General white LED current is about 20mA, but the high brightness LED 200 to 300mA current. If your product requires three to four high-brightness white LED; 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 EL7516 a typical 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.
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. The following describes a general boost chip to drive high current LEDs efficient circuit.
EL7516 lines of constant pressure to change the drive LED constant current circuit is very simple. Shown in Figure 2, the FB pin of R1 replaced by LED grow lights, change R2 can adjust the current through the LED. R2 values selected from the following formula: R2 = VFB / IledVFB FB pin voltage; that 1.3VIled is the current through the LED
For example, if the requirements of Iled 300mA; then R2 needs to 4.3Ω.
On the circuit of Figure 3, the biggest drawback is the loss of R2. R2 by 300mA, the resistance of the power consumption is close to 0.39W. Such a great power not only affect the efficiency, we also need to use the relatively large size of the resistance to accept 0.39W 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.
egardless 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 achieve this line.
A cheap TL431 added to the circuit as a 2.5V reference. As mentioned earlier, the EL7516 will remain FB 1.3V, the current through R4 is: (2.5-1.3) / 20k = 60uA. FB is a high-impedance pins, we assume that this 60? A full inflow of R5 and 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 tube is about: 0.1/0.39 = 255mA. R2, power consumption is significantly reduced to: 0.1X0.255 = 26mW.