Constant on-time control circuit of step-down circuit and step-down circuit

Abstract

The utility model provides a constant on-time control circuit of a step-down circuit and the step-down circuit, and the constant on-time control circuit generates a first current according to an input voltage, and the first current is in direct proportion to the input voltage; when the main power tube is conducted, the first current is used for charging a timing capacitor, and the timing capacitor generates a first voltage rising from an initial voltage; comparing the first voltage with the reference voltage, and when the first voltage reaches the reference voltage, controlling the main power tube to be turned off; when the main power tube is turned off, the first voltage on the timing capacitor is reset to the initial voltage, and the initial voltage is in direct proportion to the input voltage. When the input voltage changes, the switching frequency is kept stable, and the problem that the switching frequency changes greatly along with the change of the input voltage is solved.

Description

Technical Field

[0001] The utility model relates to the field of power electronics, in particular to a constant on-time control circuit of a voltage-reducing circuit and a voltage-reducing circuit. Background Art

[0002] Constant on-time (COT) is widely used in switching circuits and has the advantages of fast dynamic response and simple loop compensation. In traditional COT control, on-time is a fixed value, and the duty cycle is changed by adjusting the off-time. It is a type of PFM control, and the switching frequency is directly affected by the input voltage: for example, when the input voltage increases, the duty cycle needs to be reduced to keep the output voltage constant. Since the on-time is fixed, the duty cycle can only be reduced by increasing the off-time, which is manifested as an increase in input voltage, a longer switching cycle (on-time + off-time), and a decrease in switching frequency. However, in real applications, many scenarios require that the frequency of the switching power supply remain as stable as possible, so pseudo fixed frequency constant on-time control (Pseudo fixed frequency COT) appears. Taking the buck circuit as an example, the constant on-time (on-time) generation circuit of its main power tube is as follows: Figure 1 Shown: C TON is the timing capacitor, I TON is a and input voltage V IN Proportional to the current I TON =k1*V IN / R TON According to the charge generation formula Q = C*U = I*T, the time to trigger the comparator flip is:

[0003] C TON ·(k2·V OUT )=I TON ·T ON (1)

[0004]

[0005] where k*R TON *C TON Assume that T S =k*R TON *C TON , we can get

[0006] T ON =T S *V OUT / V IN =T S *D (3),

[0007] Among them, T ON The conduction time of the main power tube, D = V OUT / V IN is the switching duty cycle of the main power tube. Formula (3) has the same expression as that of the Buck circuit under fixed-frequency PWM control. Ideally, the switching frequency and input voltage V IN No matter, can be kept constant.

[0008] Due to the existence of comparator delay, dead time delay and switch node falling delay in the constant on-time generation circuit, the actual equivalent constant on-time (T ON_EFF ) is greater than the theoretically calculated constant on time (T ON ) is too large, that is, T ON_EFF =T ON +T D ....(4), such as Figure 2 As shown, the actual switching period T S_ACTUAL Compared with the theoretically calculated switching period T S According to formula (4), due to the turn-on delay, the actual turn-on time is longer than the theoretical turn-on time, which makes the switching frequency change with the input voltage V IN The switching frequency and input voltage V IN It has nothing to do with achieving true fixed frequency control, thus affecting the application effect of the circuit. Summary of the invention

[0009] The utility model aims to provide a constant on-time control circuit of a step-down circuit with stable switching frequency and a step-down circuit, which are used to overcome the problem that the switching frequency varies greatly with the input voltage.

[0010] The utility model also provides a constant on-time control circuit for a step-down circuit, wherein the step-down circuit comprises a main power tube and an inductor connected to each other, and the constant on-time control circuit comprises:

[0011] A current generating circuit, generating a first current according to an input voltage, wherein the first current is proportional to the input voltage;

[0012] a timing capacitor, wherein when the main power tube is turned on, the first current is used to charge the timing capacitor, and the timing capacitor generates a first voltage, and the first voltage starts to rise from an initial voltage;

[0013] A comparator, which compares the first voltage with a reference voltage, and controls the main power tube to be turned off when the first voltage reaches the reference voltage;

[0014] a reset circuit connected to the timing capacitor, and when the main power tube is turned off, the reset circuit is turned on to reset the first voltage to the initial voltage;

[0015] Wherein, when the main power is turned off, the initial voltage is adjusted according to the input voltage, and the initial voltage is proportional to the input voltage.

[0016] Optionally, a reference voltage generating circuit is further included to obtain the reference voltage according to the product of the second proportionality coefficient and the output voltage.

[0017] Optionally, the current generating circuit obtains the first current according to the product of a first proportionality coefficient and an input voltage.

[0018] Optionally, the proportionality coefficient between the initial voltage and the input voltage is adjustable, and the compensation time is adjusted by adjusting the proportionality coefficient, and the compensation time is used to compensate for the turn-on delay time of the main power tube.

[0019] Optionally, the reset circuit includes a first resistor and a controllable switch, the first resistor and the controllable switch are connected in series to form a series circuit, and the series circuit is connected to the timing capacitor; when the main power tube is turned off, the controllable switch is controlled to be turned on, and the first current flows through the first resistor to generate the initial voltage.

[0020] Optionally, the reset circuit includes a controllable voltage source, which is connected to the timing capacitor; when the main power tube is turned off, the controllable voltage source is turned on, and the input voltage controls the controllable voltage source to output the initial voltage.

[0021] Optionally, the reference voltage generating circuit also receives the input voltage, and obtains the reference voltage according to the difference between the product of the output voltage and the second proportionality coefficient and the product of the input voltage and the third proportionality coefficient.

[0022] Optionally, the current generating circuit includes a proportional regulator and a conversion resistor, the proportional regulator proportionally regulates the input voltage to obtain a regulation voltage, the conversion resistor receives the regulation voltage, and obtains the first current according to the current flowing through the conversion resistor.

[0023] The present invention also provides a step-down circuit, comprising a main power tube and an inductor connected to each other, and also comprising any one of the above-mentioned constant on-time control circuits for controlling the on-time of the main power tube to be constant.

[0024] Compared with the prior art, the utility model has the following advantages: the initial value of the voltage on the timing capacitor is controlled to be positively correlated with the input voltage to compensate for the influence of the input voltage change on the on-time change, overcome the problem that the switching frequency changes greatly with the input voltage change, ensure that the switching frequency can remain stable when the input voltage changes, and realize constant on-time control of fixed frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A circuit schematic diagram for generating a constant on-time in the prior art;

[0026] Figure 2 It is a conduction time waveform diagram of the prior art;

[0027] Figure 3 This is the principle diagram of the voltage-reducing circuit of the utility model;

[0028] Figure 4 This is a schematic diagram of a first embodiment of a conduction time control circuit of the utility model;

[0029] Figure 5 This is a schematic diagram of a second embodiment of the conduction time control circuit of the utility model;

[0030] Figure 6 This is a schematic diagram of the third embodiment of the conduction time control circuit of the utility model. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitution, modification, equivalent method and scheme made within the spirit and scope of the present invention.

[0032] In order to enable the public to have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, and those skilled in the art can fully understand the present invention without these detailed descriptions.

[0033] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all simplified and in non-precise proportions to facilitate and clearly illustrate the purpose of the embodiments of the present invention.

[0034] like Figure 3 As shown, the schematic diagram of the buck circuit of the utility model is illustrated, including a power circuit, a constant conduction time control circuit and a drive circuit. The power circuit includes a main power tube M1, a rectifier tube M2, a power inductor L1, and an output capacitor Co. The main power tube M1 and the rectifier tube M2 are connected, and their connection ends are connected to the first end of the power inductor L1, and the second end of the power inductor L2 is connected to the output capacitor Co; the switch states of the main power tube M1 and the rectifier tube M2 are complementary. The constant conduction time control circuit 01 receives an input voltage V IN , output voltage V OUTThe on-time control circuit 01 controls the constant on-time of the main power tube M1 and the off-time of the rectifier tube M2 after passing through the driving circuit 02. Specifically, when the main power tube M1 is turned on, the constant on-time control circuit 01 controls the constant on-time of the main power tube M1 and the off-time of the rectifier tube M2 according to the input voltage V IN Generates a first current I1, which is fed to the timing capacitor C TON Charging generates a first voltage V CTON , the first voltage V CTON Starts to rise from the initial value; according to the output voltage V OUT Generate reference voltage V REF , the first voltage V CTON and reference voltage V REF When the first voltage V CTON Reaching the reference voltage V REF When the main power tube M1 is turned on and off, the main power tube M1 is controlled. Under normal circumstances, the DC-DC conversion circuit is a constant voltage output. Due to the existence of the on-time delay, the main power tube M1 needs to be designed to be turned off in advance in theory. In fact, after the on-time delay, the actual on-time of the main power tube M1 can reach the ideal on-time. When the main power tube M1 is designed to be turned off in advance, in order to achieve a stable fixed frequency output, the utility model will be based on the input voltage V IN For the first voltage V CTON The initial value and / or reference voltage V REF Adjust so that the time when the main power tube M1 is turned off in advance is consistent with the input voltage V IN Avoid designing turn-on delay compensation that is affected by input voltage V IN The on-delay compensation time of the designed main power tube M1, i.e., its early shutdown time, is exactly the same as the on-delay time T of the main power tube M1. D To the first voltage V CTON For example, the first voltage V CTON The initial value and input voltage V IN The proportional coefficient is adjustable, and the compensation time for compensating the conduction delay of the main power tube is adjusted by adjusting the proportional coefficient. The larger the proportional coefficient, the larger the compensation time. The utility model overcomes the problem that the switching frequency varies greatly with the input voltage, so that the switching frequency of the step-down circuit is stable and works in a fixed frequency state.

[0035] like Figure 4 As shown, a schematic diagram of a first embodiment of the on-time control circuit of the utility model is shown, which includes a current generating circuit 101, a timing capacitor C TON, reset circuit 102, reference voltage generating circuit 103 and comparator 104, current generating circuit 101 includes conversion resistor R TON and proportional regulator k1, input voltage V IN After proportional regulation by proportional regulator k1, the regulated voltage k1*V is obtained IN , the regulated voltage is applied to the conversion resistor R TON The current flowing through the conversion resistor R TON The first current I TON , then the first current I TON With input voltage V IN Proportional, the proportionality coefficient is k1 / R TON When the main power tube M1 is turned on, the timing capacitor V CTON The reset circuit 02 connected to the first current I TON For the timing capacitor C TON Charging, timing capacitor C TON The first voltage V CTON The reference voltage generating circuit 103 increases from the initial voltage. OUT Generates an output voltage V OUT Ratio reference voltage k2*V OUT The comparator 104 converts the first voltage V CTON and reference voltage k2*V OUT When the first voltage V CTON Reach reference voltage k2*V OUT When the on-time control signal T is generated ON The main power tube M1 is controlled to be turned off, and the rectifier tube M2 is controlled to be turned on after the dead time. The shutdown control signal T of the main power tube M1 OFF / Conduction control signal T of rectifier tube M2 ONL The reset circuit 102 is controlled to be turned on. The first terminal of the reset circuit 102 is connected to the timing capacitor V CTON , whose second end is grounded, the reset circuit 102 includes a first resistor R D and the controllable switch Q1. When the main power tube M1 is turned off, the controllable switch Q1 is controlled to be turned on. The first current I TON The first resistor R D The initial voltage I TON *R D , the initial voltage I TON *R D Added to the timing capacitor C TON On, that is, the timing capacitor C TON The voltage on the INI , V INI =I TON *R D =k1*VIN *R D / R TON After Q1 is disconnected, the timing capacitor C TON The time from the upper voltage rising to the comparator flipping is:

[0036] C TON ·(k2·V OUT -V INI )=I TON ·T ON …(5)

[0037]

[0038] Comparing formula (2) and (6), we can get that formula (6) reduces R on the basis of formula (2). D *C TON , let R D *C TON =T D , where T D is the delay of the conduction time in the formula, T D With input voltage V IN irrelevant, eliminating the input voltage V IN The influence of the conduction delay time on the design can be avoided, thus avoiding the influence of the conduction delay on the frequency. In addition, the selected R D The larger the T D The larger the T ON The larger the actual delay compensation, the more appropriate R D , the actual turn-on delay can be accurately compensated, and the final actual turn-on time is the ideal turn-on time, overcoming the problem that the switching frequency changes greatly with the input voltage.

[0039] like Figure 5 As shown, the schematic diagram of the second embodiment of the on-time control circuit of the utility model is shown. The difference between the second embodiment and the first embodiment is that the implementation method of the reset circuit 102 is different. The reset circuit 102 includes a controlled voltage source V1 and a controllable switch Q1 connected in series. The input voltage V IN The size of the controllable voltage V1 (V1 is not only used to indicate the controlled voltage source, but also used to indicate the size of the controllable voltage output by the controlled voltage source) output by the controlled voltage source V1 is controlled. The controllable voltage V1 is proportional to the input voltage V IN When the main power tube M1 is turned off, the controllable switch Q1 is turned on, and the timing capacitor C TON The first voltage V CTON is reset to the controllable voltage V1. When the main power tube M1 is turned off and then turned on, the timing capacitor C TON The first voltage V CTONThe controllable voltage V1 is used as the initial value and starts to rise. The principle of the second embodiment is similar to that of the first embodiment, and both are for setting the timing capacitor C TON The initial voltage V INI Reset to the input voltage V IN proportional to the input voltage V IN When the switching frequency changes, the influence of the conduction delay can be compensated so that the switching frequency can still remain stable.

[0040] like Figure 6 As shown, the schematic diagram of the third embodiment of the on-time control circuit of the utility model is shown. The difference between the second embodiment and the first embodiment is that the implementation method of the reference voltage generating circuit 103 is different. On the basis of the first embodiment, the reference voltage generating circuit 103 is also based on the input voltage V IN Generate reference voltage V REF Specifically, the reference voltage generating circuit 103 generates a second proportional coefficient k2 and an output voltage V OUT Product k2*V OUT The third proportionality coefficient k3 and the input voltage V IN Product k3*V IN The difference k2*V OUT -k3*V IN Get the reference voltage V REF In the third embodiment, according to the input voltage V IN For the timing capacitor C TON The adjustment of the initial value of the upper voltage and the adjustment of the reference voltage VREF are equivalent to the adjustment of the input voltage V IN Only the initial value of the voltage on the timing capacitor is adjusted; in the first embodiment, the timing capacitor C TON The initial voltage value on the IN The proportional coefficient between them is relatively large. In the fourth embodiment, the initial value is proportional to the input voltage V IN The proportional coefficient between them is small. In the fourth embodiment, the compensation of the on-time by the initial value is not enough, and the reference voltage V REF Adjustment is performed to further compensate. Both embodiments are designed to adjust the input voltage V IN Adjust the timing capacitor C TON The upper voltage rises to the reference voltage V REF time to compensate for the delay time T D The effect on the conduction time is the same in principle, so as to control the switching frequency to be stable. In other embodiments, the initial value of the voltage on the timing capacitor may not be adjusted, and the input voltage V IN and output voltage V OUT Adjust the reference voltage V REF , to the conduction time T of the main power tube M1 ONWhen the timing capacitor is reset, the voltage on the timing capacitor is directly reset to zero. IN When the switching frequency changes with the input voltage V, the voltage of any one input terminal or the voltage of the two input terminals of the comparator 104 is adjusted to compensate for the delay of the conduction time of the main power tube M1, thereby overcoming the switching frequency changing with the input voltage V IN To solve the problem of large changes, fixed frequency control is implemented.

[0041] Although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0042] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. A constant on-time control circuit for a buck circuit, the buck circuit comprising a main power tube and an inductor connected to each other, characterized in that: include, A current generating circuit, generating a first current according to an input voltage, wherein the first current is proportional to the input voltage; a timing capacitor, wherein when the main power tube is turned on, the first current is used to charge the timing capacitor, and the timing capacitor generates a first voltage, and the first voltage starts to rise from an initial voltage; A comparator, which compares the first voltage with a reference voltage, and controls the main power tube to be turned off when the first voltage reaches the reference voltage; a reset circuit connected to the timing capacitor, and when the main power tube is turned off, the reset circuit is turned on to reset the first voltage to the initial voltage; The initial voltage is adjusted according to the input voltage, and the initial voltage is proportional to the input voltage.

2. The constant on-time control circuit according to claim 1, characterized in that: It also includes a reference voltage generating circuit, which obtains the reference voltage according to the product of the second proportionality coefficient and the output voltage.

3. The constant on-time control circuit according to claim 1, characterized in that: The current generating circuit obtains the first current according to the product of a first proportionality coefficient and an input voltage.

4. The constant on-time control circuit according to claim 1, characterized in that: The proportionality coefficient between the initial voltage and the input voltage is adjustable, and the compensation time is adjusted by adjusting the proportionality coefficient. The compensation time is used to compensate for the conduction delay time of the main power tube.

5. The constant on-time control circuit according to claim 3, characterized in that: The reset circuit includes a first resistor and a controllable switch, wherein the first resistor and the controllable switch are connected in series to form a series circuit, and the series circuit is connected to the timing capacitor; when the main power tube is turned off, the controllable switch is controlled to be turned on, and the first current flows through the first resistor to generate the initial voltage.

6. The constant on-time control circuit according to claim 3, characterized in that: The reset circuit includes a controllable voltage source, which is connected to the timing capacitor; when the main power tube is turned off, the controllable voltage source is turned on, and the input voltage controls the controllable voltage source to output the initial voltage.

7. The constant on-time control circuit according to claim 2, characterized in that: The reference voltage generating circuit also receives the input voltage, and obtains the reference voltage according to the difference between the product of the output voltage and the second proportionality coefficient and the product of the input voltage and the third proportionality coefficient.

8. The constant on-time control circuit according to claim 1, characterized in that: The current generating circuit includes a proportional regulator and a conversion resistor. The proportional regulator proportionally regulates the input voltage to obtain a regulation voltage. The conversion resistor receives the regulation voltage and obtains the first current according to the current flowing through the conversion resistor.

9. A step-down circuit, comprising a main power tube and an inductor connected together, characterized in that: It also includes the constant on-time control circuit described in any one of claims 1-8, which is used to control the on-time of the main power tube to be constant.

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