Datasheet FAN5613 (Fairchild) - 8

F AN561 Application Information Operational Descripton 1/F
The regulated current through each LED is a multiplica- VIN
AN5612/F
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 ISET V I4 I3 I2 I1 Reference Figure 4, the I CONTROL SET vs. VCTRL graph, to esti- CTRL
A
mate V
N5613/F
CTRL. The value of RSET is calculated according RSET FAN5613 to the formula: ON/OFF R ENABLE SET = (VCONTROL - VCTRL) / ISET GND For example, with VCONTROL = 3V, a 10mA current limit
A
through the LED results in I
N5614
SET = 50µA. That translates to an approximate value of 1.2V for VCTRL, shown in Fig- – VDROP < 0.3V ure 4. The resulting RSET value that maintains 10mA reg- – VF (at 20mA) < 3.1V (Low VF) ulation is 36kΩ. – VIN (at 20mA) =VDROP + VF = 3.4V The LED intensity can be adjusted by varying the duty – VIN (at 5mA Typical) ~ 3.1V cycle of a square wave applied to the enable pin. Fre- where VIN = Single cell Li-ion voltage. quency greater than 100Hz is best to avoid a "flickering" effect. The maximum operation frequency is 10MHz.
Key advantages:
• No boost circuit is needed for the LCD or keyboard
Efficiency Considerations
backlight. The FAN561X driver’s low-dropout architecture can sig- nificantly improve the efficiency compared to using sim- • Drivers are directly connected to a Li-ion battery. ple ballast resistors. The system efficiency, defined as • No EMI, no switching noise, no boost efficiency lost, the ratio between the LEDs’ power and the input sup- no capacitor, and no inductor. plied power, can be calculated as:
Example 2: Drive high VF white or blue LEDs from
Efficiency = (V – V ) ⁄ V IN CATHODE IN
existing bus from 4.0V to 5.5V
The lower the V High V CATHODE, the higher the system effi- F white or blue LEDs have forward voltage drop in ciency. Efficiency can be further improved by using a the range of 3.2V to 4.0V. Driving these LEDs with the higher V maximum current of 20mA for maximum brightness, IN with more LEDs, as shown in Example 3. usually requires a boost circuit for a single cell Li-ion
Application Notes
voltage range. In some cases, there is already a voltage The ultra-low voltage drop across the FAN561X series of bus in the system, which can be utilized. Due to the ultra- LED drivers allows the devices to drive white, blue, and low voltage drop of the FAN561X series of LED drivers to other color LEDs in a wide range of input voltages. The drive high-VF white or blue LEDs, the VIN needs to be driver can be used in many applications. Although only only 300mV higher than the highest VF in the circuit. the FAN5613 is shown in all three examples, any of the – VDROP < 0.3V FAN561X-series LED drivers can be used in the applica- – VF (at 20mA) < 3.3V to 4.0V (High VF) tions presented, due to their similar operation. – VIN (at 20mA) =VDROP + VF = 3.6V to 4.3V – V
Example 1: Drive low V
IN (at 5mA Typical) ~ 3.3V
F white or blue LEDs directly from single cell Li-ion
where VIN = existing bus = 3.3V to 4.3V. When using white or blue low VF LEDs, and utilizing the
Key advantages:
drivers low voltage drop, only 3.4V in VIN is needed for • No boost circuit is needed for LCD or keyboard the full 20mA LED current. At 3.1V, there is 5mA typical backlight. current available for the LEDs. The single cell Li-ion is utilized in applications like cell phones or digital still cam- • Driver utilizes the existing bus. eras. In most cases, the Li-ion battery voltage level only • Ultra-low voltage drop provides the full 20mA LED goes down to 3.0V voltage level, not down to the full dis- current at the lowest possible voltage level. charge level (2.7V) before requesting the charger. © 2003 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5611/FAN5612/FAN5613/FAN5614 Rev. 1.0.9 8