LED boost constant current circuit

Abstract

The utility model relates to an LED boost constant current circuit, comprising an LED signal processing unit and a multi-channel boost constant current unit, a lamp signal input sequentially passes through the LED signal processing unit and the multi-channel boost constant current unit to drive a load LED to be on or off; a lamp signal input end is connected with a transient voltage suppression diode TVS1 and an RC filtering protection circuit in parallel, the output of the RC filtering protection circuit is connected with a common mode inductor L1 through an anti-reverse diode D1, the other end, far away from the anti-reverse diode D1, of the common mode inductor L1 is connected with a differential mode inductor L2, and the other end, far away from the common mode inductor L1, of the differential mode inductor L2 serves as the output of an LED signal processing unit and is connected with a multi-channel boosting constant current unit. And the multi-channel boost constant current unit outputs to a load LED in a DCM mode. While the boost constant current output is designed to be stable, under the condition of open circuit or short circuit of the LED lamp, all the LED lamps can be extinguished at one time, and the load is well protected; and a more efficient DCM mode is selected for driving, so that the design complexity is reduced while the design requirements are met safely and efficiently.

Description

Technical Field

[0001] The utility model relates to an automobile lighting circuit design technology, and particularly relates to an LED boost constant current circuit. Background Art

[0002] Since the invention of LED (light emitting diode), it has attracted great attention due to its excellent performance such as small size, low energy consumption, firm and durable, long life, safe low voltage, wide application range, rich colors, and high light quality.

[0003] Nowadays, the headlights of high-end vehicles are basically composed of LEDs. In order to meet the increasingly complex functional requirements of high-end vehicles, DC-DC (direct current to direct current) chips are often used in headlight design. The commonly used architectures in DC-DC chip design are BUCK (step-down), BOOST (step-up), BUCK-BOOST (step-up / step-down), etc. And because the power consumption of the LED lamp itself is not large, its efficiency problem is thus ignored. Summary of the Utility Model

[0004] Aiming at the safety and efficiency problems of LED signal headlights, an LED boost constant current circuit is proposed.

[0005] The technical solution of the utility model is: an LED boost constant current circuit, including an LED signal processing unit and a multi-channel boost constant current unit. The lamp signal input passes through the LED signal processing unit and the multi-channel boost constant current unit in sequence to drive the load LED to light or go out;

[0006] The LED signal processing unit includes a transient voltage suppression diode, an RC filtering protection circuit, an anti-reverse diode, a common mode inductor, and a differential mode inductor; the lamp signal input terminal is connected in parallel with the transient voltage suppression diode TVS1 and the RC filtering protection circuit. The output of the RC filtering protection circuit is connected to the common mode inductor L1 through the anti-reverse diode D1. The other end of the common mode inductor L1 far from the anti-reverse diode D1 is connected to the differential mode inductor L2. The other end of the differential mode inductor L2 far from the common mode inductor L1 is the output of the LED signal processing unit, which is connected to the multi-channel boost constant current unit;

[0007] The multi-channel boost constant current unit includes a multi-channel current source DCDC chip and its peripheral circuit, and outputs in DCM mode to the load LED.

[0008] Preferably, the model of the multi-channel current source DCDC chip is MPQ7220, which is a boost converter with six-channel current sources, and the current values of each channel current source are the same.

[0009] Preferably, the load LEDs are connected in a parallel manner after the same number of LEDs are connected in series in each path. Four diodes D4 to D7 are connected in series to replace one LED to fill in the LEDs in each path with insufficient number, ensuring that the output voltage of the parallel circuit is the same.

[0010] Preferably, several channel current sources of the multi-channel current source DCDC chip provide drive current for one path of LEDs, and the current of each channel current source is evenly distributed.

[0011] Preferably, an input capacitor is further provided between the anti-reverse diode D1 and the common mode inductor L1, and the input capacitor and the common mode inductor are arranged on the same horizontal line on the PCB board.

[0012] Preferably, the GND1 of the output capacitor after the LED signal processing unit after the differential mode inductor L2 is connected to the GND1 of the output capacitor after the boost inductor L3 in the multi-channel boost constant current unit through copper plating.

[0013] Preferably, a diode D3 is connected between the boost inductor L3 and the load LEDs, and a series of R18 and R19 are connected between the negative electrode of the diode D3 and GND1. The voltage division of R18 and R19 constitutes an overvoltage protection threshold, and this threshold is set to be 30% higher than the LED voltage at -40 degrees Celsius.

[0014] The beneficial effects of the present invention are as follows: For the LED boost constant current circuit of the present invention, while designing the boost constant current output to be stable, in the case of the LED lamp being open or short-circuited, all can be turned off simultaneously, which can better protect the load; the more efficient DCM mode drive is selected, which can safely and efficiently meet the design requirements while reducing the design complexity. Description of the Drawings

[0015] Figure 1 It is the circuit diagram of the multi-channel boost constant current unit in the LED boost constant current circuit of the present invention;

[0016] Figure 2 It is the circuit diagram of the load LED unit in the LED boost constant current circuit of the present invention;

[0017] Figure 3 It is the circuit diagram of the turn signal processing unit in the LED boost constant current circuit of the present invention. Detailed Embodiments

[0018] The present invention will be described in detail below with reference to the drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0019] Such as Figure 1The circuit diagram of the multi-channel boost constant current unit is shown. The model of the DCDC chip selected for the turn signal boost constant current circuit is MPQ7220, which is specifically designed for automotive tail light design. It is a boost converter with six-channel current sources, and each channel can reach a maximum current of 100 mA, which can meet the diverse designs of the circuit; it has CCM mode and DCM mode. For light load output, DCM mode will be more efficient; it has rich protection modes, including OCP (overcurrent protection), OVP (overvoltage protection), OTP (overtemperature protection), LED string open circuit and short circuit protection, etc., to ensure the safe operation of the circuit.

[0020] Figure 1 In it, VCC is used as the power supply of the chip and is connected to the chip power supply pin VIN. The output voltage of the chip, pin 22 VCC, is the 5V output terminal of the chip, which is defined as VCC1 for distinction from the chip power supply. L3 is the boost inductor connected between VCC and SW. The SW pin is internally connected to the drain of the switching MOS. The positive pole of diode D3 is connected to the SW pin and one end of the inductor L3 far from VCC. Initially, diode D3 is not conducting, the drain of the switching MOS is at low level, and VCC charges the inductor L3; when the inductor L3 is charged and there is current flowing through, the current at both ends of the inductor cannot change suddenly, so a voltage will be induced, the voltage at the positive pole of diode D3 increases, diode D3 conducts, the output voltage increases, and at the same time the voltage at the drain of the switching MOS increases and conducts. As long as the speed of the switching MOS is fast enough (the switching frequency is high enough), and the on and off times (charge and discharge times) are well controlled, and in cooperation with the output filter capacitors (parallel C17, C183, C18, C182, C19), a basically stable output voltage can be obtained. The output voltage passes through the magnetic bead B1 that suppresses noise interference as the power supply for the LED lamp, LED+. The series-connected R201 and C184 placed beside the SW pin are used to absorb the spikes generated by the MOS switch and improve the EMC effect.

[0021] Figure 1 In it, the series-connected R18 and R19 are connected between the negative pole of diode D3 and GND1. The voltage division of R18 and R19 constitutes the overvoltage protection threshold, and the formula is V(OVP)=2*((R19+R18) / R18). This threshold is set to be 10%-30% higher than the LED voltage at low temperature (-40°C), and it is best to set it 30% higher to avoid triggering overvoltage protection during normal operation.

[0022] Figure 1 In it, the parallel-connected R20&R27 are current setting resistors, and the formula is I LED (mA)=1245 / (R20*R27 / (R20+R27)). Because the output current is small, the working mode of the chip is DCM mode, which has high efficiency.

[0023] Figure 1In the circuit, R22 and C25 are connected in series and then in parallel with C26 to form a compensation circuit. When laying out the compensation circuit connected between pin 7 (COMP) of the chip and GND1, it should be as close as possible to the compensation pin.

[0024] Figure 1 In the circuit, R10 and R12 are used for voltage division to make the EN pin at high level. R11 is a current-limiting resistor to prevent excessive input current from damaging the IC.

[0025] Figure 1 In the circuit, R21 and R26 connected in series between VCC1 and DIM, and the series connection point is connected to pin 8 (FF, fault flag pin) of the chip. R14 and R9 connected in series between VCC1 and GND1, and the series connection point is connected to pin 1 (DIM, input control switch pin) of the chip; R21, R26, R9, and R14 are used to achieve the function that when an LED fails, all LEDs go off. When the LEDs are normal, FF is at high level, DIM is at high level, and the chip works normally. When an LED is short-circuited or open-circuited, FF is pulled low, DIM is at low level, and the chip does not work.

[0026] Figure 1 In the circuit, R17 sets the short-circuit protection threshold. A current source of 18uA flows out from this pin. When V STH < 1.4V, the short-circuit protection threshold is 10 times of V STH ; when STH floats or V STH > 1.4V, the short-circuit protection threshold is 5V. MPQ7220 monitors the voltage between LEDX and GND to determine whether a short-circuit fault in series occurs. When the voltage of the LEDX pin exceeds the protection threshold, the internal counter is started. If this fault condition lasts for 7.7ms, the faulty string is marked and disabled.

[0027] As Figure 2 shown in the circuit diagram of the load LED unit, there are 19 LEDs in this scheme, designed as two parallel branches with ten in series in each branch. Since there are 19 LEDs in this case, four diodes D4~D7 are used to replace one LED in one branch during design to ensure that the output voltages of the two parallel circuits are the same. If the voltages of the two outputs are different, the voltage difference will be borne by the chip, and the chip will have the risk of being burned out due to overheating. The current of a single LED is 45mA. Therefore, the adopted design is that the current sources of every three channels of the chip are used as the driving current for one LED, and the current of each channel current source is evenly distributed, calculated as 15mA. As Figure 1 shown in the circuit, pins 14, 15, and 16 are connected as LED1-, pins 11, 12, and 13 are connected as LED2-, and filter capacitors (C20, C22, C24) are connected between LED+, LED1-, LED2- and GND1 to ensure the stability of the signals at the chip end. Figure 2In the load LED unit, two strings of 10 LEDs in series are connected between LED+ and LED1-, LED2- through detection resistors (R5, R3). Each LED is shunted with a filtering capacitor (C27~C45). Filtering capacitors (C187, C185, C186) are connected between LED+, LED1-, LED2- at the load LED unit end and GND1 to ensure the stability of the signals at the load end.

[0028] As Figure 3 shown in the circuit diagram of the turn signal processing unit, Turn represents the input of the turn signal. Generally, Turn = 13.5V. The transient voltage suppression diode TVS1 shunted between Turn and GND is used to absorb the instantaneous overvoltage or voltage pulse in the circuit; R1 and C1, C2 form an RC filtering protection circuit at the input end; the diode D1 is used for anti-reverse function. When the input current passes through RC filtering, diode anti-reverse, common mode inductor L1 and differential mode inductor L2 to filter out noise in sequence, the output of the differential mode inductor L2 is VCC, GND1. VCC is connected to the input end VIN of the chip MPQ7220 to supply power to the chip, and the chip starts to work. Filtering capacitor groups (C8-C10, C11-C13, C180, C181) are shunted at both the input and output ends of the differential mode inductor L2.

[0029] I. Introduction to the function of all off when one is off:

[0030] When the circuit is working normally, FF is at a high level. VCC1 makes DIM at a high level through voltage division by R14 and R9. At this time, the chip works normally.

[0031] All off in the short-circuit state: The STH pin of the chip monitors the voltage from the LEDx pin of the chip to GND to determine whether a short-circuit fault occurs. If one or more strings are short-circuited, the respective LEDx pins bear high voltage stress. When the voltage of the LEDx pin exceeds the protection threshold, the internal counter is started. If this fault condition lasts for 7.7ms, the fault string is marked and disabled. Once a string is marked, it is disconnected from the output voltage loop until the short circuit is removed.

[0032] The external resistor R17 of the STH pin sets the short-circuit protection threshold. A current source of 18uA flows out from this pin. When V STH <1.4V, the short-circuit protection threshold is 10 times of V STH . The measured voltage of a single LED in the circuit is between 2.7V and 2.8V. Therefore, the short-circuit protection threshold in this circuit is set to 2.7V.

[0033] When a certain LED is short-circuited, the STH pin detects that the voltage between the LEDX pin and GND is higher than 2.7V, and the fault time lasts for more than 7.7ms, then the short-circuit fault is determined to be established, and the fault information is fed back to the FF pin. The FF pin fault is pulled low, VCC1 is divided by R14 and R26, DIM is pulled low, and the IC is turned off. At this time, all LEDs are turned off, and the one-off-all-off function is completed.

[0034] All LEDs are off when the LEDs are open: Open string protection is implemented by detecting the voltage of the OVP pin and the LED1~6 pins. During operation, if a string is open, the corresponding LEDx pin voltage is pulled down to ground. The IC continues to charge the output voltage until the OVP (overvoltage protection) threshold is reached. If the OVP point is triggered, the chip stops switching and marks the faulty string with LEDx pin voltage below 100mV. Once marked, the remaining LED strings force the output voltage back to normal regulation. The string with the largest voltage drop determines the output regulation value. Every other string sends a 10μs pulse current with a delay of 500μs to detect whether the open circuit fault is eliminated and the open circuit protection can be restored.

[0035] When an LED is open-circuited, the fault information is fed back to the FF pin. The FF pin is pulled low due to the fault, VCC1 is divided by R14 and R26, DIM is pulled low, and the IC is turned off. At this time, all LEDs are turned off, and the one-off-all-off function is completed.

[0036] 2. PCB layout introduction:

[0037] Due to cost and other reasons, this solution uses a double-layer FR4 design. The TVS needs to be as close to the input connector as possible. When there is a transient overvoltage or voltage pulse in the circuit, the bidirectional TVS1 in the SMB package can absorb it instantly; there is also an input capacitor between the anti-reverse diode D1 and the common-mode inductor L1, and the input capacitor is arranged as horizontally as possible with the common-mode inductor. The output capacitor of the turn signal processing unit after the differential-mode inductor L2 and the GND1 of the output capacitor after the boost inductor L3 should be connected through a large copper plate as much as possible, which is beneficial to passing the EMC test.

[0038] The connection between the boost inductor L3 and the diode D3 at the front end of the output and the SW pin should be short. The GND1 of the output capacitor should be connected to the GND1 of pin 17 of the chip and connected to the exposed pad of the chip. The GND1 of other pins of the chip should not be connected to the exposed pad. When a large area of ​​copper is connected, please manually set the isolation.

[0039] Since this solution does not add MOS transistors at the input end as recommended by the chip circuit, when placing the electrolytic capacitor, the input VCC signal needs to pass through the electrolytic capacitor (C181) and then through the filter capacitors (parallel C11, C12, C180) before entering the input end of the chip. The GNDs of the input capacitor and the output capacitor also need to pass through the GND after the electrolytic capacitor (C181) and then enter the GND of the common-mode inductor. The electrolytic capacitor cannot be omitted, otherwise there is a risk that the PCB board will not start due to overvoltage protection caused by overvoltage during instantaneous power-on at voltages above 16V.

[0040] The double-layer board design is not very friendly to EMC tests, so an external shielding cover for the chip MPQ7220 is required. If it is a four-layer board design, the position for the shielding cover can be considered reserved. The shielding cover can be omitted if the wiring loop is in good condition. The distance between the shielding cover and the internal components, especially the higher components such as inductors and diodes, should be 1mm or more.

[0041] III. Introduction to Each Pin of the Chip

[0042] 1. The 1st pin of the chip is the input pin for the DIM signal (the control switch for the analog voltage input signal), and a PWM signal can be applied to this pin for brightness control. In the design of this solution: during normal operation, the DIM pin at the series connection point is at a high level by the voltage division of R14 and R9 connected in series between VCC1 and GND1; when a fault occurs and the 8th pin FF is at a low level, the DIM pin is at a low level by the voltage division of R14 and R26.

[0043] 2. The 3rd pin of the chip is the frequency setting pin, and the switching frequency value is set by an external resistor, with a programmable frequency up to 2.2MHz. In this circuit, R13 is the external resistor for setting the frequency, and the set frequency is 293.3KHz. The formula is f sw (kHz)=22000 / R13.

[0044] 3. The 4th pin of the chip is the EN pin of the chip. Pulling this pin to a high level enables the IC, and pulling this pin to a low level forces the IC to enter the shutdown mode. In the design of this solution, the EN pin is at a high level by the voltage division of R10 and R12 connected in series between VCC and GND1. The series connection point is connected to EN through R11, and R11 is a current-limiting resistor to prevent excessive input current from damaging the IC.

[0045] 4. The 7th pin COMP is the compensation pin of the chip, and the components of the compensation pin should be as close as possible to this pin.

[0046] 5. The 8th pin FF is the fault flag pin of the chip, which is at a high level during normal operation and is pulled low during a fault.

[0047] 6. The 9th pin of the chip is the LED current setting pin. It is connected to the ground through an external resistor from this pin. By setting the external resistor value, the channel current value can be set, and the current value of each current source channel can be programmed. The current value of each channel is the same. In this circuit, the parallel-connected R20 & R27 are used as the external resistor for current setting, and the set current is 15 mA. The formula is I LED (mA) = 1245 / (R20 * R27 / (R20 + R27)).

[0048] 7. The 11th pin to the 16th pin are the output current source channels of the LED. The negative pole of the LED is connected to these pins. The current value of each channel current source is the same, and the magnitude of the current value is set by the external resistor value of ISET. The maximum sum of all currents is 100 mA. In this circuit, the current value of each channel is set to 15 mA.

[0049] 8. The 5th, 6th, 10th, 17th, and 25th pins are all grounded.

[0050] 9. The 18th pin of the chip is the SW pin. This pin is the drain of the internal switch MOS. Connecting one end of the power inductor L3 to SW can achieve the function of storing voltage to achieve the purpose of boosting.

[0051] 10. MPQ7220 has rich protection modes to ensure the safe operation of the circuit. The protection modes include OCP (overcurrent protection), OVP (overvoltage protection), OTP (overtemperature protection), LED string open - circuit and short - circuit protection, etc. The 19th pin of the chip is the overvoltage protection pin OVP. Different - value resistors are used for voltage division to set the overvoltage protection threshold. Do not leave this pin floating. In this design, R18 and R19 are the voltage - dividing resistors for OVP. The 20th pin of the chip is the LED short - circuit protection threshold setting pin. An 18 μA current source flows out from this pin. A resistor R17 can be connected between this pin and GND to set the short - circuit protection threshold, and the resistance value is selected according to actual needs. If this pin is floating, the short - protection threshold is 5V.

[0052] 11. The 21st pin is the switch frequency spread - spectrum pin.

[0053] 12. The 22nd pin VCC of the chip is the 5V LDO output pin. VCC provides power for the internal logic and gate driver. It is defined as VCC1 to distinguish it from the chip power supply. Place a ceramic capacitor C16 as close as possible to this pin to reduce noise.

[0054] 13. The 23rd pin of the chip is SD, which is the gate drive pin of the external disconnect PMOS. It can turn off the external PMOS in the fault state. If not used, this pin can be left floating. In this solution, no external PMOS is added, so this pin is left floating.

[0055] 14. The 24th pin VIN of the chip is the power input terminal, which supplies power to the IC. An external capacitor needs to be close to this pin to ensure a clean input voltage and prevent the chip from being accidentally triggered.

[0056] The utility model discloses a turn signal boost constant current circuit solution designed based on the MPQ7220 chip, which boosts the input voltage of 13.5V to a stable constant current output voltage of 28V and retains the function requirement of all-off when one is off. Through the problems encountered in the experiment and continuous improvement in the EMC rectification, including Figure 1 the chip MPQ7220 does not use the common CCM mode but instead uses the DCM mode; Figure 3 when the electrolytic capacitor C181 is not added, at an input of 16V, the LED flashes once and then goes out, and it needs to be powered on again to be normally lit. After inspection, it is found that the overvoltage protection is activated, and it is improved by adding an electrolytic capacitor (C181); Figure 1 in the protection mode of the chip MPQ7220, the short-circuit protection threshold is selected too large to meet the all-off function, and too small is prone to accidentally trigger the short-circuit protection; without the shielding cover of the chip MPQ7220, the EMC fails, and the EMC margin is relatively small even with the shielding cover added. On this basis, an RC filter to absorb noise is added at the switching node ( Figure 1 an RC absorption circuit is added to the SW pin of the chip) and other problem points and difficulties actually encountered in the design and experimental tests are improved and solved.

[0057] The above embodiments only represent several implementation manners of the utility model, and the description is relatively specific and detailed, but it should not be construed as a limitation to the scope of the utility model patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the utility model, several deformations and improvements can still be made, and these all belong to the protection scope of the utility model. Therefore, the protection scope of the utility model patent shall be subject to the appended claims.

Claims

1. An LED boost constant current circuit, characterized in that: It includes an LED signal processing unit and a multi-channel boost constant current unit. The light signal input passes through the LED signal processing unit and the multi-channel boost constant current unit in sequence to drive the load LED to turn on or off. The LED signal processing unit includes a transient voltage suppression diode, an RC filter protection circuit, an anti-reverse diode, a common mode inductor and a differential mode inductor; the transient voltage suppression diode TVS1 and the RC filter protection circuit are connected in parallel at the lamp signal input end, the output of the RC filter protection circuit is connected to the common mode inductor L1 through the anti-reverse diode D1, the other end of the common mode inductor L1 away from the anti-reverse diode D1 is connected to the differential mode inductor L2, and the other end of the differential mode inductor L2 away from the common mode inductor L1 is the output of the LED signal processing unit, which is connected to the multi-channel boost constant current unit; The multi-channel boost constant current unit comprises a multi-channel current source DCDC chip and peripheral circuits, and outputs to the load LED in DCM mode.

2. The LED boost constant current circuit according to claim 1, characterized in that: The multi-channel current source DCDC chip model is MPQ7220, and is provided with a boost converter of six channel current sources, and the current value of each channel current source is the same.

3. The LED boost constant current circuit according to claim 2, characterized in that: The load LEDs are connected in parallel after the same number of LEDs are connected in series in each path, and four diodes D4 to D7 are connected in series to replace one LED to fill the insufficient number of LEDs in each path, so as to ensure that the output voltage of the parallel circuit is the same.

4. The LED boost constant current circuit according to claim 3, characterized in that: Several channel current sources of the multi-channel current source DCDC chip provide driving current for one LED, and the current of each channel current source flows evenly.

5. The LED boost constant current circuit according to claim 4, characterized in that: An input capacitor is also provided between the anti-reverse diode D1 and the common-mode inductor L1 , and the input capacitor and the common-mode inductor are arranged on the same horizontal line on the PCB board.

6. The LED boost constant current circuit according to claim 4, characterized in that: The output capacitor of the LED signal processing unit after the differential mode inductor L2 is connected to the GND1 of the output capacitor after the boost inductor L3 in the multi-channel boost constant current unit through copper plating.

7. The LED boost constant current circuit according to claim 6, characterized in that: A diode D3 is connected between the boost inductor L3 and the load LED, and R18 and R19 are connected in series between the cathode of the diode D3 and GND1. The voltage division of R18 and R19 constitutes an overvoltage protection threshold, which is set to be 30% higher than the LED voltage at minus 40 degrees.

Images