FAN5611 Datasheet, PDF (7/12 Pages) Fairchild Semiconductor – Low-Dropout LED Drivers for White, Blue or any Color LED

PRODUCT SPECIFICATION
FAN5611/FAN5612/FAN5613/FAN5614
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:
I_LED = 200 × (V_CONTROL – V_CTRL) ⁄ R_SET
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:
T_j = T_A + Θ_jA × I × V_F
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:
Efficiency = (V_IN – V_CATHODE) ⁄ V_IN
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 V_F 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.
V_IN
V_CONTROL
I_SET
I4
I3
I2
I1
CTRL
R_SET
FAN5613
ENABLE ^ON/OFF
GND
– 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 V_F 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.
REV. 1.0.4 6/4/04
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