A constant current control circuit for LED driving control chip

Abstract

The application claims a constant current control circuit for an LED drive control chip, belonging to the field of LED drive control, which comprises: a constant current source Ich, three transconductance operational amplifiers gm1, gm2, gm3, a comparator, an S-R latch, a first sample-and-hold circuit, a second sample-and-hold circuit, a clock block Clock, a drive, a resistor Rt, a resistor Rs, a capacitor C1, a capacitor C2, a capacitor C3, a capacitor Ct, switches SW1-SW5; the application only samples the inductor current in the switch-on stage of the control switch, generates an opening signal through an internal control algorithm, and generates an off signal by a clock block with a fixed frequency. The controlled LED current is only determined by external user-selectable parameters and internal fixed parameters, and the precision is high. The overall circuit is simple to realize, does not require an external compensation network, and has low cost. Because only the inductor current in the switch-on stage of the control switch is sampled, the conversion efficiency is high.

Description

Technical Field

[0001] The invention belongs to the technical field of LED control, and in particular to a constant current control circuit for an LED drive control chip. Background Art

[0002] An LED is a diode that converts electrical energy into light. Its essence is a PN junction, which has unidirectional conductivity. When conducting, even small voltage fluctuations can cause large current fluctuations, which in turn cause variations in the LED's output brightness. Therefore, controlling an LED requires a constant current. The rated output current is typically specified with very strict accuracy, typically less than 5% overall.

[0003] In LED lighting systems, driver control chips convert input voltage under various power supply conditions into the current required for LED illumination, making them a crucial component of the entire lighting system. Poor-performing LED driver control chips not only impact the lifespan and reliability of the lighting system, but also reduce system efficiency and cause excessive energy loss. Furthermore, the output current of the LED driver control chip affects the brightness and color variation of the LEDs. Therefore, achieving high-performance LED current control is a pressing need.

[0004] In practice, a buck converter is typically implemented in a flyback configuration, with the switch connected to ground, the cathode of the freewheeling diode connected to the supply voltage, and the LED connected between the input line and the inductor L. The inductor current is smoothed by a capacitor connected in parallel with the LED string before being fed into the LED string. The switching MOSFET is referenced to ground, making it easier to control.

[0005] Defects and shortcomings of existing technology:

[0006] There are two commonly used control methods in practice: The first is peak current mode control with fixed off time (FOT). In this control technology, when the peak current reaches the set value during the switching cycle, the power switch M is turned off and the power switch is turned on for a fixed time interval T determined by the timer circuit. OFF Then turn it on again. This method only samples the inductor current during the turn-on time, and the sampling network loss is low, but the control method is more complex and requires an external compensation network, which is costly. The second type is peak-to-peak current mode hysteresis control. This control technology sets two thresholds in the switching cycle. When the inductor peak current drops below the first set threshold, the power switch M is turned on, and it is turned off when the inductor current peak exceeds the second set threshold. This method requires sampling the inductor current throughout the entire working cycle, which has high losses and low conversion efficiency. In addition, sampling the inductor current throughout the entire working cycle requires a differential sensor with a large common-mode range, which is difficult to implement.

[0007] After searching, the application publication number CN203072210U is a constant current drive circuit, including a segmented control circuit and a constant current control circuit. The segmented control circuit includes a voltage stabilizing circuit and at least one integrated control chip. The integrated control chip of the segmented control circuit is a D trigger. The integrated control chip has a clock signal pin, and the voltage stabilizing circuit is connected to the clock signal control pin.

[0008] This patent uses a D flip-flop for segmented control, which limits the circuit's flexibility and responsiveness to rapid changes in load or input conditions. Furthermore, its voltage-stabilizing circuit requires a clock signal, making the circuit more complex and potentially sensitive to noise or variations in the clock signal, potentially affecting the overall stability and performance of the constant-current output. This invention, however, eliminates the need for a clock signal and cleverly controls the output of the transconductance operational amplifier to charge the capacitor using the on / off signals of the switching tube. This results in a highly precise constant-current value, ensuring overall stability and a faster response. Summary of the Invention

[0009] The present invention aims to solve the above problems of the prior art. It proposes a constant current control circuit for an LED driver control chip. The technical solution of the present invention is as follows:

[0010] A constant current control circuit for an LED drive control chip, comprising: a constant current source I ch , three transconductance operational amplifiers g m1 、g m2 、g m3 , comparator, SR latch, first sample and hold circuit, second sample and hold circuit, clock block Clock, a driver (here refers to the triangle connected between the SR latch and the switch tube M), resistor R t , resistor R s , capacitor C1, capacitor C2, capacitor C3, capacitor C t , switch SW1-switch SW5; wherein, the constant current source I ch After being connected to the switch SW3, one path is connected to the ground through the capacitor C2, and the other path is connected to the input end of the first sampling and holding circuit and the switch SW4. The switch SW4 is grounded, and the output end of the first sampling and holding circuit is connected to the transconductance operational amplifier g m3 The positive input terminal of the transconductance operational amplifier g is connected to m3 The negative input terminal of the transconductance operational amplifier g is grounded; m3The output end of is connected to the capacitor C3, the switch SW5, and the positive phase input end of the comparator respectively, and the capacitor C3 and the switch SW5 are grounded; the output end of the comparator is connected to the S end of the SR latch, the R end of the SR latch is connected to the clock block Clock, the Q end of the SR latch is connected to the input of the driver, and the output of the driver is connected to the gate of the switch tube M; the negative phase input end of the comparator is connected to the resistor R t , capacitor C t , one end of the switch SW2 is connected, the resistor R t , capacitor C t The other end of the switch SW2 is connected to the transconductance operational amplifier g m2 The output terminal is connected to the transconductance operational amplifier g m2 The negative input terminal of the transconductance operational amplifier g is grounded; m2 The positive input terminal of the second sampling and holding circuit is connected to one end of the second sampling and holding circuit, and the other end of the second sampling and holding circuit is connected to one end of the switch SW1, one end of the capacitor C1, and the transconductance operational amplifier g m1 The output end of the switch SW1 and the other end of the capacitor C1 are connected to the ground, and the transconductance operational amplifier g m1 The positive input terminal is connected to one end of the resistor Rs and the drain of the switch tube M. s The other end of the transconductance operational amplifier g m1 The negative input terminal is grounded.

[0011] Furthermore, when the clock block is 1, the SR latch output Q is 0, and the switch tube M is turned off; when V t2 Greater than V q2 When V t2 Refers to the voltage of capacitor C3, V q2 Refers to the capacitance C t The comparator output is 1, the SR latch output Q is 1, and the switch tube M is turned on; I L is the inductor current, and its average value is the current passing through the LED; when the switch tube M is turned on, I L Flowing through the detection resistor R s , generating a detection voltage V CS(t) , V CS(t) Connect to the positive input of the transconductance operational amplifier to generate a current I q ;I q Charge capacitor C1 to generate voltage V q1 When the SR latch output Q is 1, the switch SW1 is open, and when the SR latch output Q is 0, the switch SW1 is closed, V q1 Return to 0.

[0012] Furthermore, the voltage Vq1 The calculation formula is:

[0013]

[0014] g m1 Refers to the transconductance operational amplifier g m1 The transconductance value, C1 refers to the capacitance value of capacitor C1, Ton refers to the opening time of the switch tube M, V cs（t） Refers to the resistance R s Upper voltage value, R s Refers to the resistance R s The resistance value;

[0015] The inductor current I L(t) The following value

[0016]

[0017] I LED Refers to the current value passing through the LED, V IN Refers to the input voltage of the circuit, V LED Refers to the voltage value on the LED, L refers to the inductance value, and t is the time;

[0018] Therefore, I L(t) Bring in V q1 The value of

[0019] .

[0020] Furthermore, when the switch tube M is turned on, Q is 1, the switch SW2 is turned on, and the first sampling and holding circuit is connected to V q1 Sampling, when the switch tube M is closed, Q is 0, the switch SW2 is closed, and the second sampling and holding circuit will V q1 The maximum voltage is maintained and converted into a current through the transconductance operational amplifier to the capacitor C t Charging, generating voltage V q2 ;

[0021]

[0022] T OFF Refers to the turn-off time of the switch tube M, T s Refers to a complete working cycle time of the switch tube M, g m2 Refers to the transconductance operational amplifier g m2 The transconductance value, R t Refers to the resistance R t The resistance value;

[0023] Similarly, when the switch tube M is turned on, Q is 1, the switch SW3 is closed, and SW4 is opened, the current I provided by the constant current source is ch Charge capacitor C2 to generate voltage Vt1 , whose values ​​are as follows

[0024] .

[0025] Furthermore, when the switch tube M is turned off, Q is 0, the switch SW3 is turned on, and SW4 is turned off, the charge stored in C3 is discharged, so that the voltage of C3 in the next cycle starts from zero; V t1 Through the first sampling and holding circuit, it is sampled when the switch tube M is turned on and held when the switch tube M is turned off; when the switch tube M is turned off, SW5 is turned on, V t1 The current generated by the transconductance operational amplifier is applied to the capacitor C t Charging, the capacitor C t The voltage V is generated at both ends t2 ; can get

[0026] .

[0027] Furthermore, when When the comparator flips, that is, the SR latch output Q value is 1, and the switch M is turned on; Apply the turn-on condition to solve I LED have to:

[0028]

[0029] The DC current delivered to the LED string is determined by the external user-selectable parameter R s and internal fixed parameter I ch , capacitance value, resistance value, transconductance value, rather than the LED string voltage V LED , nor the input voltage V IN and the inductor value L.

[0030] The advantages and beneficial effects of the present invention are as follows:

[0031] The constant current control circuit proposed in the present invention only samples the inductor current during the switch-on phase, resulting in low energy loss in the sampling network and high conversion efficiency. The overall circuit is simple to implement, and the resulting LED current depends only on external user-selectable parameters and internal fixed parameters, achieving high precision while ensuring stability. No external compensation circuit is required, allowing for low costs. The present invention does not require a clock signal, but cleverly controls the output of the transconductance operational amplifier to charge the capacitor through the on / off signals of the switch tube, thereby obtaining a high-precision constant current value, ensuring overall stability and having a relatively fast response capability. It is precise in regulation, insensitive to input and output voltages and inductance values, and does not require an external correction circuit or calibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a constant current control circuit diagram for an LED drive control chip according to a preferred embodiment of the present invention;

[0033] Figure 2 It is the key waveform of the constant current control circuit proposed by the present invention;

[0034] Figure 3 This is the overall circuit working block diagram of the constant current control circuit proposed by the present invention;

[0035] Figure 4 This is a schematic diagram of the sampling and holding circuit proposed by the present invention. DETAILED DESCRIPTION

[0036] The following will describe the technical solutions in the embodiments of the present invention in detail with reference to the accompanying drawings. The described embodiments are only a part of the embodiments of the present invention.

[0037] The technical solution of the present invention to solve the above technical problems is:

[0038] like Figure 1 As shown, a constant current control circuit for LED drive control chip includes: a constant current source I ch , three transconductance operational amplifiers g m1 、g m2 、g m3 , comparator, SR latch, first sample and hold circuit, second sample and hold circuit, clock block Clock, a driver (here refers to the triangle connected between the SR latch and the switch tube M), resistor R t , resistor R s , capacitor C1, capacitor C2, capacitor C3, capacitor C t , switch SW1-switch SW5; wherein, the constant current source I ch After being connected to the switch SW3, one path is connected to the ground through the capacitor C2, and the other path is connected to the input end of the first sampling and holding circuit and the switch SW4. The switch SW4 is grounded, and the output end of the first sampling and holding circuit is connected to the transconductance operational amplifier g m3 The positive input terminal of the transconductance operational amplifier g is connected to m3 The negative input terminal of the transconductance operational amplifier g is grounded; m3 The output end of the comparator is connected to the capacitor C3, the switch SW5, and the positive input end of the comparator respectively, and the capacitor C3 and the switch SW5 are grounded; the output end of the comparator is connected to the S end of the SR latch, the R end of the SR latch is connected to the clock block Clock, and the Q end of the SR latch is connected to the gate of the switch tube M; the negative input end of the comparator is connected to the resistor R t , capacitor C t , one end of the switch SW2 is connected, the resistor R t , capacitor C tThe other end of the switch SW2 is connected to the transconductance operational amplifier g m2 The output terminal is connected to the transconductance operational amplifier g m2 The negative input terminal of the transconductance operational amplifier g is grounded; m2 The positive input terminal of the second sampling and holding circuit is connected to one end of the second sampling and holding circuit, and the other end of the second sampling and holding circuit is connected to one end of the switch SW1, one end of the capacitor C1, and the transconductance operational amplifier g m3 The output end of the switch SW1 and the other end of the capacitor C1 are connected to the ground, and the transconductance operational amplifier g m3 The non-inverting input end is connected to one end of the resistor Rs and the drain of the switch tube M, and the other end of the resistor Rs is grounded.

[0039] This patent provides a new average current control method (such as Figure 1 As shown in the figure, the control circuit samples only the inductor current during the switch-on phase. An internal control algorithm generates the turn-on signal, while a fixed-frequency clock block generates the turn-off signal. The controlled LED current is determined solely by external user-selectable parameters and internal fixed parameters, resulting in high accuracy. The overall circuit is simple to implement and requires no external compensation network, resulting in low cost. Because only the inductor current is sampled during the switch-on phase, conversion efficiency is high.

[0040] The proposed circuit structure is shown in the figure. The control part consists of a constant current source, three transconductance operational amplifiers, a comparator, an SR latch, a comparator, and two sample-and-hold circuits (which can be Figure 3 The clock block consists of a driver, a resistor, four capacitors, and five switches.

[0041] When the clock block (fixed frequency oscillator) is 1, the SR latch output Q is 0 and the switch M is off. t2 Greater than V q2 When the comparator output is 1, the SR latch output Q is 1, and the switch M is turned on. L is the inductor current, and its average value is the current passing through the LED. When M is turned on, I L Flowing through the detection resistor R s , generating a detection voltage V CS(t) , V CS(t) Connect to the positive input of the transconductance operational amplifier to generate a current I q I q Charge capacitor C1 to generate voltage V q1 When the SR latch output Q is 1, the switch SW1 is turned off and V q1 The value of is shown in the following formula. When the SR latch output Q is 0, the switch SW1 is closed, and V q1 Return to 0.

[0042]

[0043] The inductor current I L(t) The following value

[0044]

[0045] Therefore, I L(t) Bring in V q1 The value of

[0046]

[0047] When switch M is turned on, Q is 1, switch SW2 is turned on, and the sample-and-hold circuit is on V q1 Sampling, when switch M is closed, Q is 0, switch SW2 is closed, and the sampling and holding circuit will q1 The maximum voltage is maintained and converted into a current through the transconductance operational amplifier to the capacitor C t Charging, generating voltage V q2 If the time constant R t C t Than the switching period T s is much larger, making C t The voltage across the two ends V q2 The ripple superimposed on the DC value of can be ignored.

[0048]

[0049] Similarly, when switch M is turned on, Q is 1, switch SW3 is closed, SW4 is opened, and the current I provided by the constant current source is ch Charge capacitor C2 to generate voltage V t1 , whose values ​​are as follows

[0050]

[0051] When switch M is closed, Q is 0, switch SW3 is open, and SW4 is closed, the charge stored in C3 is discharged, so that the voltage of C3 starts from zero in the next cycle. t1 Through the sample-and-hold circuit, it is sampled when switch M is turned on and held when switch M is turned off. When switch M is turned off, SW5 is turned on and V t1 The current generated by the transconductance operational amplifier is applied to the capacitor C t Charging, the capacitor C t The voltage V is generated at both ends t2 . You can get

[0052]

[0053] g m3Refers to the transconductance operational amplifier g m3 The transconductance value, C3 refers to the capacitance value of capacitor C3,

[0054] when When , it is the comparator flip point, that is, the point where the SR latch output Q value is 1 and the switch M is turned on. Apply the turn-on condition and solve I LED have to:

[0055]

[0056] pass Figure 1 The DC current delivered to the LED string is determined by an external user-selectable parameter (R s ) and internal fixed parameters (I ch , capacitance, resistance, and transconductance), rather than the LED string voltage V LED , nor the input voltage V IN and the inductor value L. In the integrated circuit implementation, I ch and g m The value of T depends on the internal resistor value and can be matched; s is fixed and depends on the internal resistor and capacitor pair. Therefore, I LED The set point accuracy is expected to be sufficient to meet most market requirements.

[0057] The waveform of the key part is as follows Figure 2 shown.

[0058] Figure 3 This is the overall circuit working block diagram of the constant current control circuit proposed by the present invention;

[0059] Figure 4 Schematic diagram of the sampling and holding circuit proposed by the present invention.

[0060] Figure 2 :I L(t) The average value of is the LED current value. t2 Equal to V q2 When , it is the comparator flip point, that is, the SR latch output Q value is 1, and the switch M is turned on. Until the next cycle begins, the switch M is closed. It can be seen from the figure that I LED To obtain an accurate constant value for control.

[0061] Figure 2 The figure is a block diagram of the whole circuit. It includes the switch circuit and the circuit of the invention mentioned in this article. The switch circuit includes: inductor L, diode D, capacitor C out and the switch tube M; the cathode of the diode D is connected to the external power supply V IN and capacitor C out Connection, capacitor C outThe other end of is connected to the inductor L, the other end of the inductor L is connected to the anode of the diode D and the input end of the switch tube M at the same time, the control end of the switch tube M is connected to the output end of the inventive circuit mentioned in this article, and the output end of the switch tube M is connected to the output end of the inventive circuit mentioned in this article.

[0062] Figure 4 This is a sample-and-hold circuit. It includes operational amplifiers A1 and A2, a switch, and capacitor C. The positive terminal of operational amplifier A1 is connected to the input voltage, while the negative terminal is connected to its output and one end of the switch. The other end of the switch is connected to one end of capacitor C and the positive terminal of operational amplifier A2. The other end of capacitor C is grounded. The negative terminal of operational amplifier A2 is connected to its output.

[0063] The systems, devices, modules, or units described in the above embodiments may be implemented by computer chips or entities, or by products having certain functions. A typical implementation device is a computer. Specifically, the computer may be, for example, a personal computer, a laptop computer, a cellular phone, a camera phone, a smartphone, a personal digital assistant, a media player, a navigation device, an email device, a game console, a tablet computer, a wearable device, or a combination of any of these devices.

[0064] It should also be noted that the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, commodity, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or includes elements inherent to such process, method, commodity, or apparatus. In the absence of further limitations, an element defined by the phrase "comprises a..." does not preclude the presence of additional identical elements in the process, method, commodity, or apparatus that includes the element.

[0065] The above embodiments should be understood as merely illustrating the present invention and not as limiting the scope of protection of the present invention. After reading the contents of the present invention, technicians may make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A constant current control circuit for an LED drive control chip, characterized in that: include: Constant current source I ch , three transconductance operational amplifiers g m1 、g m2 、g m3 , comparator, SR latch, first sample and hold circuit, second sample and hold circuit, clock block Clock, a driver, resistor R t , resistor R s , capacitor C1, capacitor C2, capacitor C3, capacitor C t , switch SW1-switch SW5; wherein, the drive refers to the triangle connected between the SR latch and the switch tube M, the constant current source I ch After being connected to the switch SW3, one path is connected to the ground through the capacitor C2, and the other path is connected to the input end of the first sampling and holding circuit and the switch SW4. The switch SW4 is grounded, and the output end of the first sampling and holding circuit is connected to the transconductance operational amplifier g m3 The positive input terminal of the transconductance operational amplifier g is connected to m3 The negative input terminal of the transconductance operational amplifier g is grounded; m3 The output end of is connected to the capacitor C3, the switch SW5, and the positive input end of the comparator respectively, and the capacitor C3 and the switch SW5 are grounded; the output end of the comparator is connected to the S end of the SR latch, the R end of the SR latch is connected to the clock block Clock, the Q end of the SR latch is connected to the input of the driver, and the output of the driver is connected to the gate of the switch tube M; the negative input end of the comparator is connected to the resistor R t , capacitor C t , one end of the switch SW2 is connected, the resistor R t , capacitor C t The other end of the switch SW2 is connected to the transconductance operational amplifier g m2 The output terminal is connected to the transconductance operational amplifier g m2 The negative input terminal of the transconductance operational amplifier g is grounded; m2 The positive input terminal of the second sampling and holding circuit is connected to one end of the second sampling and holding circuit, and the other end of the second sampling and holding circuit is connected to one end of the switch SW1, one end of the capacitor C1, and the transconductance operational amplifier g m1 The output end of the switch SW1 and the other end of the capacitor C1 are connected to the ground, and the transconductance operational amplifier g m1 The non-inverting input terminal and the resistor R s One end of the resistor R is connected to the drain of the switch tube M. s The other end of the transconductance operational amplifier g m1 The negative input terminal is grounded.

2. The constant current control circuit for LED drive control chip according to claim 1, characterized in that: When the clock block is 1, the SR latch output Q is 0, and the switch tube M is turned off; when V t2 Greater than V q2 When V t2 Refers to the voltage of capacitor C3, V q2 Refers to the capacitance C t The comparator output is 1, the SR latch output Q is 1, and the switch tube M is turned on; I L is the inductor current, and its average value is the current passing through the LED; when the switch tube M is turned on, I L Flowing through the detection resistor R s , generating a detection voltage V CS(t) , V CS(t) Connect to the positive input of the transconductance operational amplifier to generate a current I q ;I q Charge capacitor C1 to generate voltage V q1 When the SR latch output Q is 1, the switch SW1 is open, and when the SR latch output Q is 0, the switch SW1 is closed, V q1 Return to 0.

3. The constant current control circuit for LED drive control chip according to claim 2, characterized in that: The voltage V q1 The calculation formula is: g m1 Refers to the transconductance operational amplifier g m1 The transconductance value, C1 refers to the capacitance value of capacitor C1, Ton refers to the opening time of the switch tube M, V cs（t） Refers to the resistance R s Upper voltage value, R s Refers to the resistance R s The resistance value; The inductor current I L(t) The following value I LED Refers to the current value passing through the LED, V IN Refers to the input voltage of the circuit, V LED Refers to the voltage value on the LED, L refers to the inductance value, and t is the time; Therefore, I L(t) Bring in V q1 The value of 。 4. The constant current control circuit for LED drive control chip according to claim 3, characterized in that: When the switch tube M is turned on, Q is 1, the switch SW2 is turned on, and the first sampling and holding circuit is connected to V q1 Sampling, when the switch tube M is closed, Q is 0, the switch SW2 is closed, and the second sampling and holding circuit will V q1 The maximum voltage is maintained and converted into a current through the transconductance operational amplifier to the capacitor C t Charging, generating voltage V q2 ; T OFF Refers to the turn-off time of the switch tube M, T s Refers to a complete working cycle time of the switch tube M, g m2 Refers to the transconductance operational amplifier g m2 The transconductance value, R t Refers to the resistance R t The resistance value; Similarly, when the switch tube M is turned on, Q is 1, the switch SW3 is closed, and SW4 is opened, the current I provided by the constant current source is ch Charge capacitor C2 to generate voltage V t1 , whose values ​​are as follows 。 5. The constant current control circuit for LED drive control chip according to claim 4, characterized in that: When the switch tube M is turned off, Q is 0, the switch SW3 is turned on, and SW4 is turned off, the charge stored in C3 is discharged, so that the voltage of C3 starts from zero in the next cycle; V t1 Through the first sampling and holding circuit, it is sampled when the switch tube M is turned on and held when the switch tube M is turned off; when the switch tube M is turned off, SW5 is turned on, V t1 The current generated by the transconductance operational amplifier is applied to the capacitor C t Charging, the capacitor C t The voltage V is generated at both ends t2 ; can get g m3 Refers to the transconductance operational amplifier g m3 The transconductance value of C3 refers to the capacitance value of capacitor C3.

6. The constant current control circuit for LED drive control chip according to claim 5, characterized in that: when When the comparator flips, that is, the SR latch output Q value is 1, and the switch M is turned on; Apply the turn-on condition to solve I LED have to: The DC current delivered to the LED string is determined by the external user-selectable parameter R s and internal fixed parameter I ch , capacitance value, resistance value, transconductance value, rather than the LED string voltage V LED , nor the input voltage V IN and the inductor value L.

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