With the development of white LED lights, it’s more widely applied. In the past, the white LED is the most common application is as a small-size color LCD backlight. White LED brightness increase, its application has spread to other areas, such as a flashlight or cell phone camera auxiliary lighting. This paper describes a general boost chip to drive the efficient high-current LED circuit.
Circuit design
General white LED current is about 20mA, but the high brightness LED 200 to 300mA current. If the product needs to use three to four high-brightness white LED, the brightness average general practice is to string them connected together. On the market, most of the white LED driver chip can only drive 20mA. Hit a series high current LED applications will have to think of other ways. Intersil’s EL7516 is a typical step-up chip, this chip operates at 1.2MHz fixed-frequency PWM mode, built-in 1.5A, 200mΩ MOSFETs. Figure 1 shows the the EL7516 typical application circuit. DC-DC boost the role of EL7516 will be 2.7 ~ 5.5V input into a constant voltage of 12V. General PWM control chip, the FB pin is connected to the error amplifier is compared with 1.3V. R1, R2 resistor value is selected to set the output voltage.
The EL7516 Boost Application Circuit
EL7516 voltage circuit into a constant-current LED drive circuit is very simple. Shown in Figure 2, the FB pin of R1 replaced by LED, change R2 can adjust the current through the LED. The value of R2 can be selected from the following formula: R2 = VFB / Iled where VFB is the FB pin voltage, that is, 1.3V; Iled current through the LED.
Standard LED driver circuit
EL7516 used to drive four high-brightness white LED tube
R2 by 300mA, the resistance of the power consumption is close to 0.39W. Such a great power not only affect the efficiency, but also need to use the relatively large size of the resistance in order to accept the 0.39W of heat. In general, these applications are battery-powered, efficiency and line PCB space requirements are more stringent now let us look at how to improve the efficiency of this line.
Regardless of the circuit changes how EL7516 will adjust the duty cycle so that the FB voltage to maintain 1.3V. The assumption D1 positive voltage drop is 0.6V, the voltage drop across R2 is about 0.7V. To maintain a 300mA LED current, R2 can be used to 2.3Ω. R2 power consumption also dropped from the original 0.39W 0.21W. Although the power consumption has been reduced, but R2 have to use a half-watt resistor to this circuit.
A cheap TL431 added to the circuit as a 2.5V reference. As mentioned earlier, the EL7516 will keep the FB in 1.3V; so the current through R4 is: (2.5-1.3) / 20k = 60mA. FB is a high-impedance pins, we assume that this 60mA full inflow of R5, made the same 1.2V voltage drop. 0.1V, the rest of the R2 to complete. In order to facilitate the purchase, R2 is selected as 0.39Ω. So the current through the LED is about: 0.1/0.39 = 255mA. R2, the power consumption is significantly reduced to: 0.1 × 0.255 = 26mW.
Conclusion
Can be seen from the above experiments, minor changes can greatly improve the efficiency of high-current LED panel in the circuit. Experiments, we use a 5V input. The practical application is likely to be a single lithium battery power supply. The EL7516 minimum operating voltage of 2.3V, so apply to a single lithium battery power supply. EL7516 built-in MOSFET peak current (1.3A min) protection, such as the lithium battery voltage dropped below 3V, drive four LEDs may trigger the current protection. Such as the need to 3V below work best to reduce the output for the three LEDs.