Design of LED lighting driver based on MAX168O1/168O2

As the latest lighting technology, LED has advantages not only in terms of luminous quality, but also in terms of production, manufacturing, and ease of use, which greatly surpass traditional light sources such as incandescent lamps and fluorescent lamps. Inspired by the principle of fluorescent lamp luminescence, LED manufacturers add a layer of phosphor on the high-brightness blue LED core and use blue light to excite the phosphor to emit white light. In addition, by using different phosphors, a variety of white light LEDs with color temperatures of 4500-10000K and 2850-3800K can be emitted. The luminous efficiency of white light LEDs has mostly exceeded 301m/W, and some products have exceeded 50lm/W, which has the foundation for formal large-scale practical application. The comprehensive performance of RGB three-color LED synthetic white light is good. Under high color rendering index, the lumen efficiency may be as high as 2001m/w. The main technical problem to be solved is to improve the electro-optical conversion efficiency of green light LEDs, which is currently only about 13%. For LED-excited phosphors to emit light, three-primary-color mixing can avoid the former's shortcomings such as discontinuous spectral distribution and poor color rendering. At the same time, the human eye comfort of three-primary-color mixing can be greatly improved. After three-primary-color PWM modulation, multi-color and full-color lighting can be achieved in the same light source as needed.

1 Design Concept and Solution
To design a high-power semiconductor driver, we must first consider the three aspects of light-emitting chip selection and light source realization; drive circuit design, secondary optical design; and device packaging.
In LED lighting, there are successful design cases of monochromatic LEDs stimulating phosphors to emit light, but considering that this solution is difficult to achieve full spectrum and high color rendering, this design uses an RGB three-primary color mixed light source, and drives the three monochromatic LED chips of red, green and blue separately to emit light separately to achieve the completion of the light source. In order to achieve stable illumination under high-power output, it is necessary to compensate for the temperature attenuation of the LED, and also to compensate for the surge pulses and unstable current fluctuations at startup. In the secondary optical design, the three-primary color mixing is mainly considered. In the comparison between integrating sphere mixing and fiber coupling, fiber coupling is selected, and the light output of the three-color chip is mixed through fiber coupling and then output.

2 LED optical and electrical characteristics
For the characteristics of ultra-bright LEDs, when the forward voltage exceeds a certain threshold (about 2V), which is usually called the on-voltage, it can be approximately considered that IF is proportional to VF. The current maximum IF of ultra-bright LEDs can reach 1A, while VF is usually 2 to 4V. Since the optical characteristics of LEDs are usually described as a function of current rather than voltage, constant current source driving can better control the brightness. In addition, the forward voltage drop of LEDs has a large range of variation (up to 1V or more), and a small change in VF will cause a large change in IF, thereby causing a large change in brightness. Therefore, the use of a constant voltage source drive cannot guarantee the consistency of LED brightness, and affects the reliability, life and light decay of the LED. Therefore, ultra-bright LEDs are usually driven by a constant current source. The luminous flux of LEDs is inversely proportional to the temperature. The luminous flux at 85°C is half that at 25°C, and the light output at -40°C is 1.8 times that at 25°C. Temperature changes also have a certain impact on the wavelength of LEDs, so good heat dissipation is the guarantee for LEDs to maintain constant brightness.

3 Light mixing scheme
Additive light mixing of three primary colors means that the three primary colors of light can be mixed to obtain white light source and gray light. For LED, according to the chromaticity diagram released by the International Commission on Illumination (CIE), the color of light is related to the ratio of the three primary colors R, G, and B, and r(λ)+g(λ)+b(λ)=1. The main problem with the method of using RGB to synthesize white light is the low conversion efficiency of green light. Now the conversion efficiency of red, green and blue LEDs has reached 30%, 10% and 25% respectively. Due to the different color temperatures and color rendering indexes required for synthesized white light, the lumen efficiency of each color LED for synthesized white light is also different. When different proportions of pulse width current are provided to the three light-emitting chips, different grayscale lighting sources can be output. If multi-level grayscale calculation is implemented for the mixing ratio, full-color dimming can be achieved. In order to improve the mixing efficiency, commonly used solutions include integrating sphere mixing and mixing rod mixing. In order to reduce the device packaging volume, this solution uses fiber-coupled light mixing, that is, the light emitted by the three chips is injected into the optical fiber at the focal point through a convex lens, and mixed in the optical fiber. The mixed light is then sent out through the optical fiber to the concave lens for output.

4 Driving circuit design
The whole circuit is divided into three parts: a. Switching power supply, which realizes the conversion of AC power to low-voltage constant current LED adaptive current; b. PWM modulation circuit, which realizes the control of LED light output efficiency; c. Control circuit, which realizes the connection and control of switching power supply and PWM, and realizes the control of light output grayscale after mixing.
4.1 AC/DC switching power supply based on MAXl6801/16802 The
high-brightness (HB) LED driver control integrated chip MAXl680l/16802 basically meets the requirements of the key circuit of the actual LED driver, and can be used for high-brightness lighting and display applications. It can be used for 85~265V AC rectified voltage input. When the LED current needs to be accurately adjusted, the on-chip error amplifier and the reference with an accuracy of 1% can be used. A wider brightness adjustment range can also be achieved through on-chip PWM brightness adjustment.
The internal functions of MAXl6801/16802 are shown in Figure 1.

The driving circuit of a single LED chip is shown in FIG2 .

Description of the functions between the various ports of the circuit:
UVLO/EN: Externally programmable undervoltage lockout. UVLO sets the input startup voltage. Connecting UVLO to GND can disable the device from working. DIM/FB: Low-frequency PWM brightness adjustment input/error amplifier inverting input. COMP: Error amplifier output. In high-brightness LED current regulation applications, connect the compensation component between DIM/FB and COMP.
CS: Current sensing signal connection terminal for current regulation. Ring current sense resistor high end. RC filter can be thrown to remove burrs on the leading edge. GND: Power ground. NDRV: External N-channel MO-SET gate connection terminal. Vcc: Gate drive power supply. Internally obtained by stepping down from IN. Connect a decoupling capacitor of 10nF or more between Vcc and GND. IN: IC power supply. Connect a decoupling capacitor of 10nF or more between IN and GND. In the bootstrap mode of MAXl6801, a startup resistor can be connected between the input power supply and IN. The bias winding power supply is connected to this point. For MAXl6802, IN is directly connected to the +10.8~+24V power supply.

4.2 PWM control circuit of a single LED chip
Since the electro-optical conversion efficiency of the three LEDs of red, green and blue is inconsistent, and the ratio of the three primary colors of red, green and blue is different when mixing light, the three color chips must be modulated separately. The modulation chip is NCP-1200, which is a current-type PWM controller launched by ON Semiconductor. Its application circuit only needs to use a few peripheral components, making the design more compact. In addition, the chip integrates an output short-circuit protection circuit to further reduce the cost. There are two types of feedback in the module: the first is output voltage feedback, the output voltage sampling value VSS is compared with the set value provided by the microcontroller, the voltage of the positive pin of the NCP1200 chip is controlled by the optical coupler, and the pulse width of the PWM output by the DRV pin is adjusted to control the on and off time of the field effect tube, thereby achieving the purpose of adjusting the output voltage value. The other feedback is the current limiting feedback. When the sampled output current value ISS exceeds the maximum current limiting value IPWM provided by the microcontroller, the comparator outputs a positive voltage to make the optocoupler conduct at the maximum, pull down the FB pin voltage, and reduce the PWM pulse width output by NCP1200, thereby achieving the purpose of current limiting. When the output current is less than the current limiting
value provided by the microcontroller, the current limiting feedback does not work.
The block diagram of the monochrome control circuit is shown in Figure 3.

4.3 Real-time control of three-primary color circuit
In the control of NCP1200, the ATmega8515 single-chip microcomputer is used. The input end of ATmega8515 is user settings, temperature feedback and light intensity feedback, and the output end is the control information of the PWM modulation
pulse width in NCP1200. The I/O port of ATmega8515 can realize multiple control signal settings through the keyboard and dial switch. It is also possible to program various pulse width ratios of full color into a single chip according to the calculated three-color mixing ratio and the three-chip PWM table to achieve rapid color change.
4.4 Overall framework
In order to ensure that the light intensity attenuation caused by heating of LED is compensated, temperature feedback and light intensity feedback are added to the output end to correct the light attenuation caused by heating. The overall structure is shown in Figure 4.

5 Conclusion
In order to realize high-power full-color LED lighting, a RG-BLED lighting driver design scheme is designed with MAXl6801/16802 as the core switch power supply, and the red, green and blue three-primary color chips are powered by PWM adjustment. This design takes into account the actual problems of luminous attenuation, output wavelength drift, lumen attenuation, etc. caused by the temperature rise of LEDI chips during operation, and makes corresponding compensation. A certain degree of grayscale adjustment is achieved. Under the control system with ATmega8515 microcontroller as the core, temperature sensors and photoelectric sensors are used as feedback information sources to achieve adaptive control. Experiments have proved that this drive circuit works well and can achieve a certain degree of multi-color lighting, which can be used in outdoor lighting and landscape lighting.