FAN5608_06 Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – Serial / Parallel LED Driver with Current-Regulated, Step-Up DC/DC Converter

decreases due to the resistive loss in the switching
power FET. Using L = 3.3μΗ and increasing the load to
12 LED x 20mA (P_OUT = 800mW) requires V_IN > 3.5V to
maintain a constant 20mA current through LEDs. An
inductance L = 4.7μH ensures proper operation for 2 x 4
white LEDs with regulated 20mA current if V_IN > 2.7V.
For any lighter load or higher V_IN, the inductance may be
increased to improve the system efficiency. Application
examples are given in Figures 16 - 25.
The peak current in the inductor depends on the
maximum battery voltage and the inductance, according
to the equation:
I _pk
=
1.4μS ×V_INmax
L
EQ 7
which gives the maximum rated current for the inductor.
For L =4.7μH and V_IN = 4.2V, the inductor saturation
current should be at least 1A.
Capacitors Selection
Low ESR capacitors should be used to minimize the
input and output ripple voltage. Use of a C_IN >
4.7μF/6.3V and C_OUT = 4.7μF/25V type X5R/X7R multi-
layer ceramic capacitor is recommended.
A larger value input capacitor placed as close as
possible to FAN5608 may be needed to reduce the input
voltage ripple in noise-sensitive applications. An
additional LC filter between the battery and the
FAN5608 input can help reduce the battery ripple to the
level required by a particular application.
Driving Higher Current LEDs
To increase the LED current range to 50mA, the CH1
and CH2 outputs may be connected, as shown below:
CIN
2.7V to 5.5V
L = 4.7µH
INDB ^INDA
GND
DAC Inputs
A₁
B₁
CH1
A₂
CH2
B₂
4.7µF
Figure 14. Digital Brightness Control
C_IN
2.7V to 5.5V
L = 4.7µH
VEXT
INDB ^INDA
GND
VIN
A₁
CH1
A2
CH2
4.7µF
The current feeding the string of LEDs is the sum of the
currents programmed for each channel in digital or
analog mode. Using all four inputs in digital mode, the
LED current can be programmed within the 0 to 40mA
range, according to the following table:
Driving Two Independent Strings of LEDs
For best efficiency, the two strings of LEDs driven by the
FAN5608 should have close forward voltages. This
recommendation is met when the same number and
type of LEDs are serially connected in each channel. If
the application requires the two channels be significantly
unbalanced, an external resistor may be added in series
with the shorter string of LEDs to reduce the voltage
difference between the two channels and enhance
performance. The maximum differential LEDs connected
in series to CH1 and CH2 is not recommended to
exceed three LEDs.
Specific Layout Recommendations
The FAN5608 is available in a dual Die Attach Pad
(DAP) package. One DAP is connected to GND and
another to V_OUT, making it unnecessary to provide any
external connection to the DAPs. Since the internal
power dissipation is low, the 4x4mm MLP package is
capable of dissipating maximum power without requiring
an extra copper field on the printed circuit board. When
viewing the bottom of the dual-DAP device package, two
electrically isolated exposed metal islands are visible.
The two IND pins should be connected externally.
Depending on the application noise sensitivity, an
external filter may be required between the battery and
the IC input.
Figure 15. Analog Brightness Control
© 2006 Fairchild Semiconductor Corporation
FAN5608 Rev. 1.0.5 • 8/8/06
9
www.fairchildsemi.com