lt3591 Linear Technology Corporation, lt3591 Datasheet - Page 8

• Manufacturer Part Number: lt3591
• Description: White Led Driver With Integrated Schottky In 3mm X 2mm Dfn
• Manufacturer: Linear Technology Corporation

LT3591
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
at a very low frequency to minimize input current. The V
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.
INRUSH CURRENT
The LT3591 has a built-in Schottky diode. When supply
voltage is applied to the V
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.
8
CAP
500mA/DIV
is clamped at 42V (typ). The LT3591 will then switch
20V/DIV
V
CAP
I
Figure 4. Output Open-Circuit Waveform
L
V
CIRCUIT OF
FRONT PAGE
APPLICATION
IN
= 3.6V
LEDs DISCONNECTED
AT THIS INSTANT
IN
500 s/DIV
pin, an inrush current fl ows
3591 F04
CAP
For low DCR inductors, which is usually the case for this
application, the peak inrush current can be simplifi ed as
follows:
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
PROGRAMMING LED CURRENT
The feedback resistor (R
(V
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
are shown below.
I
PK
CAP
V
R
IN
4.2
4.2
4.2
4.2
SENSE
2
(V)
– V
V
r
•
L C
IN
L
L
1
•
LED
– .
•
0 6
–
) control the LED current.
200
4 •
r (Ω)
I
0.71
0.58
0.3
1.6
LED
r r
• exp –
2
L
mV
2
L (µH)
SENSE
22
22
15
15
•
2
) and the sense voltage
C
OUT
2.2
2.2
1
1
(µF)
SENSE
selection
I
1.06
0.96
0.83
0.68
P
(A)
3591f