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 LED. The brightness of white LED is proportional to the current . AP3036 provides a series LED architecture, which can provide the voltage required for 8 white LEDs in series . The biggest advantage of AP3036 is that it can use PWM signal 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 the EMI is better.

LED = Light Emitting Diode, a solid-state semiconductor device that can convert electrical energy into visible light; LED = Large Electronic Display; LED = Lupus erythematosus disseminatus, 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 uses SOT23-6 and TSOT-23-6 packages , and has a 200mV version and a 250mV version (AP3036A). Both versions are currently 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 8 LEDs, 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 use 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.

[page]Application Circuit Refer to Figure 1. The maximum LED current is determined by a resistor RISET from FB to ground . Because the LED adopts a series structure, the current of each LED can be guaranteed to be the same and has nothing to do with the change of the conduction voltage of each LED . The relationship between LED current and RISET is expressed by the following formula: RISET=VFB/ILED, 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, it will have adverse effects on the system and battery. AP3036 integrates a soft start function, which starts slowly 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.

Figure 5. LED Current vs. PWM Duty

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