CN110943429A

CN110943429A - Undervoltage protection circuit and control method thereof

Info

Publication number: CN110943429A
Application number: CN201911406642.0A
Authority: CN
Inventors: 李鑫坤; 尹显泓; 刘杰
Original assignee: Mornsun Guangzhou Science and Technology Ltd
Current assignee: Mornsun Guangzhou Science and Technology Ltd
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2020-03-31
Anticipated expiration: 2039-12-31
Also published as: CN110943429B

Abstract

The invention discloses an undervoltage protection circuit, which comprises a sampling circuit 100, a control circuit 200, a switch circuit 300 and a hysteresis circuit 400; compared with the prior art, the voltage at two ends of the resistor R4 is used as the input of the switch circuit, and the voltage stabilizing diode D1 is connected in series with the input loop of the switch circuit, so that the loss of the undervoltage protection circuit is reduced; the collector of the PNP triode is connected in series with a diode D2 and a resistor R6 to form stable, accurate and easily-adjusted hysteresis voltage. The invention has the advantages of accurate on-off control of the undervoltage protection circuit, low loss, accurate and easily-adjusted hysteresis voltage and the like, thereby improving the overall reliability of the applied circuit.

Description

Undervoltage protection circuit and control method thereof

Technical Field

The present invention relates to switching power supplies, and more particularly, to an under-voltage protection circuit of a switching power supply and a control method thereof.

Background

Most of the existing electronic products, such as: DC/DC converters, etc., all need an under-voltage protection circuit to avoid the problem that the electronic product is damaged due to the abnormal working state when the input voltage is too low.

Fig. 1 is a diagram of a conventional under-voltage protection circuit, where the sampling circuit 100 of the under-voltage protection circuit shown in fig. 1 samples an input voltage Vin and outputs a sampling signal, and then the sampling signal is input to a reference pin of a TL431 of the control circuit 200, and the TL431 outputs a control signal according to the sampling signal and passes through a zener diode Z1 to a switching tube Q2 of a hysteresis circuit 300 and a switching tube Q1 of a switching circuit; the transistor Q2 of the hysteretic circuit 300 provides the hysteretic voltage under the condition of under-voltage protection by turning on the reference pin voltage of the pull-down TL 431; the transistor Q1 of the switch circuit 400 is turned on to pull down the COMP pin level of the switch power supply control IC, thereby turning off the switch power supply.

The hysteresis voltage is as follows: and the voltage difference value of the undervoltage protection point of the input voltage and the input voltage point recovered again after the undervoltage protection of the input voltage.

The COMP pin of the IC is: the voltage error amplifier of the switch power supply control IC outputs, and when the COMP pin is pulled low, the switch power supply control IC does not output.

The input undervoltage protection realized by adopting the circuit mainly has the following defects:

1. the transistor Q2 of the hysteresis circuit of the circuit is connected with the transistor Q1 of the switch circuit in parallel, and due to the fact that base conduction voltage drops of the two transistors cannot be completely consistent due to batch difference, conduction time of the two transistors is inconsistent, even only a single transistor is conducted, and the output of a product is unstable.

2. Because the voltage stabilizing diode has certain leakage current and the error amplifier TL431 has a fuzzy area near the critical value, the undervoltage circuit can not completely release the level of the power supply control end, which shows that the product can have restarting or unstable output.

The chinese patent application No. 201610101819.6 provides an input under-voltage protection circuit and method, which solves the problem that the output of the product is unstable or the product is restarted due to the existing input under-voltage technology. Fig. 2 shows an under-voltage protection circuit, in which a sampling circuit samples an input voltage and outputs a sampling signal, a control circuit receives the sampling signal and outputs a control signal to a switching circuit and a hysteresis circuit, the switching circuit pulls down a Vcc pin level of a switching power supply control IC when an under-voltage occurs, a feedback circuit detects the Vcc pin level and provides a feedback signal to the control circuit, so that the Vcc pin level is quickly pulled down, a product enters under-voltage protection, and the hysteresis circuit pulls down the sampling signal to generate a hysteresis voltage of the under-voltage protection, thereby making the under-voltage protection of the product more reliable. Switch circuit stops to draw low Vcc pin level when undervoltage resumes, and feedback circuit detects Vcc pin level and provides feedback signal to control circuit, makes switch circuit release Vcc pin level rapidly, and Vcc pin level is charged to control IC operating voltage scope by outside starting circuit, and the work of resuming smoothly to the product has eliminated the hiccup phenomenon of product in the recovery process.

The voltage at the point A of the under-voltage protection state needs to meet the following conditions at the same time: the voltage of the point A is larger than the TR1 conduction threshold voltage, the voltage of the point A is larger than the voltage of a D4 voltage regulator tube, the voltage of the point A is larger than the 431 working voltage, the voltage of the point A is smaller than the VCC voltage, and the voltage of the point A is smaller than the IC starting voltage. And the voltage at which the VCC voltage is pulled low is difficult to determine.

Although the invention 201610101819.6 improves the problems of the prior art, the circuit for realizing the undervoltage protection has the following disadvantages due to the limitation of the operating conditions:

1. because the voltage of the pulled-down VCC voltage cannot be determined and the turn-on voltage is influenced by the VCC voltage, the control of the turn-on and turn-off voltages is inaccurate;

2, the abnormal working phenomenon of the comparator is easy to occur after the Vcc voltage is pulled down;

3. in order to meet the requirement that the operating voltage range of the point A needs a small value of R5, the current flowing through R5 is large after the undervoltage protection circuit is in an undervoltage recovery state, and the loss of the corresponding undervoltage protection circuit is large.

Disclosure of Invention

Therefore, the invention aims to solve the problems of large loss, inaccurate control voltage, product restart, abnormal working of the comparator and the like of the undervoltage protection circuit. The undervoltage protection circuit with high precision, low loss and high reliability is provided.

The invention conception of the application is as follows: the sampling circuit 100 samples input voltage and outputs a sampling signal, the control circuit 200 receives the sampling signal and compares the sampling signal with a reference signal to output a control signal, the control signal triggers the switch circuit 300, the switch circuit is switched on to enable the EN and the hysteresis circuit to act, and the hysteresis circuit 400 compensates the sampling circuit in the under-voltage cancellation process to reduce the under-voltage turn-off value. The switching circuit is composed of a triode, a voltage stabilizing tube and a current limiting resistor, so that the selection of the control end differential input resistor is large, and the current required by the undervoltage protection circuit is reduced. The hysteresis circuit acts during the undervoltage recovery to provide a compensation current, the same value is reached, the open-circuit voltage is reduced, a hysteresis effect is formed, the turn-off voltage value is reduced, the hysteresis voltage of undervoltage protection is generated, and the undervoltage protection of the product is more reliable. Therefore, the undervoltage protection circuit applied by the invention has the advantages of low loss, high precision of the starting voltage, easy regulation of the hysteresis voltage and high undervoltage reliability.

Based on the inventive concept, the invention provides the following specific schemes:

an undervoltage protection circuit, comprising: the circuit comprises a sampling circuit, a control circuit, a switch circuit and a hysteresis circuit, wherein the sampling circuit is used for sampling input voltage and outputting a sampling signal; the control circuit is used for receiving the sampling signal and outputting a control signal to trigger the switch circuit after comparing the sampling signal with a reference signal, and comprises a comparator U1, a resistor R3 and a resistor R4, wherein a reference end of the comparator U1 is used as an input end of the control circuit to be connected with an output end of the sampling circuit and an output end of the hysteresis circuit, an anode of a comparator U1 is used as a grounding end of the control circuit to be connected with the ground, a cathode of a comparator U1 is connected with a resistor R3, the other end of a resistor R3 is used as an output end of the control circuit to be connected with a control end of the switch circuit and a resistor R4, and the; the switching circuit is used for receiving and outputting a control signal of the control circuit, the input end of the switching circuit is connected with the output end of the control circuit and comprises a resistor R5, a resistor R7, a voltage stabilizing diode D1 and a switching tube Q1, the anode of the voltage stabilizing diode D1 is connected with an external power supply Vcc, the cathode of the voltage stabilizing diode D1 is connected with the emitter of a PNP triode Q1, the collector of the PNP triode Q1 is used as the feedback end of the switching circuit to be connected with the input end of the hysteresis circuit and a resistor R7, the other end of the resistor R7 is used as the output end of the switching circuit and is used for being connected with the enable end EN of the chip, the base of the PNP triode Q865; the hysteresis circuit is used for compensating the sampling circuit in the undervoltage withdrawal process and comprises a diode D2 and a resistor R6, the anode of the diode D2 is used as the input end of the hysteresis circuit to be connected with the feedback end of the switch circuit, the cathode of the diode D2 is connected with the resistor R6, and the other end of the resistor R6 is used as the output end of the hysteresis circuit to be connected with the output end of the sampling circuit and the control input end of the control circuit.

Preferably, the sampling circuit includes a resistor R1 and a resistor R2, one end of the resistor R1 is a sampling circuit input end and is connected to the input voltage Vin, the other end is connected to the resistor R2, a control circuit control input end, and a hysteresis circuit output end, one end of the resistor R2 is connected to the resistor R1, the control circuit input end, and the other end of the hysteresis circuit output end is connected to ground.

Preferably, the switch circuit further includes a resistor R8, wherein one end of the resistor R8 is connected to the resistor R7, and the other end is connected to the ground GND.

The invention also provides an undervoltage protection control method, which comprises the following steps of generating and transmitting a control signal in the starting process: when the sampling signal is greater than the built-in reference Vref of the comparator U1, the cathode current of the comparator U1 is rapidly increased, the cathode voltage is reduced, the voltage of the connection point of a resistor R3 and a resistor R4 of the control circuit is reduced along with the cathode voltage of the comparator U1, the voltage difference between the voltage of the connection point of a resistor R3 and a resistor R4 and the external power supply voltage Vcc serves as the output signal of the control circuit, and the switching circuit is triggered to be switched on; generating a switching signal and establishing a hysteresis signal in the starting process: when the input signal of the switching circuit is increased, the current of a loop formed by the voltage-stabilizing diode D1, the emitter and the base of the PNP triode Q1 and the resistor R5 is increased, the current of the base of the PNP triode Q1 is increased, the current of the collector is increased, and the collector is output to the enable end EN of the chip through the resistor R7 to control the chip to normally start and work; meanwhile, the collector voltage of the PNP triode Q1 is increased, the voltages of the input end and the output end of the hysteresis circuit are increased, the diode D2 is conducted, and a certain current is provided for the sampling resistor R2 through the resistor R6 to form a hysteresis signal; the method comprises the following steps of generating and transmitting a control signal in the turn-off process: when the sampling signal superposition hysteresis signal is still smaller than the built-in reference Vref of the comparator U1, the current of the cathode of the comparator U1 is rapidly reduced, the voltage of the cathode is increased, the voltage at the connection point of the resistor R3 and the resistor R4 is increased along with the voltage of the cathode of the comparator U1, and the voltage difference between the voltage at the connection point of the resistor R3 and the resistor R4 and the external power supply voltage Vcc is used as the output signal of the control circuit to be connected with the input signal of the switching circuit and is switched; generating a switching signal and turning off a hysteresis signal in the turning-off process: when the input signal of the switching circuit is reduced, the voltage stabilizing diode D1 is cut off, the PNP triode Q1 is in a cut-off region, and the collector is output to the chip enable end EN through the resistor R7 to control the chip to be turned off; meanwhile, the collector voltage of the PNP triode Q1 is reduced, the voltages of the input end and the output end of the hysteresis circuit are reduced, the diode D2 is cut off in the reverse direction, and the hysteresis signal formed by the sampling resistor R2 is cancelled through the resistor R6.

Preferably, the turn-on voltage vin (on) of the turn-on process is:

wherein, Vref is the built-in reference voltage of the comparator U1, R1 is the resistance of the resistor R1, and R2 is the resistance of the resistor R2.

Preferably, the turn-off voltage vin (off) of the turn-off process is:

the Vref is a built-in reference voltage of the comparator U1, Vcc is a power supply terminal voltage, Vc is a voltage of an input terminal of the hysteresis circuit, R1 is a resistance value of the resistor R1, R2 is a resistance value of the resistor R2, and R6 is a resistance value of the resistor R6.

Preferably, the hysteresis voltage vin (on) -vin (off) is:

The invention further provides an undervoltage protection circuit, which comprises a sampling circuit, a control circuit, a switch circuit and a hysteresis circuit, wherein the sampling circuit comprises a resistor R1 and a resistor R2 which are sequentially connected in series between an input voltage end Vin and a ground end GND, and the connection point of the resistor R1 and the resistor R2 is used as the output end of the sampling circuit; the control circuit comprises a comparator U1, a resistor R3 and a resistor R4, wherein the resistor R4 and the resistor R3 are connected in series between a power supply terminal Vcc and a cathode of the comparator U1, the series connection point of the resistor R4 and the resistor R3 is used as the output end of the control circuit, a reference terminal of the comparator U1 is used as the input end of the control circuit and is connected with the output end of the sampling circuit, and the anode of the comparator U1 is connected with a ground terminal GND; the switching circuit comprises a switching tube Q1, a voltage stabilizing diode D1, a resistor R5 and a resistor R7, wherein the base electrode of the switching tube Q1 is connected with the output end of the control circuit through a resistor R5, the emitter electrode of the switching tube Q1 is connected with the anode electrode of the voltage stabilizing diode D1, the cathode electrode of the voltage stabilizing diode D1 is connected with a power supply end Vcc, the collector electrode of the switching tube Q1 serving as the feedback end of the switching circuit is connected with one end of the resistor R7, and the other end of the resistor R7 serving as the output end of the switching circuit is used for outputting an under; the hysteresis circuit comprises a diode D2 and a resistor R6, wherein the anode of the diode D2 is connected with the feedback end of the switch circuit, the cathode of the diode D2 is connected with one end of a resistor R6, and the other end of the resistor R6 is connected with the input end of the control circuit and the sampling output end.

Preferably, the switch circuit further includes a resistor R8, one end of the resistor R8 is connected to the other end of the resistor R7, and the other end of the resistor R8 is connected to the ground GND.

Compared with the prior art, the invention has the following remarkable effects:

1. the advantage of accurate starting voltage can be obtained by directly utilizing the voltage division of the resistors R1 and R2 to compare with the built-in reference Vref of the control comparator U1 during starting;

2. the voltage at two ends of a control circuit resistor R4 is used as a switch circuit input signal, and a voltage stabilizing diode D1 is connected in series with a switch circuit input loop, so that the situation that the working current of a comparator U1 is controlled in the existing circuit, and the switch circuit is triggered by mistake when the resistance value of R4 is large can be avoided, so that the resistance value of R4 is large, and the input current of the undervoltage protection circuit in an undervoltage cancellation state is still reduced (the current value required by the undervoltage protection control circuit part after undervoltage cancellation is about (Vcc-Vref)/R4, and the power of the undervoltage protection part is about Vcc (Vcc-Vref)/R4), thereby reducing the loss of the undervoltage protection circuit and realizing that;

3, the Vcc end supplies power and utilizes more stable voltage, the voltage difference between the collector of the triode Q1, the resistor R7, the input end voltage of the hysteresis circuit and the Vref voltage is not easily interfered, the hysteresis voltage is accurate, the compensation current of the hysteresis voltage is in direct proportion to the resistor R6, and accurate adjustment after calculation and adjustment can be realized.

Drawings

FIG. 1 is a diagram of a conventional under-voltage protection circuit;

FIG. 2 is a circuit diagram of an under-voltage protection circuit of the prior art patent 201610101819.6;

FIG. 3 is a schematic circuit diagram of a first embodiment of the under-voltage protection circuit of the present invention;

FIG. 4 is a waveform diagram illustrating the under-voltage cancellation process of the under-voltage protection circuit according to the first embodiment of the present invention;

FIG. 5 is a waveform diagram illustrating the turn-off process of the under-voltage protection circuit according to the first embodiment of the present invention;

FIG. 6 is a schematic circuit diagram of a second embodiment of the under-voltage protection circuit of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

First embodiment

Fig. 3 is a schematic circuit diagram of an under-voltage protection circuit according to a first embodiment of the present invention, and the difference between the scheme of the present invention shown in fig. 3 and the circuit schemes shown in the conventional under-voltage protection circuit diagram of fig. 1 and the conventional 201610101819.6 patent under-voltage protection circuit diagram of fig. 2 is that: the voltage at two ends of a resistor R4 is used as the input of a switch circuit, the anode of a voltage stabilizing diode D1 is connected with Vcc, the cathode of a voltage stabilizing diode D1 is connected with an emitter of a PNP triode Q1, the output is controlled to be a chip enable EN pin, a diode D2 is added, and a resistor R6 is added.

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

An undervoltage protection circuit, comprising: the circuit comprises a sampling circuit, a control circuit, a switching circuit and a hysteresis circuit.

And the sampling circuit is used for sampling the input voltage and outputting a sampling signal, the input end of the sampling circuit is connected with the input voltage Vin, and the output end of the sampling circuit is connected with the input end of the control circuit and the output end of the hysteresis circuit.

The control circuit is used for receiving the sampling signal and outputting a control signal to trigger the switch circuit after comparing the sampling signal with a reference signal, and comprises a comparator U1, a resistor R3 and a resistor R4, wherein a reference end of the comparator U1 is used as an input end of the control circuit to be connected with an output end of the sampling circuit and an output end of the hysteresis circuit, an anode of a comparator U1 is used as a grounding end of the control circuit to be connected with the ground, a cathode of a comparator U1 is connected with a resistor R3, the other end of a resistor R3 is used as an output end of the control circuit to be connected with a control end of the switch circuit and a resistor R4, and the.

The switching circuit is used for receiving and outputting a control signal of the control circuit, the input end of the switching circuit is connected with the output end of the control circuit, and the switching circuit comprises a resistor R5, a resistor R7, a voltage stabilizing diode D1 and a switching tube Q1, the anode of the voltage stabilizing diode D1 is connected with an external power supply Vcc, the cathode of the voltage stabilizing diode D1 is connected with an emitter of a PNP triode Q1, a collector of the PNP triode Q1 is used as the feedback end of the switching circuit to be connected with the input end of the hysteresis circuit and a resistor R7, the other end of the resistor R7 is used as the output end of the switching circuit and is used for being connected with an enable end EN of a chip, the base of the PN.

The hysteresis circuit is used for compensating the sampling circuit in the undervoltage withdrawal process and comprises a diode D2 and a resistor R6, the anode of the diode D2 is used as the input end of the hysteresis circuit to be connected with the feedback end of the switch circuit, the cathode of the diode D2 is connected with the resistor R6, and the other end of the resistor R6 is used as the output end of the hysteresis circuit to be connected with the output end of the sampling circuit and the control input end of the control circuit.

The under-voltage protection control principle of the circuit of the invention is as follows,

generating and transmitting a sampling signal: in the starting process, the input end of the sampling circuit is connected with Vin, current flows through the resistor R1, the resistor R2, and the connection position of the resistor R1 and the resistor R2 outputs a sampling signal to the control circuit.

The method comprises the following steps of generating and transmitting a control signal in the starting process: and the control circuit receives the sampling signal, and when the sampling signal is greater than the built-in reference Vref of the comparator U1, the current of the cathode of the comparator U1 is rapidly increased, and the voltage of the cathode is reduced. The voltage of the connection point of the resistor R3 and the resistor R4 is reduced along with the voltage of the cathode of the comparator U1, and the voltage difference between the voltage of the connection point of the resistor R3 and the resistor R4 and the external power supply voltage Vcc is used as an input signal of the control circuit output signal connection switch circuit to be switched on.

Generating a switching signal and establishing a hysteresis signal in the starting process: the high-voltage input end (input Vin end) of the switch circuit is connected with a voltage stabilizing diode D1, the low-voltage end (input GND end) of the switch circuit is connected with a resistor R5, and the output signal of the control circuit, namely the input signal of the switch circuit, is increased in the starting process, so that the current of a loop formed by the voltage stabilizing diode D1, an emitter and a base of a PNP triode Q1 and the resistor R5 is increased. The base current of the PNP triode Q1 is increased to cause the collector current to be increased, the collector is output to the current control type enable EN input through the resistor R7, and the chip is started normally to work. The collector voltage of the PNP triode Q1 rises, the voltage of the input end and the output end of the hysteresis circuit increases, the diode D2 is conducted, a certain current is provided for the sampling resistor R2 through the resistor R6, and according to the circuit superposition theorem, the voltage of the Vin end required by the same Verf is reduced to form a hysteresis signal.

The method comprises the following steps of generating and transmitting a control signal in the turn-off process: the control circuit receives the sampling signal, and when the Vin-end sampling signal superposition hysteresis signal is still smaller than the built-in reference Vref of the comparator U1, the current of the cathode of the comparator U1 is rapidly reduced, and the voltage of the cathode is increased. The voltage at the connection point of the resistor R3 and the resistor R4 is increased along with the voltage of the cathode of the comparator U1, the voltage difference between the voltage at the connection point of the resistor R3 and the resistor R4 and the external power supply voltage Vcc is reduced, the switch circuit inputs a turn-off signal, and the switch circuit is turned off.

Generating a switching signal and turning off a hysteresis signal in the turning-off process: the high-voltage end of the input of the switch circuit is connected with a voltage-stabilizing diode D1, the low-voltage end of the input of the switch circuit is connected with a resistor R5, and the output signal of the control circuit, namely the input signal of the switch circuit, is reduced in the turn-off process, so that the voltage-stabilizing diode D1 is cut off. The PNP triode Q1 is in a cut-off region, the collector is output to the current control type enabling EN input through the resistor R7, and the chip is closed to work. The collector voltage of the PNP triode Q1 is reduced, the voltages of the input end and the output end of the hysteresis circuit are reduced, the diode D2 is cut off in the reverse direction, and the hysteresis signal formed by the sampling resistor R2 is cancelled through the resistor R6.

Fig. 4 is a voltage waveform diagram of the under-voltage protection circuit according to the first embodiment of the present invention during the start-up process, and it is easy to know from the above principle that the set value of the start-up voltage vin (on) is:

The collector of the triode Q1, the resistor R6, and the input end of the hysteresis circuit are recorded as Vc, fig. 5 is a voltage waveform diagram of the undervoltage protection circuit according to the first embodiment of the present invention in the turn-off process, and the set values of the turn-off voltage vin (off) are:

the collector of the triode Q1, the resistor R6 and the input end of the hysteresis circuit are recorded as Vc, and the value of the hysteresis voltage difference is:

table 1 shows the under-voltage protection circuit according to the first embodiment of the present invention, and according to the experimental results of testing the turn-on and turn-off when the selected Vref is 2.5V and the circuit is composed of R1, R2, and R6, it can be found from the experimental results that the error of the turn-on and turn-off voltages of the scheme of the present invention is less than 0.1V, and the input current of the under-voltage protection current is less than 2 mA.

Table 1 shows the results of the on and off tests

R1

R2

R6

Turn-on voltage

Off voltage

Hysteresis pressure difference

Input current

51K

3.9K

200K

35.2V

34.0V

1.2V

1.5mA

51K

3.9K

100K

35.2V

32.8V

2.4V

1.5mA

Second embodiment

As shown in fig. 6, in the second embodiment of the present invention, a resistor R8 is added to the switch circuit, one end of the resistor R8 is connected to the resistor R7, and the other end is connected to the ground GND. The voltage division of the resistor R7 and the resistor R8 is realized by inputting the connection point of the R7 and the R8 as a voltage type control chip, and the voltage division ratio of the resistors R7 and R8 is set to realize the voltage control type enable EN.

The embodiments of the present invention are not limited thereto, and according to the above-mentioned contents of the present invention, the specific implementation circuit of the present invention can be modified, replaced or changed in various other forms without departing from the basic technical idea of the present invention. Furthermore, although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. An undervoltage protection circuit, comprising: a sampling circuit, a control circuit, a switch circuit and a hysteresis circuit,

the sampling circuit is used for sampling the input voltage and outputting a sampling signal, the input end of the sampling circuit is connected with the input voltage Vin, and the output end of the sampling circuit is connected with the input end of the control circuit and the output end of the hysteresis circuit;

the control circuit is used for receiving the sampling signal and outputting a control signal to trigger the switch circuit after comparing the sampling signal with a reference signal, and comprises a comparator U1, a resistor R3 and a resistor R4, wherein a reference end of the comparator U1 is used as an input end of the control circuit to be connected with an output end of the sampling circuit and an output end of the hysteresis circuit, an anode of a comparator U1 is used as a grounding end of the control circuit to be connected with the ground, a cathode of a comparator U1 is connected with a resistor R3, the other end of a resistor R3 is used as an output end of the control circuit to be connected with a control end of the switch circuit and a resistor R4, and the;

the switching circuit is used for receiving and outputting a control signal of the control circuit, the input end of the switching circuit is connected with the output end of the control circuit and comprises a resistor R5, a resistor R7, a voltage stabilizing diode D1 and a switching tube Q1, the anode of the voltage stabilizing diode D1 is connected with an external power supply Vcc, the cathode of the voltage stabilizing diode D1 is connected with the emitter of a PNP triode Q1, the collector of the PNP triode Q1 is used as the feedback end of the switching circuit to be connected with the input end of the hysteresis circuit and a resistor R7, the other end of the resistor R7 is used as the output end of the switching circuit and is used for being connected with the enable end EN of the chip, the base of the PNP triode Q865;

2. The undervoltage protection circuit of claim 1, wherein: the sampling circuit comprises a resistor R1 and a resistor R2, one end of the resistor R1 is the input end of the sampling circuit and is connected with the input voltage Vin, the other end of the resistor R2 is connected with the control input end of the control circuit and the output end of the hysteresis circuit, and one end of the resistor R2 is connected with the resistor R1, the input end of the control circuit and the other end of the output end of the hysteresis circuit and is connected with the ground.

3. The undervoltage protection circuit of claim 1 or 2, wherein: the switch circuit further comprises a resistor R8, one end of the resistor R8 is connected with the resistor R7, and the other end of the resistor R8 is grounded to GND.

4. An undervoltage protection control method comprises the following steps:

the method comprises the following steps of generating and transmitting a control signal in the starting process: when the sampling signal is greater than the built-in reference Vref of the comparator U1, the cathode current of the comparator U1 is rapidly increased, the cathode voltage is reduced, the voltage of the connection point of a resistor R3 and a resistor R4 of the control circuit is reduced along with the cathode voltage of the comparator U1, the voltage difference between the voltage of the connection point of a resistor R3 and a resistor R4 and the external power supply voltage Vcc serves as the output signal of the control circuit, and the switching circuit is triggered to be switched on;

generating a switching signal and establishing a hysteresis signal in the starting process: when the input signal of the switching circuit is increased, the current of a loop formed by the voltage-stabilizing diode D1, the emitter and the base of the PNP triode Q1 and the resistor R5 is increased, the current of the base of the PNP triode Q1 is increased, the current of the collector is increased, and the collector is output to the enable end EN of the chip through the resistor R7 to control the chip to normally start and work; meanwhile, the collector voltage of the PNP triode Q1 is increased, the voltages of the input end and the output end of the hysteresis circuit are increased, the diode D2 is conducted, and a certain current is provided for the sampling resistor R2 through the resistor R6 to form a hysteresis signal;

the method comprises the following steps of generating and transmitting a control signal in the turn-off process: when the sampling signal superposition hysteresis signal is still smaller than the built-in reference Vref of the comparator U1, the current of the cathode of the comparator U1 is rapidly reduced, the voltage of the cathode is increased, the voltage at the connection point of the resistor R3 and the resistor R4 is increased along with the voltage of the cathode of the comparator U1, and the voltage difference between the voltage at the connection point of the resistor R3 and the resistor R4 and the external power supply voltage Vcc is used as the output signal of the control circuit to be connected with the input signal of the switching circuit and is switched;

generating a switching signal and turning off a hysteresis signal in the turning-off process: when the input signal of the switching circuit is reduced, the voltage stabilizing diode D1 is cut off, the PNP triode Q1 is in a cut-off region, and the collector is output to the chip enable end EN through the resistor R7 to control the chip to be turned off; meanwhile, the collector voltage of the PNP triode Q1 is reduced, the voltages of the input end and the output end of the hysteresis circuit are reduced, the diode D2 is cut off in the reverse direction, and the hysteresis signal formed by the sampling resistor R2 is cancelled through the resistor R6.

5. The undervoltage protection control method according to claim 4, wherein: the turn-on voltage vin (on) of the turn-on process is:

6. The undervoltage protection control method according to claim 4, wherein: the turn-off voltage vin (off) of the turn-off process is:

7. The undervoltage protection control method according to claim 4, wherein: the hysteresis voltage vin (on) -vin (off) is:

8. An under-voltage protection circuit is characterized by comprising a sampling circuit, a control circuit, a switching circuit and a hysteresis circuit;

the sampling circuit comprises a resistor R1 and a resistor R2 which are sequentially connected in series between an input voltage end Vin and a ground end GND, and the connection point of the resistor R1 and the resistor R2 serves as the output end of the sampling circuit;

the control circuit comprises a comparator U1, a resistor R3 and a resistor R4, wherein the resistor R4 and the resistor R3 are connected in series between a power supply terminal Vcc and a cathode of the comparator U1, the series connection point of the resistor R4 and the resistor R3 is used as the output end of the control circuit, a reference terminal of the comparator U1 is used as the input end of the control circuit and is connected with the output end of the sampling circuit, and the anode of the comparator U1 is connected with a ground terminal GND;

the switching circuit comprises a switching tube Q1, a voltage stabilizing diode D1, a resistor R5 and a resistor R7, wherein the base electrode of the switching tube Q1 is connected with the output end of the control circuit through a resistor R5, the emitter electrode of the switching tube Q1 is connected with the anode electrode of the voltage stabilizing diode D1, the cathode electrode of the voltage stabilizing diode D1 is connected with a power supply end Vcc, the collector electrode of the switching tube Q1 serving as the feedback end of the switching circuit is connected with one end of the resistor R7, and the other end of the resistor R7 serving as the output end of the switching circuit is used for outputting an under;

the hysteresis circuit comprises a diode D2 and a resistor R6, wherein the anode of the diode D2 is connected with the feedback end of the switch circuit, the cathode of the diode D2 is connected with one end of a resistor R6, and the other end of the resistor R6 is connected with the input end of the control circuit and the sampling output end.

9. The undervoltage protection circuit of claim 8, wherein: the switch circuit further comprises a resistor R8, one end of the resistor R8 is connected with the other end of the resistor R7, and the other end of the resistor R8 is grounded to GND.