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FAN5611 데이터시트(PDF) 8 Page - Fairchild Semiconductor

부품명 FAN5611
상세설명  Low-Dropout LED Drivers for White, Blue, or any Color LED
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제조업체  FAIRCHILD [Fairchild Semiconductor]
홈페이지  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

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© 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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