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FAN5612 датащи(PDF) 7 Page - Fairchild Semiconductor |
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FAN5612 датащи(HTML) 7 Page - Fairchild Semiconductor |
7 / 12 page PRODUCT SPECIFICATION FAN5611/FAN5612/FAN5613/FAN5614 REV. 1.0.4 6/4/04 7 Application Information Setting the LED Current The current flowing into LEDs is approximately 200 times greater then the current ISET. The LED current is controlled by VCONTROL and RSET according to the formula: For VCONTROL = 3V and a specified LED current, the RSET value can be evaluated using the diagram shown in the Typi- cal Performance Characteristics section. For any other option, ISET value can be determined using the graph ISET vs. VCTRL. The LED’s brightness can also be adjusted by driving ENABLE or the CTRL pin with a PWM signal. The driving signal frequency should be greater than 100Hz to avoid flickering, increasing to more than 1MHz, if necessary. LEDs are very sensitive to temperature. In most cases the maximum allowed junction temperature is 100°C. This tem- perature is the result of adding to the ambient temperature the over temperature due to power dissipation, is described by as follows: where Tj is the LED junction temperature, TA is the ambient temperature, ΘjA is the junction to ambient thermal resis- tance, I is the LED current and VF is the LED forward voltage. The FAN5611 to FAN5614 are designed to reduce the current through LEDs, when the temperature rises and the cathode voltage increases, as can be seen from typical the "LED Current vs. LED Cathode Voltage" graph under the Typical Performance Characteristics section. Efficiency Considerations The FAN561X driver’s low dropout architecture can signifi- cantly improve the efficiency compared to using simple ballast resistors. The system efficiency, defined as the ratio between the LEDs power and the input supplied power can be calculated as follows: The lower the VCATHODE, the higher the system efficiency. Efficiency can be further improved 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, allow 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 of LED drivers can be used in the applications presented in this document, 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 still 5mA typical current available for the LEDs. The single cell Li-ion is utilized in most applications like cell phones or digital still cameras. In most cases, the Li-ion battery voltage level only goes down to 3.0V voltage level, and not down to the full discharge 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 needed for the LCD or keyboard backlight • Drivers 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. To drive these LEDs with the maximum current of 20mA for maximum brightness, usually requires a boost circuit for a single cell Li-ion voltage range. I LED 200 V CONTROL V CTRL – () × R SET ⁄ = T j T A Θ jA IV F × × + = Efficiency V IN V CATHODE – () V IN ⁄ = ISET RSET I1 I2 I3 I4 VIN GND FAN5613 CTRL V CONTROL ENABLE ON/OFF |
Аналогичный номер детали - FAN5612 |
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Аналогичное описание - FAN5612 |
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