LM3519 Datasheet, PDF (12/14 Pages) National Semiconductor (TI) – High Frequency Boost White LED Driver with High-Speed PWM Brightness Control

Application Information
CAPACITOR SELECTION
To minimize output and input voltage ripple, low equivalent
series resistance (ESR) ceramic capacitors are the best
choice to use for the input and output filters. For most display
applications, a 4.7µF capacitor is recommended for C_IN and
1µF for C_OUT .
Larger output capacitors can be used to reduce ripple volt-
age. To guarantee good performance, a minimum of 0.47µF
C_OUT is required to trade off for large ripple voltage. Care
must be taken to account for the true capacitance of a
multilayer ceramic capacitor. Smaller case size capacitors
typically have less capacitance for a given bias voltage as
compared to a larger case size capacitor with the same bias
voltage. Please confirm with capacitor manufacturer data
before selecting the capacitor.
Some recommended capacitor manufacturers include but
are not limited to:
Manufacturer
Description
Case Size
06033D105MAT-25V
0603
AVX
06036D475MAT-6.3V
0603
TDK
C2012X5R1A475M-10V 0805
Taiyo Yuden TMK212BJ105KG-J
0805
EM212BJ475MG-16V
0805
muRata
GRM40-034B105K25
0805
GRM39X5R475K6.3
0603
INDUCTOR SELECTION
In order to maintain sufficient inductance, the saturation
current rating of the inductor used with the LM3519 should
be higher than the peak inductor current in the target appli-
cation. Inductors with low DCR values have less power loss
and higher efficiency. Larger inductor values such as 2.2µH
and 3.3µH can be used to optimize efficiency, frequency and
peak current. If 1µH is used, the peak inductor current,
frequency will be higher and the efficiency will be lower. Note
that the switching frequency ranges will be higher at lower
inductance. Typical frequency range is between 4 to 8MHz
for 1µH, 2 to 5MHz for 2.2µH and 2 to 4MHz for 3.3µH over
the input range. Below is a sample list of low profile induc-
tors.
Some recommended inductor manufacturers include but
are not limited to:
Manufacturer
CoilCraft:
DO3314-102
DO3314-222
L
1µH
2.2µH
Case Size
3.3x3.3x1.4mm
I_SAT
2.1A
1.6A
DO3314-332 3.3µH
1.4A
Coilcraft:
LPO3310-102ML
LPO3310-222ML
1µH
2.2µH
3.3x3.3x1.0 mm
1.6A
1.1A
LPO3310-332ML 3.3µH
0.95A
Cooper:
SD31121R0
SD3114-2R2
1µH
2.2µH
3.1x3.1x1.4 mm
2.07A
1.48A
SD3114-3R3 3.3uH
1.15A
Taiyo Yuden:
NR3015T1R0N
NR3015T2R2M
NR3015T3R3M
1µH
2.2µH
3.3µH
2.1A
3.0x3.0x1.5 mm
1.48A
1.21A
DIODE SELECTION
Diodes with low forward voltage ratings (V_F) and low junction
capacitance magnitudes (C_J or C_T or C_D) are conducive to
high efficiency. The chosen diode must have a reverse
breakdown voltage rating (V_R and/or V_RRM) that is larger
than the output voltage. The following criteria should be
followed when choosing a diode:
1. V_R (Diode Blocking Voltage Range) and V_RRM (Diode
Peak Repetitive Reverse Voltage Rating) > V_OUT (Out-
put Voltage)
2. I_F or I_O (Diode Average Forward Current Rating) ≥ I_LOAD
(Load Current)
3. I_FRM (Diode Peak Repetitive Forward Current Rating) ≥
I_Lpeak (Peak Inductor Current)
Some recommended diode manufacturers include but are
not limited to:
Maufacturer
Description
Vishay
SS12(1A/20V)
SS14(1A/40V)
SS16(1A/60V)
Central Semiconductor CMSH1- 40M(1A/40V)
ONSemi
MBRS1540T3(1.5A/40V)
PWM DIMMING
The LED current is set internally by the LM3519 to 20mA
(typical); dimming control may be realized by applying a
pulse width modulated(PWM) signal to the En pin. For ex-
ample, a 50% duty cycle waveform will produce an average
current of 10mA. A control signal frequency between 17kHz
and 30kHz is suitable for dimming.
Although the LM3519 is capable of operation outside this
frequency range, it is not recommended to operate below
17kHz for the following reasons: 1) frequency below 100Hz
is likely to cause visible flicker in the light emitted by the LED
string. 2) frequency below 17kHz may induce audible noise
due to combinations of some capacitance/PCB. A PWM
frequency above 30kHz is possible but the current linearity
vs duty cycle will be affected.
If it is not possible to operate the dimming control above
17kHz, audible noise emission may be minimized by using
capacitors with low susceptibility to piezoelectric induced
stresses, such as poly film designs. Minimum audible noise
is most likely to occur when the PWM frequency is less than
2kHz. It is recommended that any application using a PWM
control signal below 17kHz be thoroughly evaluated for un-
desirable audible or visible noise.
DRIVING 2 LEDs
The LM3519 is optimized to drive up to 4LEDs. When driving
2LEDs, a minimum inductance of 2.2µH is required to main-
tain good loop regulation and current accuracy. If a smaller
inductor is used, the LED current will have more variation
with input voltage than a typical application. The following
curve illustrates the behavior.
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