Datasheet

LT3599
11
3599fe
The companies below offer Schottky diodes with high
voltage and current ratings. Standard silicon diodes (PN
junction diodes) should not be used.
Table 3. Suggested Diodes
PART
MAX
CURRENT
(A)
MAX REVERSE
VOLTAGE
(V) MANUFACTURER
B250A
DFLS240
B240A
B350A
B340A
2
2
2
3
3
50
40
40
50
40
Diodes, Inc.
www.diodes.com
HSM150G
HSM150J
HSM350G
1
1
3
50
50
50
Microsemi
www.microsemi.com
Overvoltage Protection
The LT3599 uses the FB pin to provide overvoltage protec-
tion. A resistor divider is connected between the V
O_SW
pin and ground (Figure 2). There is an internal PMOS
switch between V
OUT
and V
O_SW
, which is controlled by
the PWM signal. The PMOS switch addition prevents the
feedback resistor divider from draining the output capaci-
tor during PWM off-period, allowing for a higher dimming
range without falsely tripping the OPENLED fl ag. It also
reduces the system current in shutdown. This PMOS has
about 1k resistance, so select FB resistor values taking
this resistance into account.
To set the maximum output voltage, select the values
of R1 and R2 (see Figure 2) according to the following
equation:
V
OUT(MAX)
=1.223V 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
The output voltage should be set 10% higher than the
normal LED string operating voltage. Under normal
operation, LED1 to LED4 pin voltages are monitored and
provide feedback information to the converter for output
voltage regulation given the programmed LED current.
The output voltage regulation loop is activated only when
all LEDs are open.
Programming Maximum LED Current
Maximum LED current can be programmed by placing a
resistor between the I
SET
pin and ground (R
ISET
). The I
SET
pin resistor can be selected from 11k to 44.2k.
The LED current can be programmed according to the
following equation:
I
LED
≈
1330
R
ISET
Amps
(
)
(CTRL >1V)
See Table 4 and Figure 3 for resistor values and corre-
sponding programmed LED current.
LED current can also be adjusted by programming the
CTRL pin voltage.
Table 4. R
ISET
Value Selection for LED Current
LED CURRENT (mA) RESISTOR ON I
SET
PIN (k)
30mA 44.2
50mA 26.7
99mA 13.3
120mA 11
APPLICATIONS INFORMATION
Figure 2. Overvoltage Protection Voltage Feedback Connections
Figure 3. R
ISET
Value Selection for LED Current
020
40
60 80 120100 140 160
R
ISET
(k)
I
LED
(mA)
80
60
40
20
0
120
100
3599 F03
3599 F02
LT3599
V
O_SW
FB
V
OUT
R2
R1