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FAN5611 데이터시트(PDF) 8 Page - Fairchild Semiconductor |
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FAN5611 데이터시트(HTML) 8 Page - Fairchild Semiconductor |
8 / 14 page © 2003 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5611/FAN5612/FAN5613/FAN5614 Rev. 1.0.9 8 Application Information Operational Descripton The regulated current through each LED is a multiplica- tion of the ISET current. The ISET value is determined by the RSET value. The ISET value can be calculated as: ISET = ILED / Current Gain Reference Figure 4, the ISET vs. VCTRL graph, to esti- mate VCTRL. The value of RSET is calculated according to the formula: RSET = (VCONTROL - VCTRL) / ISET For example, with VCONTROL = 3V, a 10mA current limit through the LED results in ISET = 50µA. That translates to an approximate value of 1.2V for VCTRL, shown in Fig- ure 4. The resulting RSET value that maintains 10mA reg- ulation is 36k Ω. The LED intensity can be adjusted by varying the duty cycle of a square wave applied to the enable pin. Fre- quency greater than 100Hz is best to avoid a "flickering" effect. The maximum operation frequency is 10MHz. Efficiency Considerations The FAN561X driver’s low-dropout architecture can sig- nificantly improve the efficiency compared to using sim- ple ballast resistors. The system efficiency, defined as the ratio between the LEDs’ power and the input sup- plied power, can be calculated as: The lower the VCATHODE, the higher the system effi- ciency. Efficiency can be further improved by using a higher VIN with more LEDs, as shown in Example 3. Application Notes The ultra-low voltage drop across the FAN561X series of LED drivers allows the devices to drive white, blue, and other color LEDs in a wide range of input voltages. The driver can be used in many applications. Although only the FAN5613 is shown in all three examples, any of the FAN561X-series LED drivers can be used in the applica- tions presented, due to their similar operation. Example 1: Drive low VF white or blue LEDs directly from single cell Li-ion When using white or blue low VF LEDs, and utilizing the drivers low voltage drop, only 3.4V in VIN is needed for the full 20mA LED current. At 3.1V, there is 5mA typical current available for the LEDs. The single cell Li-ion is utilized in applications like cell phones or digital still cam- eras. In most cases, the Li-ion battery voltage level only goes down to 3.0V voltage level, not down to the full dis- charge level (2.7V) before requesting the charger. –VDROP < 0.3V –VF (at 20mA) < 3.1V (Low VF) –VIN (at 20mA) =VDROP + VF = 3.4V –VIN (at 5mA Typical) ~ 3.1V where VIN = Single cell Li-ion voltage. Key advantages: • No boost circuit is needed for the LCD or keyboard backlight. • Drivers are directly connected to a Li-ion battery. • No EMI, no switching noise, no boost efficiency lost, no capacitor, and no inductor. Example 2: Drive high VF white or blue LEDs from existing bus from 4.0V to 5.5V High VF white or blue LEDs have forward voltage drop in the range of 3.2V to 4.0V. Driving these LEDs with the maximum current of 20mA for maximum brightness, usually requires a boost circuit for a single cell Li-ion voltage range. In some cases, there is already a voltage bus in the system, which can be utilized. Due to the ultra- low voltage drop of the FAN561X series of LED drivers to drive high-VF white or blue LEDs, the VIN needs to be only 300mV higher than the highest VF in the circuit. –VDROP < 0.3V –VF (at 20mA) < 3.3V to 4.0V (High VF) –VIN (at 20mA) =VDROP + VF = 3.6V to 4.3V –VIN (at 5mA Typical) ~ 3.3V where VIN = existing bus = 3.3V to 4.3V. Key advantages: • No boost circuit is needed for LCD or keyboard backlight. • Driver utilizes the existing bus. • Ultra-low voltage drop provides the full 20mA LED current at the lowest possible voltage level. Efficiency V IN V CATHODE – () V IN ⁄ = ISET RSET I1 I2 I3 I4 VIN GND FAN5613 CTRL V CONTROL ENABLE ON/OFF |
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