FAN5333A Datasheet, PDF (8/11 Pages) Fairchild Semiconductor – High Efficiency, High Current Serial LED Driver with 30V Integrated Switch

Inductor Selection
The inductor parameters directly related to device performances
are saturation current and dc resistance. The FAN5333A/
FAN5333B operates with a typical inductor value of 10µH. The
lower the dc resistance, the higher the efficiency. Usually a
trade-off between inductor size, cost and overall efficiency is
needed to make the optimum choice.
The inductor saturation current should be rated around 1A, in
an application having the LED current near the maximum cur-
rent as indicated in “Typical Performance Characteristics”. The
peak inductor current is limited to 1.5A by the current sense
loop. 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 auto-
matic turn-off of the switching transistor, resulting in higher rip-
ple and reduced efficiency.
Some recommended inductors are suggested in the table
below:
Inductor
Value
10µH
10µH
10µH
Vendor
TDK
MURATA
COOPER
Com-
Part Number ment
SLF6025&-100M1R0
LQH66SN100M01C Highest
Efficiency
SD414-100
Small
Size
Table 1: Recommended Inductors
Capacitors Selection
For best performance, low ESR input and output capacitors are
required. Ceramic capacitors of C_IN = 10µF and C_OUT = 1µF
placed as close to the IC pins, are required for the maximum
load(65mA). For the lighter load (≤ 20mA) the capacitances may
be reduced to C_IN = 4.7µF and C_OUT = 0.47µF or even to 0.1µF,
if higher ripple is acceptable. The output capacitor voltage rating
should be according to the V_OUT setting. Some capacitors are
suggested in the table below.
Capacitor
Value
0.47µF
1µF
10µF
Vendor
Panasonic
Murata
Murata
Part Number
ECJ-3YB1E474K
GRM21BR61E105K
GRM21BR61A106K
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.
Brightness Control
1. Dimming Using PWM Logic Signal
A PWM signal applied to SHDN Table 5 on page 8 can control
the LED’s brightness in direct dependence with the duty cycle.
The maximum frequency should not exceed 1kHz to ensure a
linear dependence of the LED’s average current. The amplitude
of the PWM signal should be suitable to turn the FAN5333 ON
and OFF.
Alternatively, a PWM logic signal can be used to switch a FET
ON/OFF to change the resistance that sets the LED’s current
Table 6 on page 8. Adjusting the duty cycle from 0% to 100%
results in varying the LED’s current between I_MIN and I_MAX.
Where
I_MIN = _R--V---M-^F---_I-^B-_N-- and I_MAX = _R-----M----_I--_N-V----^F---R^B----_S----_E---_T--
FAN5333
SHDN
Figure 5. Dimming Using a PWM Signal
FAN5333
FB
R_MIN
R_SET
Figure 6. Dimming Using a PWM Logic Signal
2. Dimming Using DC Voltage
An external adjustable DC voltage Table 7 on page 8 between
0V to 2V can control the LED’s current from 15mA to 0mA,
respectively.
FAN5333A
FB
V_DC
1.6KΩ
90KΩ
5Ω
15Ω
FAN5333B
FB
4.7KΩ
V_DC
90KΩ
Figure 7. Dimming Using DC Voltage
8
FAN5333A/FAN5333B Rev. 1.0.1
www.fairchildsemi.com