LT3474/LT3474-1
APPLICATIONS INFORMATION
BOOST and BIAS Pin Considerations
can be tied to the input (Figure 4b). The circuit in Figure
4a is more efficient 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.
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-
ficiency. For outputs of 2.8V or higher, the standard circuit
(Figure 4a) is best. For lower output voltages, the BIAS pin
C3
BIAS
BOOST
SW
LT3474
V
V
V
OUT
IN
IN
Programming LED Current
GND
– V ≈ V
OUT
3474 F04a
The LED current can be set by adjusting the voltage on
the V pin. For a 1A LED current, either tie V to REF
V
BOOST
SW
MAX V
≈ V + V
IN OUT
ADJ
ADJ
BOOST
or to a 1.25V source. For lower output currents, program
(4a)
the V
using the following formula:
ADJ
1A •VADJ
1.25V
C3
BIAS
BOOST
ILED
=
LT3474
V
V
SW
V
OUT
IN
IN
Voltages less than 1.25V can be generated with a voltage
divider from the REF pin, as shown in Figure 5.
GND
3474 F04b
V
– V ≈ V
SW
BOOST
IN
IN
MAX V
≈ 2V
BOOST
REF
(4b)
R1
R2
LT3474
GND
V
> 3V
IN2
IN
V
ADJ
C3
BIAS
BOOST
SW
LT3474
V
V
V
OUT
IN
3474 F04
GND
Figure 5. Setting VADJ with a Resistor Divider
3474 F04c
V
– V ≈ V
SW IN2
BOOST
BOOST
MAX V
≈ V + V
IN2 IN
MINIMUM VALUE FOR V = 3V
IN2
In order to have accurate LED current, precision resistors
are preferred (1% or better is recommended). Note that
(4c)
the V
pin sources a small amount of bias current, so
ADJ
use the following formula to choose resistors:
C3
BIAS
BOOST
SW
LT3474
VADJ
1.25V – VADJ
V
IN
V
IN
V
OUT
R2 =
GND
– V ≈ V
+ 50nA
3474 F04d
R1
V
– V
Z
BOOST
MAX V
SW
OUT
≈ V + V
IN
– V
OUT Z
BOOST
(4d)
Figure 4. Generating the Boost Voltage
3474fd
15
Linear [ Linear ]
