FAN5608 Fairchild Semiconductor, FAN5608 Datasheet - Page 8

• Manufacturer Part Number: FAN5608
• Description: Serial/Parallel LED Driver with Current-Regulated/ Step-Up DC/DC Converter
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
Application Information
Inductor Selection
The inductor is one of the main components required by the
boost converter to store energy. The amount of energy stored
in the inductor and transferred to the load is controlled by the
regulator using PWM and pulse skipping techniques. In most
cases the FAN5608 operates the inductor in discontinuous
conduction mode.
To ensure proper operation of the current regulator over the
entire range of conditions, select the inductor based on the
maximum required power (P
voltage (V
where units of L, V
respectively and F = 0.4 is a factor depending upon the
FAN5608 architecture.
The above relation is applicable up to P
L = 4.3µΗ , or greater. At lower inductor values the effi-
ciency decreases due to the resistive loss in the switching
Power FET. Using L = 4.3µΗ and increasing the load to 12
LED x 20mA (P
maintain a constant 20mA current through LEDs. The induc-
tor L = 4.3µH ensures proper operation for 2 x 4 white LEDs
with 20mA at 3.5V for V
For any lighter load or higher V
increased to improve the system efficiency. Application
examples are given in Figure 1 through Figure 4.
The peak current in the inductor is:
which gives the maximum rated current for the inductor. For
L = 4.3µH, T
inductor saturation current should be at least 1A.
Capacitor Selection
Low ESR capacitors should be used to minimize the input
and output ripple voltage. Use of C
C
capacitor is recommended.
8
OUT
= 4.7µF/25V type X5R/X7R multi layer ceramic
IN
).
ON_Max
OUT
IN
I
=
= 800mW) requires V
, and P
= 1.25µS and V
T
------------------------------------------------ -
L
ON_Max
IN
<
(
--------------------------
> 2.8V.
V
OUT
OUT
P
IN
OUT
L
×
IN
)
2
) and the minimum input
are in µH, Volt, and Watt,
V
, the inductance can be
×
IN_Max
IN
F
IN_Max
= 4.7µF/6.3V and
OUT
IN
= 0.6W and
= 4.2V, the
> 3.5V to
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 to
further reduce the battery ripple to the level required by a
particular application.
Schottky Diode Selection
The FAN5608HMPX and FAN56508MPX require the use of
an external Schottky diode. This diode should be rated at
200mA to 500mA average rectified current and 20V
maximum repetitive reverse voltage.
The MBR0520L (Fairchild) Schottky diode is
recommended.
Driving Higher Current LEDs
To increase the LED current range to 50mA, the CH1 and
CH2 outputs may be connected, as shown below:
2.7V to 5.5V
2.7V to 5.5V
V
EXTERNAL
C
C
DAC Inputs
IN
IN
L = 4.7µH
L = 4.7µH
Analog Brightness Control
Digital Brightness Control
IND
V
A
A
IND
A
B
A
B
IN
1
2
1
1
2
2
GND
GND
CH1
CH2
CH1
CH2
IND
IND
REV. 1.0.3 6/28/04
4.7µF
4.7µF
FAN5608