LT3595
OPERATION
Discontinuous Current Mode
The LT3595 can drive a 10-LED string at 15mA LED cur-
rent operating in continuous conduction mode using the
recommended external components shown in the ap-
plication circuit on page 1 of this data sheet. As current
is further reduced, the regulator enters discontinuous
conduction mode. The photo in Figure 1 details circuit
operation driving ten LEDs at 10mA load. The inductor
current reaches zero during the discharge phase and the
SW pin exhibits ringing. The ringing is due to the LC tank
circuit formed by the inductor in combination with the
switch and diode capacitance. This ringing is not harmful;
far less spectral energy is contained in the ringing than in
the switch transitions.
V
SW
20V/DIV
I
L
20mA/DIV
400ns/DIV
V
IN
= 45V
10 LEDS AT 10mA
3595 F01
Figure 1. Switching Waveforms
TYPICAL APPLICATIONS
Inductor Selection
A 100μH inductor is recommended for most LT3595
applications. Although small size and high efficiency are
major concerns, the inductor should have low core losses
at 2MHz and low DCR (copper wire resistance). Some
inductors that meet these criteria are listed in Table 1. An
efficiency comparison of different inductors is shown in
Figure 2.
100
COILCRAFT 1812FS-104KLB
COILCRAFT LPS4012-104MLB
TOKO A915AY-101M
TDK VLCF4020T-101MR26
COILTRONICS SD3812-101
COILTRONICS SD52-101
Table 1. Inductor Manufacturers
INDUCTANCE
RANGE (μH)
RELEVANT TO
LT3595
100 to 680
100 to 680
100 to 680
100 to 390
100 to 680
100 to 680
100 to 330
VENDOR
Coilcraft
www.coilcraft.com
PART
SERIES
DO1605
LPS4012
1812FS
MSS5131
CDC4D20
D53LC
VLCF4020T
DIMENSIONS
(mm)
5.4
×
4.2
×
1.8
4
×
4
×
1.2
5.8
×
4.9
×
3.8
5.1
×
5.1
×
3.1
4.8
×
4.8
×
2
5.2
×
5.4
×
3
4
×
4
×
2
Sumida
www.sumida.com
Toko
www.tokoam.com
TDK
www.component.
tdk.com
Coiltronics
www.cooperet.com
EFFICIENCY (%)
95
SD3812
SD52
LQH32M
LQH43M
100 to 330
100 to 330
100 to 560
100 to 680
4
×
4
×
1.2
5.6
×
5.2
×
2
3.2
×
2.5
×
2
4.5
×
3.2
×
2
90
Murata
www.murata.com
Capacitor Selection
85
0.01
V
IN
= 45V
10 LEDS
0.1
1
I
LED
(mA)
10
100
3595 F02
L = 100μH
Figure 2. Efficiency Comparison of Different Inductors
The small size of ceramic capacitors make them ideal for
LT3595 applications. Only X5R and X7R types should be
used because they retain their capacitance over wider
voltage and temperature ranges than other types such
as Y5V or Z5U. Typically, 10μF capacitors on V
IN
and
V
CC
are sufficient. The output capacitor used across the
3595f
9
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