FAN5611 Fairchild Semiconductor, FAN5611 Datasheet - Page 7

• Manufacturer Part Number: FAN5611
• Description: Low-Dropout LED Drivers for White/ Blue or any Color LED
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
Application Information
Setting the LED Current
The current flowing into LEDs is approximately 200 times
greater then the current I
by V
For V
value can be evaluated using the diagram shown in the Typi-
cal Performance Characteristics section. For any other
option, I
vs. V
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 T
temperature, Θ
tance, I is the LED current and V
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 V
Efficiency can be further improved using a higher V
more LEDs as shown in example 3.
REV. 1.0.4 6/4/04
Efficiency
T
I
j
LED
=
CONTROL
CTRL
CONTROL
T
=
A
j
SET
is the LED junction temperature, T
+
200
. The LED’s brightness can also be adjusted by
Θ
=
value can be determined using the graph I
jA
×
(
and R
jA
×
V
= 3V and a specified LED current, the R
CATHODE
(
IN
V
I V
is the junction to ambient thermal resis-
×
CONTROL
–
SET
V
F
CATHODE
SET
according to the formula:
, the higher the system efficiency.
. The LED current is controlled
–
V
CTRL
) V
F
⁄
is the LED forward
IN
)
⁄
R
SET
A
is the ambient
IN
SET
with
SET
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
directly from single cell Li-ion
When using white or blue low V
drivers low voltage drop, only 3.4V in V
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.
Where V
Key advantages:
• No boost circuit needed for the LCD or keyboard
• Drivers directly connected to a Li-ion battery
• No EMI, no switching noise, no boost efficiency lost, no
Example 2: Drive high V
from existing bus from 4.0V to 5.5V
High V
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.
– V
– V
– V
– V
backlight
capacitor, and no inductor.
V
CONTROL
DROP
F (at 20mA)
IN (at 20mA)
IN (at 5mA Typical)
F
white or blue LEDs have forward voltage drop in
IN
= Single cell Li-ion Voltage
< 0.3V
ENABLE
V
IN
R
I
SET
< 3.1V (Low V
SET
=V
DROP
~ 3.1V
CTRL
ON/OFF
FAN5611/FAN5612/FAN5613/FAN5614
I4
+ V
F
F
F
white or blue LEDs
F
F
= 3.4V
white or blue LEDs
LEDs, and utilizing the
)
I3
FAN5613
GND
IN
I2
is needed for the
I1
7