Datasheet

LT3474/LT3474-1
15
3474fd
APPLICATIONS INFORMATION
BOOST and BIAS Pin Considerations
The capacitor and internal diode tied to the BOOST pin
generate a voltage that is higher than the input voltage.
In most cases, a 0.22μF capacitor will work well. Figure 4
shows three ways to arrange the boost circuit. The BOOST
pin must be more than 2.5V above the SW pin for full ef-
fi ciency. For outputs of 2.8V or higher, the standard circuit
(Figure 4a) is best. For lower output voltages, the BIAS pin
can be tied to the input (Figure 4b). The circuit in Figure
4a is more effi cient because the BOOST pin current comes
from a lower voltage source. The BIAS pin can be tied to
another source that is at least 3V (Figure 4c). For example,
if a 3.3V source is on whenever the LED is on, the BIAS
pin can be connected to the 3.3V output. For LT3474-1
applications with higher output voltages, an additional
Zener diode may be necessary (Figure 4d) to maintain the
BOOST pin voltage below the absolute maximum. In any
case, be sure that the maximum voltage at the BOOST pin
is both less than 51V and the voltage difference between
the BOOST and SW pins is less than 25V.
Programming LED Current
The LED current can be set by adjusting the voltage on
the V
ADJ
pin. For a 1A LED current, either tie V
ADJ
to REF
or to a 1.25V source. For lower output currents, program
the V
ADJ
using the following formula:
I
LED
=
1A • V
ADJ
1.25V
Voltages less than 1.25V can be generated with a voltage
divider from the REF pin, as shown in Figure 5.
Figure 4. Generating the Boost Voltage
REF
V
ADJ
GND
LT3474
3474 F04
R1
R2
Figure 5. Setting V
ADJ
with a Resistor Divider
In order to have accurate LED current, precision resistors
are preferred (1% or better is recommended). Note that
the V
ADJ
pin sources a small amount of bias current, so
use the following formula to choose resistors:
R
V
VV
R
nA
ADJ
ADJ
2
125
1
50
=
+
.–
BIAS
V
IN
3474 F04c
BOOST
V
IN
V
IN2
> 3V
SW
GND
LT3474
(4c)
V
BOOST
– V
SW
≈ V
IN2
MAX V
BOOST
≈ V
IN2
+ V
IN
MINIMUM VALUE FOR V
IN2
= 3V
V
OUT
C3
BIAS
V
IN
3474 F04b
BOOST
V
IN
SW
GND
LT3474
(4b)
V
BOOST
– V
SW
≈ V
IN
MAX V
BOOST
≈ 2V
IN
V
OUT
C3
BIAS
V
IN
3474 F04a
BOOST
V
IN
SW
GND
LT3474
(4a)
V
BOOST
– V
SW
≈ V
OUT
MAX V
BOOST
≈ V
IN
+ V
OUT
V
OUT
C3
BIAS
V
IN
3474 F04d
BOOST
V
IN
SW
GND
LT3474
(4d)
V
BOOST
– V
SW
≈ V
OUT
–V
Z
MAX V
BOOST
≈ V
IN
+ V
OUT
–V
Z
V
OUT
C3