FAN5331_05 Datasheet, PDF (7/10 Pages) Fairchild Semiconductor – High Efficiency Serial LED Driver and OLED Supply with 20V Integrated Switch

The inductor saturation current should be rated around 1A,
which is the threshold of the internal current limit circuit. This
limit is reached only during the start-up and with heavy load
condition; when this event occurs the converter can shift over in
discontinuous conduction mode due to the automatic turn-off of
the switching transistor, resulting in higher ripple and reduced
efficiency.
Some recommended inductors are suggested in the table
below:
Inductor
Value Vendor Part Number Comment
10µH
Panasonic ELL6GM100M
Lower Profile
(1.6mm)
10µH
Murata LQS66SN100M03L Highest
Efficiency
10µH
Coilcraft DO1605T-103Mx Small Size
PCB Layout Recommendations
The inherently high peak currents and switching frequency of
power supplies require careful PCB layout design. Therefore,
use wide traces for high current paths and place the input
capacitor, the inductor, and the output capacitor as close as
possible to the integrated circuit terminals. The resistor divider
that sets the output voltage should be routed away from the
inductor to avoid RF coupling. A four layer PCB with at least one
ground plane connected to the pin 2 of the IC is recommended.
This ground plane acts as an electromagnetic shield to reduce
EMI and parasitic coupling between components.
Table 1: Recommended Inductors
Capacitors Selection
For best performance, low ESR input and output capacitors are
required. Ceramic capacitors in the range 4.7µF to 10µF, placed
as close to the IC pins, are recommended for the lower input
and output ripple. The output capacitor voltage rating should be
according to the V_OUT setting.
A feed forward capacitor C_F, is required for stability. The recom-
mended value (R₁ x C_F) is around 18µS. Some capacitors are
suggested in the table below.
Capacitor
Value
4.7µF
4.7µF
Vendor
Panasonic
Murata
Part Number
ECJ3YB1C475K
GRM31CR61C475
Table 2: Recommended Capacitors
Diode Selection
The external diode used for rectification is usually a Schottky
diode. Its average forward current and reverse voltage maxi-
mum ratings should exceed the load current and the voltage at
the output of the converter respectively. A barrier Schottky diode
such as BAT54 is preferred, due to its lower reverse current over
the temperature range.
Care should be taken to avoid any short circuit of V_OUT to GND,
even with the IC disabled, since the diode can be instantly dam-
aged by the excessive current.
Thermal Shutdown
When the die temperature exceeds 150°C, a reset occurs and
will remain in effect until the die cools to 130°C, at that time the
circuit will be allowed to restart.
Figure 5. Recommended Layout
Application Examples
1. LED Driver
One or more serial LED strings can be driven with a constant
current, set by the series resistor, given by
I_LED = ¹----^.-_R-²---³¹----^V--
V_IN
2.7V to 5.5V
C_IN
4.7µF
5
V
IN
L
10µH
SW ¹
BAT54
4.7µF
C_OUT
3
FB
V_OUT
ON
OFF
4
SHDN
2
GND
R1
R2
Figure 6. Low Noise Boost LED Driver
7
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FAN5331 Rev. 1.0.1