Datasheet

LT3598
11
3598fb
Overvoltage Protection
The LT3598 uses the FB pin to provide regulated overvoltage
protection when all LED strings are open. 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 capacitor 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 resistors 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:
VV
R
R
OUT MAX()
.=+
⎛
⎝
⎜
⎞
⎠
⎟
1 230 1
2
1
APPLICATIONS INFORMATION
Figure 2. Overvoltage Protection
Voltage Feedback Connections
The output voltage should be set 15% higher than the
normal LED string operating voltage. Under normal
operation, LED1 to LED6 pin voltages are monitored and
provide feedback information to the converter for output
voltage regulation given the programmed LED current.
The maximum output regulation loop is activated only
when all LEDs are open.
Programming Maximum LED Current
Maximum LED current is programmed by placing a resistor
between the I
SET
pin and ground (R
ISET
). The I
SET
pin
resistor can be selected from 10k to 100k.
The LED current is programmed according to the following
equation:
I
V
R
LED
ISE T
≈
294
See Table 4 and Figure 3 for resistor values and corr-
esponding 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)
3mA 97.6
10mA 29.4
20mA 14.7
30mA 9.76
LED Current Dimming
Two different types of dimming control can be used with
the LT3598. The LED current can be set by modulating
the CTRL pin or the PWM pin.
For some applications, a variable DC voltage that adjusts
the LED current is the preferred method of brightness
control. The CTRL pin voltage can be modulated to set
the dimming of the LED string (see Figures 4 and 5). As
the voltage on the CTRL pin increases from 0V to 1.0V,
the LED current increases from 0 to the programmed LED
current level. As the CTRL pin voltage increases beyond
1V, it has no effect on the LED current.
Figure 3. R
ISET
Value Selection for LED Current
3598 F02
LT3598
V
O_SW
FB
V
OUT
R2
R1
R
ISET
(k)
0
LED CURRENT (mA)
20
25
15
10
40 80
20
60 100
5
0
30
3598 F03