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LT3474/LT3474-1
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-
?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 ef?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
BIAS
BOOST
C3
is both less than 51V and the voltage difference between
LT3474
the BOOST and SW pins is less than 25V.
V IN
V IN
GND
SW
V OUT
Programming LED Current
V BOOST – V SW ≈ V OUT
MAX V BOOST ≈ V IN + V OUT
(4a)
3474 F04a
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:
V IN
BIAS BOOST
LT3474
V IN SW
C3
V OUT
I LED =
1A ? V ADJ
1.25V
Voltages less than 1.25V can be generated with a voltage
GND
V BOOST – V SW ≈ V IN
MAX V BOOST ≈ 2V IN
(4b)
3474 F04b
divider from the REF pin, as shown in Figure 5.
REF
V IN2 > 3V
R1
V ADJ
LT3474
BIAS BOOST
LT3474
C3
R2
GND
V IN
V IN
GND
SW
V OUT
3474 F04c
3474 F04
Figure 5. Setting V ADJ with a Resistor Divider
V BOOST – V SW ≈ V IN2
MAX V BOOST ≈ V IN2 + V IN
MINIMUM VALUE FOR V IN2 = 3V
(4c)
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
BIAS
BOOST
C3
use the following formula to choose resistors:
V IN
V IN
LT3474
GND
SW
V OUT
3474 F04d
R 2 =
V ADJ
1. 25 V – V ADJ
R 1
+ 50 nA
V BOOST – V SW ≈ V OUT – V Z
MAX V BOOST ≈ V IN + V OUT – V Z
(4d)
Figure 4. Generating the Boost Voltage
3474fd
15
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