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LT3591EDDB#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT3591
7
3591f
INDUCTOR SELECTION
A 22µH inductor is recommended for most LT3591 ap-
plications. Although small size and high effi ciency are
major concerns, the inductor should have low core losses
at 1MHz and low DCR (copper wire resistance). Some
small inductors in this category are listed in Table 1. The
effi ciency comparison of different inductors is shown in
Figure 3.
Table 1. Recommended Inductors
PART
L
(µH)
CURRENT
RATING
(mA)
MAX
DIMENSION
L × W × H
(mm) VENDOR
VLF4012AT-
220MR51
22 510
4
× 3.8 × 1.2
TDK
www.tdk.com
VLCF4018T-
220MR49-2
22 490
4.1
× 4.1 × 1.8
VLCF4020T-
220MR56
22 560
4.1
× 4.1 × 2
LQH43CN220K03 22 420
4.8
× 3.4 × 2.8
Murata
www.murata.com
NR4018T220M 22 590
4.2
× 4.2 × 1.8
Taiyo Yuden
www.t-yuden.com
NR4012T220M 22 510
4.2
× 4.2 × 1.2
CDRH3D18-
220NC
22 600
4 × 4
× 2
Sumida
www.sumida.com
B82470-A1223-M 22 480
4.8
× 4.8 × 1.2
Epcos
www.epcos.com
APPLICATIONS INFORMATION
CAPACITOR SELECTION
The small size of ceramic capacitors make them ideal for
LT3591 applications. Use only X5R and X7R types because
they retain their capacitance over wider temperature
ranges than other types such as Y5V or Z5U. A 1µF input
capacitor and a 50V, 2.2µF output capacitor are suffi cient
for most applications.
A limited number of manufacturers produce small 50V
capacitors. Table 2 shows a list of several recommended
50V capacitors. Consult the manufacturer for detailed
information on their entire selection of ceramic parts.
Table 2. Recommended Output Capacitors
PART
C
(µF)
VOLTAGE CASE SIZE
VENDORTEMP.
HEIGHT
(mm)
GRM21BR71H105KA12L 1 50V 0805 Murata
www.murata.com
X7R 1.25 ± 0.15
GRM31MR71H105KA88 1 50V 1206
X7R 1.15 ± 0.1
GRM31CR71H225KA88 2.2 50V 1206
X7R 1.6 ± 0.2
GRM31CR71H475KA12L 4.7 50V 1206
X7R 1.6 ± 0.2
UMK316BJ475KL-T 4.7 50V 1206 Taiyo Yuden
www.t-yuden.com
X7R 1.6 ± 0.2
Figure 3. Effi ciency Comparison of Different Inductors
LED CURRENT (mA)
0
50
EFFICIENCY (%)
55
85
80
65
5
10
3591 F03
70
75
60
15
20
TAIYO YUDEN NR4018T220M
TDK VLCF4018T-220MR49-2
TAIYO YUDEN NR4012T220M
TDKVLCF4012AT-220MR51
MURATA LQH43CN220K03
TDK VLCF4020T-220MR56
SUMIDA CDRH3D18-220NC
EPCOS B82470-A1223-M
V
IN
= 3.6V
10 LEDs
LT3591
8
3591f
APPLICATIONS INFORMATION
SCHOTTKY DIODE
The LT3591 has a built-in Schottky diode. The internal
schottky saves board space in space constrained appli-
cations. In less space sensitive applications, an external
schottky diode connected between the SW node and the
CAP node increases effi ciency one to two percent. It is
important to use a properly rated schottky diode that can
handle the peak switch current of the LT3591. In addition,
the schottky diode must have a breakdown voltage of at least
40V along with a low forward voltage in order to achieve
higher effi ciency. One recommended external schottky
diode for the LT3591 is the Phillips PMEG4005AEA.
OVERVOLTAGE PROTECTION
The LT3591 has an internal open-circuit protection circuit.
In the cases of output open circuit, when the LEDs are
disconnected from the circuit or the LEDs fail open circuit,
V
CAP
is clamped at 42V (typ). The LT3591 will then switch
at a very low frequency to minimize input current. The V
CAP
and input current during output open circuit are shown in
the Typical Performance Characteristics. Figure 4 shows
the transient response when the LEDs are disconnected.
For low DCR inductors, which is usually the case for this
application, the peak inrush current can be simplifi ed as
follows:
I
V
L
r
L
LC
PK
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
=
=
–.
•
• exp – •
•
•
–
06
2
2
1
ω
α
ω
π
α
ω
rr
L
2
2
4•
where L is the inductance, r is the DCR of the inductor
and C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Currents
V
IN
(V) r (Ω)L (µH) C
OUT
(µF) I
P
(A)
4.2 0.3 22 2.2 1.06
4.2 0.71 22 2.2 0.96
4.2 0.58 15 1 0.83
4.2 1.6 15 1 0.68
PROGRAMMING LED CURRENT
The feedback resistor (R
SENSE
) and the sense voltage
(V
CAP
– V
LED
) control the LED current.
The CTRL pin controls the sense reference voltage as
shown in the Typical Performance Characteristics. For
CTRL higher than 1.5V, the sense reference is 200mV,
which results in full LED current. In order to have accurate
LED current, precision resistors are preferred (1% is rec-
ommended). The formula and table for R
SENSE
selection
are shown below.
R
mV
I
SENSE
LED
=
200
V
CAP
20V/DIV
I
L
500mA/DIV
V
IN
= 3.6V
CIRCUIT OF
FRONT PAGE
APPLICATION
LEDs DISCONNECTED
AT THIS INSTANT
500µs/DIV
3591 F04
Figure 4. Output Open-Circuit Waveform
INRUSH CURRENT
The LT3591 has a built-in Schottky diode. When supply
voltage is applied to the V
IN
pin, an inrush current fl ows
through the inductor and the Schottky diode and charges
up the CAP voltage. The Schottky diode inside the LT3591
can sustain a maximum current of 1A.
LT3591
9
3591f
APPLICATIONS INFORMATION
Table 4. R
SENSE
Value Selection for 200mV Sense
I
LED
(mA) R
SENSE
(Ω)
540
10 20
15 13.3
20 10
DIMMING CONTROL
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL pin
with a DC voltage, a fi ltered PWM signal or directly with
a PWM signal.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL pin voltage can be modulated to set the dim-
ming of the LED string. As the voltage on the CTRL pin
increases from 0V to 1.5V, the LED current increases from
0 to I
LED
. As the CTRL pin voltage increases beyond 1.5V,
it has no effect on the LED current.
The LED current can be set by:
I
mV
R
when V V
I
V
LED
SENSE
CTRL
LED
CTRL
≈>
≈
200
15
6
,.
.
225
125
•
,.
R
when V V
SENSE
CTRL
<
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
Using a Filtered PWM Signal
A fi ltered PWM signal can be used to control the
brightness of the LED string. The PWM signal is fi ltered
(Figure 5) by a RC network and fed to the CTRL pin.
The corner frequency of R1, C1 should be much lower
than the frequency of the PWM signal. R1 needs to be
much smaller than the internal impedance of the CTRL
pin which is 10MΩ (typ).
Direct PWM Dimming
Changing the forward current fl owing in the LEDs not only
changes the intensity of the LEDs, it also changes the color.
The chromaticity of the LEDs changes with the change in
forward current. Many applications cannot tolerate any
shift in the color of the LEDs. Controlling the intensity of
the LEDs with a direct PWM signal allows dimming of the
LEDs without changing the color. In addition, direct PWM
dimming offers a wider dimming range to the user.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second.
By increasing the PWM frequency to ~80Hz or higher,
the eye will interpret that the pulsed light source is con-
tinuously on. Additionally, by modulating the duty cycle
(amount of “on-time”), the intensity of the LEDs can be
controlled. The color of the LEDs remains unchanged in
this scheme since the LED current value is either zero or
a constant value.
Figure 6 shows a Li-Ion powered driver for ten white LEDs.
Direct PWM dimming method requires an external NMOS
tied between the cathode of the lowest LED in the string
LT3591
CTRL
C1
0.1µF
PWM
10kHz TYP
3591 F05
R1
100k
Figure 5. Dimming Control Using a Filtered PWM Signal
CTRL
PWM
FREQ
V
IN
L1
22µH
V
IN
3V TO
5V
R
SENSE
10Ω
3591 F06
LT3591
SW
CAP
LED
100k
GND
0V
5V
C2
2.2µF
Q1
Si2308
C1
1µF
Figure 6. Li-Ion to Ten White LEDs with Direct PWM Dimming
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT3591EDDB#TRMPBF

Mfr. #:
Buy LT3591EDDB#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers White LED Driver w/ Integrated Schottky in DFN (3x2)
Lifecycle:
New from this manufacturer.
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