Datasheet

LT3596
12
3596fa
applicaTions inForMaTion
In UVLO an internal 5.1µA (typical) pull-down current
source is connected to the pin for programmable UVLO
hysteresis. The hysteresis is set according to the follow-
ing equation:
V
UVLO(HYST)
= 5.1µA • R2
Care must be taken if too much hysteresis is programmed,
the pin voltage might drop too far and cause the current
source to saturate.
Once the EN/UVLO pin goes below 0.4V, the part enters
shutdown.
Programming Maximum LED Current
Maximum LED current is programmed by placing a resistor
(R
ISETn
) between the I
SETn
pin and ground. R
ISETn
values
between 20k and 100k can be chosen to set the maximum
LED current between 100mA and 20mA respectively.
The LED current is programmed according to the follow-
ing equation:
I V
R
LED
ISET
1-3
=1
2000
•
n
where R
ISETn
is in kΩ and I
LEDn
is in mA. See Table 5 and
Figure 3 for resistor values and corresponding programmed
LED current.
Table 5. R
ISETn
Value for LED Current
R
ISETn
VALUE (kΩ) LED CURRENT (mA)
20 100
24.9 80
33.2 60
49.9 40
100 20
R
ISETn
(kΩ)
0
LED CURRENT (mA)
40
60
100
3596 F03
20
0
25
50
75
100
80
Figure 3. R
ISETn
Value for LED Current
LED Current Dimming
The LT3596 has two different types of dimming control.
The LED current is dimmed using the CTRLn pin or the
PWMn pin.
For some applications, a variable DC voltage that adjusts
the LED current is the preferred method for brightness
control. In this case, the CTRLn pin is modulated to set
the LED dimming. As the CTRLn pin voltage rises from
0V to 1V, the LED current increases from 0mA to the
maximum programmed LED current in a linear fashion
(see Figure 4). As the CTRLn pin increases beyond 1V,
the maximum programmed LED current is maintained.
If this type of dimming control is not desired, the CTRLn
pin can be connected to V
REF
.
CTRLn VOLTAGE (V)
0
0
LED CURRENT (mA)
25
50
75
100
0.25 0.5 0.75 1
3596 F04
1.25 1.5
R
ISETn
= 20k
Figure 4. LED Current vs CTRLn Voltage
+
–
1.51V
3596 F02
EN/UVLO
LT3596
R2
V
IN
R1
Figure 2. EN/UVLO Control