Datasheet

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.