Design of high-efficiency LED backlight driver power supply solution

At present, the LCD screen backlight of portable electronic products is mainly based on white light LED. The brightness of white light LED is proportional to the current. AP3036 provides a series LED architecture, which can provide the voltage required for up to 8 white light LEDs in series. The biggest advantage of AP3036 is that it can use PWM signals to adjust the LED brightness. The dimming frequency range is very wide, up to 100KHz. When AP3036 uses a frequency of more than 20KHz to adjust the LED brightness, it can skip the audio range, so the audio interference is smaller and EMI is better.

LED = Light Emitting Diode, a solid-state semiconductor device that can convert electrical energy into visible light, can directly convert electricity into light; LED = Large Electronic Display, large electronic display; LED = Lupus erythematosus disseminatus, disseminated lupus erythematosus, a chronic, idiopathic autoimmune disease; led is the past tense and past participle of lead, meaning "lead"; IATA code for Pulkovo Airport in Russia. This entry mainly introduces light-emitting diodes.

Figure 1. AP3036/A typical application

The conversion efficiency of AP3036 can reach up to 82%, and the minimum operating voltage is 2.5V, which can reduce current consumption and extend battery life. The built-in Schottky diode reduces the components and complexity of the system. The built-in soft start function can reduce the inrush current, and the overvoltage protection (OVP) and overcurrent protection (OCP) can ensure that the system works in normal state.

AP3036 adopts two packages, SOT23-6 and TSOT-23-6, and has 200mV version and 250mV version (AP3036A). Both versions are now in mass production and samples and demo boards are available.

AP3036 Introduction

AP3036 is the latest LED backlight dedicated driver control chip launched by BCD. It adopts inductive structure current mode boost control to provide ideal LED constant current function and excellent regulation rate. AP3036 adopts series LED structure to drive up to 8LEDs, and realizes LED constant current function through a current detection resistor. The input voltage range reaches 2.5V to 16V, and a single battery or multiple batteries in series can be used for system power supply.

The maximum operating frequency of AP3036 reaches 1.0MHz, so the parameters and size of external devices can be significantly reduced. The typical 1mm 22uH inductor and 0.22uF output capacitor can meet normal usage conditions.

8LED 20mA backlight driver solution

Most of the LCD screen backlights for portable electronic products are below 8 LEDs, and the LEDs used are in a small package structure of 20 mA, so this solution can meet the application needs of most such occasions.

LED (Light-Emitting-Diode) is a semiconductor that can convert electrical energy into visible light. It changes the principle of tungsten filament light emission of incandescent lamps and trichromatic powder light emission of energy-saving lamps, and adopts electric field light emission. According to analysis, the characteristics of LED are very obvious, with long life, high light efficiency, no radiation and low power consumption. The spectrum of LED is almost entirely concentrated in the visible light band, and its luminous efficiency can exceed 150lm/W (2010). Comparing LED with ordinary incandescent lamps, spiral energy-saving lamps and T5 trichromatic fluorescent lamps, the results show that the luminous efficiency of ordinary incandescent lamps is 12lm/W, and the life is less than 2000 hours, the luminous efficiency of spiral energy-saving lamps is 60lm/W, and the life is less than 8000 hours, and the luminous efficiency of T5 fluorescent lamps is 96lm/W, and the life is about 10,000 hours. The luminous efficiency of white light LED with a diameter of 5 mm can exceed 150lm/W, and the life can be more than 100,000 hours. Some people also predict that the upper limit of LED life in the future will be infinite.

Refer to Figure 1 for the application circuit. The maximum LED current is determined by a resistor RISET from FB to ground. Because the LEDs are connected in series, the current of each LED can be guaranteed to be the same and has nothing to do with the change in the on-state voltage of each LED. The relationship between the LED current and RISET is expressed by the following formula: RISET=VFB/ILED, where VFB can be found in the data sheet. At the moment of system startup, the input will have a relatively large inrush current, in order to avoid this situation from having adverse effects on the system and battery. The AP3036 integrates a soft-start function that slowly starts within 0.7ms to eliminate this effect.

The corresponding Demo Board PCB is as follows:

Figure 3 PCB Layout

Figure 4: Physical diagram of the solution

In order to avoid audio interference, AP3036 has added a high-frequency dimming function. The frequency of PWM dimming can be as high as 100KHz, which effectively avoids audio interference below 20KHz. And it ensures perfect dimming linearity and system stability, refer to Figure 5 and Figure 6 below.

图5. LED Current vs. PWM Duty

Figure 6. PWM dimming waveform (VIN=3.6V; ILED=20mA; 8LED; FPWM=100KHz; Duty=50%)