Utilizing the EL7516 as an Efficient, High-Current White LED Driver

Foreword

　　With the development of white light LED, its applications are becoming more and more widespread. In the past, the most common application of white LEDs was as backlight for small-size LCD color screens. Nowadays, the brightness of white LED has increased, and its application has spread to other aspects, such as flashlights or mobile phone camera auxiliary lighting. This article introduces a high-efficiency circuit that uses a common boost chip to drive high-current LEDs.

Circuit design:

　　Generally, the current of white LED is about 20mA, but high-brightness LED requires 200~300mA current. If the product requires three to four high-brightness white LEDs, the general approach is to connect them in series for average brightness. Most white light LED driver chips on the market can only drive about 20mA. When encountering the application of series-connected high-current LEDs, another solution must be found. Intersil's EL7516 is a typical boost chip. This chip operates in 1.2MHz fixed frequency PWM mode and has a built-in 1.5A, 200mΩ MOSFET. Figure 1 shows a typical application circuit of the EL7516. Through DC-DC boosting effect, EL7516 converts 2.7~5.5V input into a constant voltage of 12V. Like general PWM control chips, the FB pin is connected to the error amplifier that compares with 1.3V. Set the output voltage by selecting the resistor values ​​of R1 and R2.

Figure 1 Boost application circuit of EL7516

　　It is very simple to change the constant voltage circuit of EL7516 into a constant current circuit for driving LED. As shown in Figure 2, as long as R1 at the FB end is replaced by an LED, and R2 is changed, the current through the LED can be adjusted. The R2 value can be selected from the following formula: R2=VFB/Iled where VFB is the voltage of the FB pin, which is 1.3V; Iled is the current through the LED.

Figure 2 Standard LED drive circuit

　　If the requirement of Iled is 300mA, then R2 requires 4.3W, as shown in Figure 3.

Figure 3 EL7516 is used to drive 4 high-brightness white LEDs.

　　Regarding the circuit in Figure 3, the biggest disadvantage is the loss of R2. When R2 passes 300mA, the resistor's power dissipation is close to 0.39W. Such a large power consumption not only affects efficiency, but also requires a relatively large resistor to withstand 0.39W of heat. Generally speaking, these applications are battery-powered and have strict efficiency and circuit PCB space requirements. Now let's see how to improve the efficiency of this circuit.

　　Figure 4 shows the improved circuit. Comparing Figure 3, Figure 4 adds two components—R3 and D1. No matter how the circuit is modified, EL7516 will adjust the duty cycle to maintain the voltage of FB at 1.3V. Assuming that the forward voltage drop of D1 is 0.6V, the voltage drop of R2 is about 0.7V. To maintain 300mA LED current, R2 can be selected as 2.3Ω. The power consumption of R2 has also been reduced from the original 0.39W to 0.21W. Although the power consumption has been reduced, R2 still has to use a half-watt resistor to implement this circuit.

Figure 4 Improvement Line 1

　　Figure 5 shows further improvement methods. A cheap TL431 was added to the circuit as a 2.5V reference source. As mentioned before, EL7516 will keep FB at 1.3V; so the current through R4 is: (2.5-1.3)/20k=60mA. Since FB is a high-impedance pin, we assume that all 60mA flows into R5 and creates the same voltage drop of 1.2V. The remaining 0.1V will be completed by R2. For the convenience of purchase, we selected R2 as 0.39Ω. So the current through the LED is approximately: 0.1/0.39 = 255mA. The power consumption of R2 is also significantly reduced to: 0.1×0.255 = 26mW.

Figure 5 Improved Line 2

　　Through experiments, we obtained the efficiency comparison results of Figure 3 and Figure 5 (Table 1).

Conclusion

　　From the above experiments, we can see that as long as slight changes are made to the circuit, the working efficiency of high-current LEDs can be greatly improved. In the experiment, we used 5V input. The actual application is likely to be powered by a single lithium battery. The minimum operating voltage of EL7516 is 2.3V, so it is suitable for power supply by a single lithium battery. However, the EL7516's built-in MOSFET has peak current (1.3A min) protection. If the lithium battery voltage drops below 3V, driving four LEDs may trigger the current protection. If you need to work below 3V, it is best to reduce the output to three LEDs.

References:

1. Intersil EL7516 datasheet
2. Luxpia LUXPHERION LL1.0 High Power LED datasheet