Circuit structure for realizing LED dimming

Abstract

The present invention relates to a circuit structure for realizing LED dimming, which includes a PWM module, a duty cycle detection module, a frequency modulation module, a first DAC module, and a constant current control module. The input end of the duty cycle detection module is connected to the output end of the PWM module. The output end of the duty cycle detection module is respectively connected to the input end of the frequency modulation module and the input end of the first DAC module. The input end of the constant current control module is connected to the frequency modulation module and the first DAC module. By adopting the circuit structure for realizing LED dimming of the present invention, a very low minimum brightness can be achieved, and a very high dimming accuracy can be achieved between the minimum brightness and the maximum brightness. At the same time, the output current of the linear dimming circuit of the present invention does not contain the frequency component of the PWM signal, so there is no noise and no stroboscopic phenomenon.

Description

Technical Field

[0001] The present invention relates to the field of Internet of Things, and particularly to the field of LED dimming, and specifically refers to a circuit structure for realizing LED dimming. Background Art

[0002] Two dimming methods taking the Buck structure as an example:

[0003] (1) Linear dimming: By changing the peak current of the inductor to change the LED drive current, the inductor current always works in the critical conduction mode, and the specific signal waveform is as Figure 1 shown. Among them, VREF_CS is the reference voltage controlled by an external analog voltage, and IPK is the peak current of the inductor. GD is the gate voltage of the MOS transistor, which is turned on at a high level and turned off at a low level.

[0004] (2) The PWM dimming method changes the output current by controlling the duty cycle of the PWM wave. When the PWM is high, the power circuit of the circuit works, and when the PWM is low, the power circuit of the circuit stops working.

[0005] As Figure 2 shown, in the figure, the PWM signal frequency is close to the power transistor switching frequency, and the duty cycle of the PWM1 signal is less than that of the PWM2 signal. However, since the falling edge of the PWM occurs at the demagnetization moment, the output current size will not be changed, affecting the dimming accuracy. Summary of the Invention <9000022>The purpose of the present invention is to overcome the above-mentioned disadvantages and provide a circuit structure for realizing LED dimming that satisfies the requirements of simple operation, high dimming accuracy, and wide application range.

[0007] In order to achieve the above purpose, the circuit structure for realizing LED dimming of the present invention is as follows:

[0008] The circuit structure for realizing LED dimming, its main feature is that the circuit structure includes a PWM module, a duty cycle detection module, a frequency modulation module, a first DAC module, and a constant current control module. The input end of the duty cycle detection module is connected to the output end of the PWM module, the output end of the duty cycle detection module is simultaneously connected to the input end of the frequency modulation module and the input end of the first DAC module, and the output end of the frequency modulation module and the output end of the first DAC module are both connected to the input end of the constant current control module.

[0009] The PWM module outputs a PWM signal to the duty cycle detection module, and the duty cycle detection module outputs a digital signal. The frequency modulation module, the first DAC module, and the constant current control module control the output current of the LED based on the digital signal, and the output current has a linear relationship with the duty cycle of the PWM signal.

[0010] Preferably, a first comparator is further included between the output terminal of the first DAC module and the constant current control module. The non-inverting input terminal of the first comparator is connected to the output terminal of the first DAC module, the inverting input terminal of the first comparator is connected to a first variable voltage, and the output terminal of the first comparator is connected to the input terminal of the constant current control module.

[0011] Preferably, the duty cycle detection module outputs a digital signal, and the frequency modulation module, the first DAC module, and the constant current control module control the output current of the LED based on the digital signal. It further includes:

[0012] The frequency modulation module outputs a delay time signal to the constant current control module according to the digital signal. The first DAC module outputs a peak comparison voltage to the first comparator according to the digital signal. The first comparator outputs a peak current control signal to the constant current control module. The constant current control module controls the output current of the LED based on the delay time signal and the peak current control signal.

[0013] Preferably, the delay time signal is the delay time after the inductor demagnetization ends.

[0014] Preferably, the circuit structure is further connected to an external MOS transistor. The MOS transistor cooperates with the circuit structure to control the brightness of the LED. The output signal of the constant current control module is the gate voltage of the MOS transistor, and the delay time signal is the delay time from the end of inductor demagnetization to the MOS transistor being turned on again.

[0015] Preferably, the output current has a linear relationship with the duty cycle of the PWM signal. It further includes:

[0016] The specific relationship formula between the output current and the duty cycle is:

[0017]

[0018] where, I LED is the output current, D is the duty cycle, V REF is the first reference voltage, and Rcs is the external sampling resistance value.

[0019] Preferably, the digital signal is D <n:1>, the digital signal D <n:1>The relational expression with the duty cycle D is specifically as follows:

[0020]

[0021] Preferably, the frequency modulation module includes a variable resistor, a first switch, a second switch, a fixed resistor, a capacitor, and a second comparator. The output terminal of the fixed resistor is connected to the first switch, the output terminal of the variable resistor is connected to the second switch, both the first switch and the second switch are connected to the non-inverting terminal of the second comparator, one end of the capacitor is connected to the non-inverting terminal of the second comparator, and the other end is grounded. The output terminal of the second comparator is connected to the constant current control module, and the inverting terminal of the second comparator is connected to a second reference voltage.

[0022] The fixed resistor generates a charging current according to a third reference voltage V1, the digital signal generates a second variable voltage, and the variable resistor generates a discharging current based on the second variable voltage and the digital signal; by controlling the closing and opening of the first switch and the second switch, the charging and discharging of the capacitor are alternately controlled.

[0023] Preferably, when the first switch is closed and the second switch is open, the charging current charges the capacitor; when the demagnetization of the inductor ends, the first switch is open and the second switch is closed, and the capacitor starts to discharge to generate the discharging current; when the voltage at the non-inverting terminal of the second comparator is less than the second reference voltage, the first switch is closed, the second switch is open, and the capacitor starts to charge.

[0024] Preferably, the relational expressions of the charging current and the discharging current with the duty cycle D are specifically as follows:

[0025]

[0026] Wherein, I₁ is the charging current, I₂ is the discharging current, and D is the duty cycle.

[0027] By adopting the circuit structure for realizing LED dimming of the present invention, a very low minimum brightness can be achieved, and a very high dimming accuracy can be achieved between the minimum brightness and the maximum brightness. At the same time, the output current of the linear dimming circuit of the present invention does not contain the frequency component of the PWM signal, so there is no noise and no stroboscopic phenomenon. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the specific signal waveform of linear dimming for an embodiment.

[0029] Figure 2 It is a waveform diagram of the PWM dimming method of linear dimming for an embodiment.

[0030] Figure 3 Schematic diagram of the circuit structure for realizing LED dimming in an embodiment.

[0031] Figure 4 Schematic diagram of the circuit structure of the frequency modulation module of the circuit structure for realizing LED dimming in an embodiment.

[0032] Figure 5 Schematic diagram of the implementation of I1 and I2 in the frequency modulation module of the circuit structure for realizing LED dimming in an embodiment. Detailed implementation manners

[0033] In order to more clearly describe the technical content of the present invention, the following will be further described in conjunction with specific embodiments.

[0034] With the development and popularization of the Internet of Things and smart homes, the demand for light source dimming is increasing, and the quality requirements for dimming are also getting higher and higher. Therefore, the dimming technology of LEDs needs to keep progressing in order to give full play to its own advantages and conform to the development trend of smart homes.

[0035] Currently, the commonly used dimming technologies include linear dimming, PWM dimming, and thyristor dimming, etc. Among them, the linear dimming circuit is relatively simple. It changes the peak current of the inductor by changing the voltage or resistance to change the LED drive current. The disadvantage is that when the peak current becomes smaller, it will increase the operating frequency of the drive circuit and increase the switching loss.

[0036] The existing PWM dimming method changes the output current by controlling the duty cycle of the PWM wave. When the duty cycle of the PWM wave is high, the power loop of the circuit works. When the duty cycle of the PWM wave is low, the power loop of the circuit stops working. When the PWM frequency is low, the LED light source will flicker and emit noise that can be perceived by the human ear. When the frequency is high, it is difficult to achieve high dimming accuracy and depth.

[0037] The present invention proposes a PWM dimming technology that modulates both the peak current of the inductor and the operating frequency of the circuit, increases the turn-off time of the switching tube to change the duty cycle of the turn-on time of the switching tube, and finally realizes high-precision and high-depth dimming of the LED.

[0038] The PWM dimming circuit of the present invention can achieve a very small minimum brightness and realize very fine dimming accuracy between the minimum brightness and the maximum brightness. At the same time, the output current of the dimming circuit of the present invention does not contain the frequency component of the PWM signal, so there is no noise and no flicker.

[0039] The circuit structure for realizing LED dimming includes a PWM module, a duty cycle detection module, a frequency modulation module, a first DAC module, and a constant current control module. The input end of the duty cycle detection module is connected to the output end of the PWM module. The output end of the duty cycle detection module is simultaneously connected to the input ends of the frequency modulation module and the first DAC module. The output ends of the frequency modulation module and the first DAC module are both connected to the input end of the constant current control module.

[0040] The PWM module outputs a PWM signal to the duty cycle detection module. The duty cycle detection module outputs a digital signal. The frequency modulation module, the first DAC module, and the constant current control module control the output current of the LED based on the digital signal. The output current is linearly related to the duty cycle of the PWM signal.

[0041] Convert the duty cycle D into an N-bit digital signal D <n:1>, the first DAC module according to the digital signal D <n:1>Generate a peak comparison voltage VREF_CS, and the frequency modulation module is based on the digital signal D <n:1>Generate a delay time signal T2 to control the delay time for the MOS transistor to turn on, so that the output current ILED and the duty cycle D of the PWM signal are linearly related. The MOS transistor cooperates with the circuit structure of this solution to control the LED brightness. It is located on the periphery of the circuit structure and can be fabricated on the same chip or different chips as the circuit structure in the embodiment. The output signal GD of the constant current control module is the gate voltage of the MOS transistor. The delay time signal T2 is the delay time from the end of inductor demagnetization to the MOS transistor turning on again.

[0042] A first comparator is further included between the output terminal of the first DAC module and the constant current control module. The non-inverting input terminal of the first comparator is connected to the output terminal of the first DAC module. The inverting input terminal of the first comparator is connected to a first variable voltage. The output terminal of the first comparator is connected to the input terminal of the constant current control module.

[0043] The duty cycle detection module outputs a digital signal. The frequency modulation module, the first DAC module, and the constant current control module control the output current of the LED based on the digital signal. It further includes:

[0044] The frequency modulation module outputs the delay time signal T2 to the constant current control module according to the digital signal. The first DAC module outputs a peak comparison voltage to the first comparator according to the digital signal. The first comparator outputs a peak current control signal to the constant current control module. The constant current control module controls the output current of the LED based on the delay time signal T2 and the peak current control signal. The peak current control signal is used to control the magnitude of Ipk.

[0045] Preferably, the frequency modulation module includes a variable resistor R2, a first switch, a second switch, a fixed resistor R1, a capacitor C1, and a second comparator COMP. The output terminal of the fixed resistor R1 is connected to the first switch. The output terminal of the variable resistor R2 is connected to the second switch. Both the first switch and the second switch are connected to the non-inverting input terminal of the second comparator COMP. One end of the capacitor C1 is connected to the non-inverting input terminal of the second comparator COMP, and the other end is grounded. The output terminal of the second comparator COMP is connected to the constant current control module.

[0046] The fixed resistor R1 generates a charging current I1 according to the third reference voltage V1, by the digital signal D <n:1>Generate a second variable voltage V2, where the variable resistor R2 is based on a digital signal D <n:1>and a second variable voltage V2, and generate a discharge current I2; by controlling the closing and opening of the first switch and the second switch, the charging and discharging of the capacitor C1 are alternately controlled.

[0047] As a preferred embodiment of the present invention, when the first switch is closed and the second switch is open, the charging current I1 charges the capacitor C1; when the demagnetization of the inductor ends, the first switch is open and the second switch is closed, the capacitor C1 starts to discharge, generating a discharge current I2; when the voltage VA at the non-inverting terminal of the second comparator is less than the second reference voltage at the inverting input terminal of the second comparator, the first switch is closed and the second switch is open, and the capacitor C1 starts to charge.

[0048] A PWM dimming circuit, in which the LED output current ILED and the duty cycle D of the PWM signal exhibit a strict linear relationship.

[0049] The PWM signal calculates the duty cycle D of the PWM signal through an internal duty cycle detection circuit and converts it into an N-bit digital signal D <n:1>。Signal D <n:1>On the one hand, the peak comparison voltage VREF_CS of the sampling resistor Rcs is controlled, and at the same time, the delay time T2 after the inductor demagnetization ends is controlled. According to the calculation formula (11) in the specific implementation manner, the LED output current has a linear relationship with D.

[0050] The output current of the linear dimming circuit of the present invention does not contain the frequency component of the PWM signal, so there is no noise and stroboscopic phenomenon.

[0051] The accuracy and the lowest brightness of the linear dimming circuit of the present invention can both reach 1 / 2 N 。

[0052] In the Buck structure, the average output current of the LED is determined by formula (1):

[0053]

[0054] Where Rcs is the value of the external sampling resistor, VREF_CS is the peak voltage on the sampling resistor Rcs in each switching cycle, T1 is the sum of the inductor conduction time (Ton) and the demagnetization time (Toff), T2 is the delay from the end of inductor demagnetization to the MOS transistor being turned on again, and ILED is the average output current of the LED.

[0055] The circuit structure of the present invention is as Figure 3 shown. The PWM signal passes through the duty cycle detection module, and the duty cycle D is converted into an N-bit digital signal D <n:1> ，D <n:1>The relational expression with the duty cycle D is as shown in Formula (2). Among them, VREF is the first reference voltage inside the chip, and CS in the figure is the first variable voltage.

[0056]

[0057] D <n:1>The peak comparison voltage VREF_CS of CS is generated by the first DAC module, and the equivalent formula of the circuit is as follows:

[0058]

[0059] D <n:1>Generate a delay time T2 through the frequency modulation module to control the delay time for the MOS transistor to turn on.

[0060] The circuit structure of the frequency modulation module is as Figure 4 shown within the dashed box in

[0061] When D > 87.5%, T2 = 0; when D ≤ 87.5%, the third reference voltage V1 and the fixed resistor R1 generate a charging current I1; by D <n:1>The generated second variable voltage V2 and variable resistor R2 generate a discharge current I2.

[0062] Within time T1, switch 1 is closed and switch 2 is open, and the charging current I1 charges the capacitor C1; when the demagnetization of the inductor ends, switch 1 is open and switch 2 is closed, and the capacitor C1 starts to discharge, and the discharge current is I2. When the voltage at the non-inverting terminal of the second comparator is less than the second reference voltage, switch 1 is closed again and switch 2 is open again, and the capacitor C1 starts to charge again. Then the relationship between I1 and I2 is as shown in Equations (4) and (5).

[0063] I1×T1 = ΔU×C1 = I2×T2...(4)

[0064] I1 / I2 = T2 / T1...(5)

[0065] The specific implementation circuits of the charging current I1 and the discharge current I2 in the frequency modulation module are as Figure 5 shown. In the implementation circuit of I1 in the frequency modulation module, the third reference voltage V1 is input to the non-inverting terminal of the third comparator, the inverting terminal is connected to the gate of the first MOS transistor M1, the output terminal of the third comparator is connected to the source of the first MOS transistor M1, and the source is grounded through a fixed resistor R1. The drain of the first MOS transistor M1 is connected to a first symmetric circuit composed of four field effect transistors, and the symmetric circuit outputs the charging current I1. In the implementation circuit of I2 in the frequency modulation module, the first DAC module receives the third reference voltage V1 and the digital signal D <n:1>, the first DAC module outputs a second variable voltage V2 to the non-inverting terminal of the fourth comparator. The inverting terminal is connected to the gate of the second MOS transistor M2. The output terminal of the fourth comparator is connected to the source of the second MOS transistor M2, and the source is grounded through a variable resistor R2. The drain of the second MOS transistor M2 is connected to ground in series with a second symmetric circuit composed of four field effect transistors and a third symmetric circuit composed of four field effect transistors. Among them, the sources of two field effect transistors in the third symmetric circuit are grounded, and the third symmetric circuit also outputs a discharge current I2.

[0066] Where V1 is the third reference voltage, R1 is a fixed resistor, V2 is the second variable voltage, and R2 is a variable resistor. The relationships between the charging current I1, the discharge current I2, and the duty cycle D of the PWM signal are as follows:

[0067] I1 = V1 / R1...(6)

[0068] V2 = V1 × D...(7)

[0069]

[0070]

[0071] Substituting formulas (3), (5), and (10) into formula (1) and simplifying, we get:

[0072]

[0073] Then, when D ranges from 0 to 100%, we have:

[0074] I LED = V REF × D...(12)

[0075] From the above analysis, it can be seen that the output current ILED of the LED dimming circuit of the present invention has a linear relationship with the duty cycle D, and both the accuracy and the lowest brightness can reach 1 / 2 N .

[0076] By adopting the circuit structure for realizing LED dimming of the present invention, a very small lowest brightness can be achieved, and a very high dimming accuracy can be achieved between the lowest brightness and the highest brightness. At the same time, the output current of the linear dimming circuit of the present invention does not contain the frequency component of the PWM signal, so there is no noise and no stroboscopic phenomenon.

[0077] In this specification, the present invention has been described with reference to its specific embodiments. However, it is obvious that various modifications and transformations can still be made without departing from the spirit and scope of the present invention. Therefore, the specification and the drawings should be regarded as illustrative rather than restrictive. < / n:1>

Claims

1. A circuit structure for realizing LED dimming, characterized in that The described circuit structure includes a PWM module, a duty cycle detection module, a frequency modulation module, a first DAC module, and a constant current control module. The input end of the duty cycle detection module is connected to the output end of the PWM module. The output end of the duty cycle detection module is simultaneously connected to the input ends of the frequency modulation module and the first DAC module. The output ends of the frequency modulation module and the first DAC module are both connected to the input end of the constant current control module. The PWM module outputs a PWM signal to the duty cycle detection module. The duty cycle detection module outputs a digital signal. The frequency modulation module, the first DAC module, and the constant current control module control the output current of the LED based on the digital signal. The output current has a linear relationship with the duty cycle of the PWM signal. The frequency modulation module includes a variable resistor, a first switch, a second switch, a fixed resistor, a capacitor, and a second comparator. The output end of the fixed resistor is connected to the first switch. The output end of the variable resistor is connected to the second switch. The first switch and the second switch are both connected to the non-inverting input end of the second comparator. One end of the capacitor is connected to the non-inverting input end of the second comparator, and the other end is grounded. The output end of the second comparator is connected to the constant current control module. The inverting input end of the second comparator is connected to a second reference voltage. The fixed resistor generates a charging current according to a third reference voltage. The digital signal generates a second variable voltage. The variable resistor generates a discharging current based on the second variable voltage and the digital signal. By controlling the closing and opening of the first switch and the second switch, the charging and discharging of the capacitor are alternately controlled.

2. The circuit structure for realizing LED dimming according to claim 1, wherein A first comparator is further included between the output end of the first DAC module and the constant current control module. The non-inverting input end of the first comparator is connected to the output end of the first DAC module. The inverting input end of the first comparator is connected to a first variable voltage. The output end of the first comparator is connected to the input end of the constant current control module.

3. The circuit structure for realizing LED dimming according to claim 2, wherein The duty cycle detection module outputs a digital signal. The frequency modulation module, the first DAC module, and the constant current control module control the output current of the LED based on the digital signal. It further includes: The frequency modulation module outputs a delay time signal to the constant current control module according to the digital signal. The first DAC module outputs a peak comparison voltage to the first comparator according to the digital signal. The first comparator outputs a peak current control signal to the constant current control module. The constant current control module controls the output current of the LED based on the delay time signal and the peak current control signal.

4. The circuit structure for realizing LED dimming according to claim 3, wherein, The delay time signal is the delay time after the inductor demagnetization ends.

5. The circuit structure for realizing LED dimming according to claim 3, characterized in that, The described circuit structure is also connected to the peripheral MOS transistor. The MOS transistor cooperates with the circuit structure to control the brightness of the LED. The output signal of the constant current control module is the gate voltage of the MOS transistor, and the delay time signal is the delay time from the end of inductor demagnetization to the next turn-on of the MOS transistor.

6. The circuit structure for realizing LED dimming according to claim 1, characterized in that, The output current has a linear relationship with the duty cycle of the PWM signal, and further includes: The specific relationship between the output current and the duty cycle is: Wherein, I LED is the output current, D is the duty cycle, V REF is the first reference voltage, and Rcs is the external sampling resistance value.

7. The circuit structure for realizing LED dimming according to claim 1, characterized in that, The digital signal described is D <n:1>, the digital signal D <n:1>The relationship with the duty cycle D is specifically: < / n:1> 8. The circuit structure for realizing LED dimming according to claim 1, wherein, When the first switch is closed and the second switch is open, the charging current charges the capacitor; at the end of inductor demagnetization, the first switch is open and the second switch is closed, and the capacitor starts to discharge, generating the discharge current. When the voltage at the non-inverting terminal of the second comparator is less than the second reference voltage, the first switch is closed, the second switch is open, and the capacitor starts to charge.

9. The circuit structure for realizing LED dimming according to claim 1, wherein, The specific relationships between the charging current, the discharge current and the duty cycle are: Where, I1 is the charging current, I2 is the discharge current, and D is the duty cycle.

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