of the first SR trigger SRFF1 is also connected with the S input end of the second SR trigger SRFF2 in the power tube off time control module 30, and the Q output end of the second SR trigger SRFF2 is connected with the second output end of the driver U1 for driving the power tube Ml, then when the first comparator COMP1 outputs the overcurrent protection signal OCP, the off driving signal OFF is formed, and the off driving signal OFF is transmitted to the driver U1 to control the power tube Ml to turn off, the power tube Ml is turned off for the second time, and the overcurrent protection of the power tube Ml is realized.

[0049] Before the third time the power tube M1 is turned on, the Q terminal of the first SR flip-flop SRFF1 needs to be high level, and is not turned until the RST1 signal is high level, thus in the second period, when the OCP disappears, the Q terminal of the first SR flip-flop SRFF1 will not be immediately turned, but is turned when the RST1 signal is high level, and then based on the principle that the input terminal of the NAND gate NAND1 has 0 to 1, the levels of the VA control signal and the VB control signal are turned, i.e. the VA control signal becomes high level, the VB control signal becomes 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, and since the capacitance of the first capacitor C1 is far greater than the capacitance of the second capacitor C2, thus after the power tube M1 is turned off in the second period, the voltage VCS2 on the second capacitor C2 = VCS1 = VCS, i.e. the first voltage is transmitted to the second capacitor C2; since the voltage on the second capacitor C2 is increased, thus is necessarily greater than the second VREF2, the output terminal current of the first transconductance amplifier OTA1 is gradually increased, thus the voltage of the inverting input terminal (-) of the second comparator COMP2 is gradually increased, until the fourth capacitor C4 is charged to the same voltage as the inverting input terminal (-) of the second comparator COMP2, the voltage of the Q output terminal of the second comparator COMP2 is turned, the power tube M1 is turned on for the third time, i.e. enters the third period.

[0050] Obviously, the embodiment of the application is through the power tube M1 at the moment after the N+1 time is turned on, the current on the power tube M1 is sampled, then the voltage corresponding to the current is transmitted to the power tube turn-off time control module 30, and the second reference voltage corresponding to the target setting value (or the setting value or the ideal minimum value of the inductor current) of the preset inductor current is compared and calculated, then the VOTA voltage is output, and then the high and low of the VOTA voltage is used to adjust the charging time of the fourth capacitor C4, and the high and low of the VOTA voltage is also used to adjust the turn-off time of the N+1 time the power tube M1 is turned off.

[0051] Further, the application embodiment also proposes how to control the power tube M1 turn-off time by using the second trans-impedance amplifier OTA2 in the power tube turn-off time control module 30 when the DC-DC whole system is just started or in the special situation of short circuit; specifically, because the voltage at the output voltage feedback pin FB is small when the DC-DC whole system is just started or in the special situation of short circuit, and the third reference voltage VREF3 at the inverting input end (-) of the second trans-impedance amplifier OTA2 is a set value, therefore, when the DC-DC whole system is just started or in the special situation of short circuit, the voltage at the non-inverting input end (+) of the second trans-impedance amplifier OTA2 in the power tube turn-off time control module 30 is smaller than the voltage at the inverting input end (-), thus the current can be pulled from the constant current source I1, so that when the fourth capacitor C4 starts to charge after the power tube M1 is turned off in a certain period, the charging current of the fourth capacitor C4 connected with the constant current source I1 can be reduced by the pulling current, thereby indirectly achieving the purpose of controlling the turn-off time of the power tube M1.

[0052] Based on the working principle of the voltage drop switching power supply system, the power tube M1 is turned on first and then turned off in each period to realize voltage conversion. At present, in the prior art, a fixed turn-off time is usually set, but the fixed turn-off time cannot release the energy on the inductor or be understood as the current on the inductor dropping to a set value; in the application embodiment, after the power tube M1 is turned off by the overcurrent protection signal OCP in the last period, the interval time from the current period to the next period, i.e. the turn-off time of the power tube M1, can be controlled by the power tube current collection module 10, the power tube current limiting control module 20 and the power tube turn-off time control module 30, so that the energy stored on the inductor in the last period is fully released, thereby avoiding increasing the starting current of the power tube M1 at the beginning of the next period, and further adjusting the difference between the starting current of the power tube M1 at the beginning of the next period collected by the power tube current collection module 10 and the preset minimum current I L , by adjusting the parameters of one or more components in the power tube current collection module 10, the power tube current limiting control module 20 or the power tube turn-off time control module 30, such as the gain parameter of the instrumentation amplifier INA1, thereby dynamically adjusting the starting current of the power tube M1 at the beginning of the next period, and further reducing the starting current of the power tube M1 at the beginning of the next period to a set value from the first period to multiple periods.

[0053] For convenience of description, the current value corresponding to the VCS2 voltage can be considered as I L , and the calculation formula of I L is as follows:

[0054] ;

[0055] VCS2=VCS1=VCS; that is, by means of negative feedback to control the voltage of VCS2, the current of I L can be controlled, and then the average current of I AVG of the inductor can be controlled, and then the energy of the whole system can be controlled; and the calculation formula of the off time of the power tube M1 is:

[0056] T OFF = (C4*V OTA ) / (I1-I OTA2 );

[0057] wherein, 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, and I OTA2 is the current value of the current pulled from the constant current source by the second transconductance amplifier.

[0058] In summary, in the fixed on-time structure current limiting circuit provided by the application, the current (current) on the power tube (upper tube) for adjusting the output voltage is detected in real time by the power tube current collection module, and when the current on the power tube reaches the current limiting point (or current limiting threshold), an overcurrent protection signal is output by the power tube current limiting control module 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 of the next turn-on of the power tube, and the starting current is taken as the minimum current of the inductor after the power tube is turned off this time; then the power tube off time control module controls the off time of the power tube in the next off state by using the difference between the minimum current and the preset minimum inductor current, that is, the purpose of adjusting the off time of the power tube and the discharge time of the inductor current is achieved, and after several cycles, through negative feedback adjustment, the minimum value of the inductor current after triggering the overcurrent protection signal reaches the set value, so that the upper tube can be turned off quickly and overcurrent protection can be performed in the case of overcurrent of the upper tube, and high-efficiency fixed on-time structure current limiting can be achieved without PWM signal, and the precision, timeliness and reliability of the fixed on-time structure current limiting are improved.

[0059] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", or "a specific example" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any suitable manner in any one or more embodiments. In addition, those skilled in the art can combine and integrate different embodiments or examples described in the present specification.

[0060] The above are only preferred embodiments of the present application, and do not have any limiting effect on the present application. Any person skilled in the art can make any form of equivalent replacement or modification, etc. to the technical solutions and technical contents disclosed in the present application without departing from the scope of the technical solutions of the present application, which still belongs to the protection scope of the present application.

Claims

1. A current limiting circuit of a fixed conduction time configuration, characterized by, The application relates to a current limiting control applied to a voltage drop switching power supply system, the voltage drop switching power supply system comprising a switching power supply chip, the switching power supply chip having an output voltage feedback pin FB for feeding back output voltage variation and a power tube for regulating output voltage, and the current limiting circuit comprising: a power tube current collection module connected to the power tube, for collecting current flowing through the power tube in each period and outputting a first voltage converted from the current flowing through the power tube in each period and amplified; a power tube current limiting control module connected to the power tube current collection module, for outputting an overcurrent protection signal and controlling the power tube to be periodically turned off when the current flowing through the power tube reaches a current limiting threshold in each period; a power tube turn-off time control module connected to the power tube current collection module and the power tube current limiting control module, for adjusting the turn-off time of the power tube after the power tube is turned off in a previous period, and adjusting the starting current flowing through the power tube in a next period until the starting current flowing through the power tube is adjusted to a preset value after multiple periods; wherein the power tube turn-off time control module comprises a second transconductance amplifier, a constant current source and a fourth capacitor; the non-inverting input end of the second transconductance amplifier is connected to the output voltage feedback pin FB, the inverting input end of the second transconductance amplifier is connected to a third reference voltage, and the output end of the constant current source is connected to the first end of the fourth capacitor and the output end of the second transconductance amplifier, so that the charging current of the fourth capacitor connected to the constant current source is reduced by means of the output end of the second transconductance amplifier drawing current from the constant current source when the voltage drop switching power supply system is just started or short-circuited and after the power tube is turned off in a certain period, and the turn-off time of the power tube is indirectly controlled.

2. The fixed on-time configuration current limit circuit of claim 1, wherein, The driving signal comprises an on driving signal and an off driving signal; the power tube current limiting control module comprises a driver, for controlling the power tube to be turned on at the beginning of each period when the on driving signal is connected, and for controlling the power tube to be turned off based on the off driving signal generated by the power tube turn-off time control module when the current flowing through the power tube reaches the current limiting threshold in the period.

3. The fixed on-time configuration current limit circuit of claim 2, wherein, The power tube current limiting control module is connected to the first voltage, and is used for outputting an overcurrent protection signal to the power tube turn-off time control module when the first voltage is greater than a first reference voltage in each period, and controlling the power tube to be periodically turned off through the off driving signal output by the power tube turn-off time control module.

4. The fixed on-time configuration current limit circuit of claim 3, wherein, The power tube turn-off time control module is used for receiving the first voltage corresponding to the starting current flowing through the power tube when the power tube is turned on, which is collected by the power tube current collection module in a period, and adjusting the turn-off time of the power tube in the period by using the voltage difference between the first voltage corresponding to the starting current and a second reference voltage, so as to adjust the starting current flowing through the power tube in a next period 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.

5. The fixed on-time configuration current limit circuit of claim 2, wherein, The power tube current acquisition module comprises an instrument amplifier, a first transmission gate, a first capacitor, a second transmission gate and a second capacitor; the first transmission gate comprises an input end, an output end and a control end, the input end of the first transmission gate is connected with the output end of the instrument amplifier to receive the first voltage, and the output end of the first transmission gate is connected with the first end of the first capacitor to be turned on at the beginning of the conduction of the power tube in each cycle and store the first voltage corresponding to the starting current when the power tube is conducted in the first capacitor, and the second end of the first capacitor is connected with a reference ground end; The second transmission gate is connected in series with the first transmission gate and comprises an input end, an output end and a control end, the input end of the second transmission gate is connected with 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 with the first end of the second capacitor to be turned on after the first voltage corresponding to the starting current when the power tube is conducted is acquired and transfer the potential difference of the first voltage from the first capacitor to the second capacitor, and the second end of the second capacitor is connected with the reference ground end.

6. The fixed on-time configuration current limit circuit of claim 5, wherein, The power tube current limiting control module comprises a first comparator, a first SR flip-flop, a pulse generator, an NAND gate and a NOT gate; The first comparator comprises a non-inverted input end, an inverted input end and an output end, the non-inverted input end is used as the input end of the power tube current limiting control module and is connected with the output end of the instrument amplifier to access the first voltage, the inverted input end is connected with a first reference voltage, the output end is connected with the S input end of the first SR flip-flop and outputs an overcurrent protection signal when the first voltage is greater than the first reference voltage; The first SR flip-flop comprises an S input end and a Q output end, the S input end is connected with the output end of the first comparator to access the overcurrent protection signal when the first voltage is greater than the first reference voltage, the Q output end is connected with one input end of the NAND gate, the input end of the pulse generator is connected with the first input end of the driver and simultaneously accesses an opening driving signal, the output end of the driver is connected with the power tube, the output end of the pulse generator is connected with the other input end of the NAND gate, the output end of the NAND gate is connected with the input end of the NOT gate, and the output end of the NAND gate is connected with the output end of the NOT gate, respectively, to correspondingly connect the control ends of the first transmission gate and the second transmission gate to control the conduction and turn-off of the first transmission gate and the second transmission gate.

7. The fixed on-time configuration current limit circuit of claim 6, wherein, The power tube turn-off time control module comprises a first transconductance amplifier and a third capacitor; the non-inverted input end of the first transconductance amplifier is connected with the first end of the second capacitor to access the first voltage, the inverted input end of the first transconductance amplifier is connected with a second reference voltage, and the output end of the first transconductance amplifier outputs a first error voltage.

8. The fixed on-time configuration current limit circuit of claim 7, wherein, The power tube turn-off time control module further comprises a second comparator and a second SR flip-flop; The second comparator comprises a non-inverted input end, an inverted input end and an output end, the inverted input end is connected with the output end of the first trans-impedance amplifier and the first end of the third capacitor to access the first error voltage, the non-inverted input end is connected with the first end of the fourth capacitor, a constant current source and the output of the second trans-impedance amplifier, and outputs a high level when the voltage value of the first end of the fourth capacitor rises to the first error voltage, otherwise, outputs a low level, the second ends of the third capacitor and the fourth capacitor are connected with a reference ground end; the R input end of the second SR flip-flop is connected with the output end of the second comparator, the S input end is connected with the output end of the first comparator, and the Q output end is connected with the second input end of the driver.

9. The fixed on-time configuration current limit circuit of claim 8, wherein, The calculation formula of the turn-off time of the power tube is: T OFF = (C4*V OTA ) / (I1-I OTA2 ); Wherein, T OFF is the turn-off time of the power tube, C4 is the capacitance value of the fourth capacitor, V OTA is the output voltage of the first trans-impedance amplifier, I1 is the current of the constant current source, I OTA2 is the current value of the current pulled by the second trans-impedance amplifier from the constant current source.

10. The fixed on-time configuration current limit circuit of claim 1, wherein, The power tube comprises a PMOS tube or an NMOS tube.

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