LT3474 Datasheet, PDF (15/20 Pages) Linear Technology – Step-Down 1A LED Driver

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-
ficiency. For outputs of 2.8V or higher, the standard circuit
(Figure 4a) is best. For lower output voltages, the BIAS pin
BIAS BOOST
C3
LT3474
V_IN
V_IN
SW
GND
V_BOOST – V_SW ≈ V_OUT
MAX V_BOOST ≈ V_IN + V_OUT
(4a)
V_OUT
3474 F04a
BIAS BOOST
C3
LT3474
V_IN
V_IN
SW
GND
V_BOOST – V_SW ≈ V_IN
MAX V_BOOST ≈ 2V_IN
(4b)
V_IN2 > 3V
BIAS BOOST
C3
LT3474
V_IN
V_IN
SW
GND
V_BOOST – V_SW ≈ V_IN2
MAX V_BOOST ≈ V_IN2 + V_IN
MINIMUM VALUE FOR V_IN2 = 3V
(4c)
V_OUT
3474 F04b
V_OUT
3474 F04c
BIAS BOOST
C3
LT3474
V_IN
V_IN
SW
V_OUT
GND
V_BOOST – V_SW ≈ V_OUT – V_Z
MAX V_BOOST ≈ V_IN + V_OUT – V_Z
(4d)
3474 F04d
Figure 4. Generating the Boost Voltage
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.
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.
REF
R1
LT3474
V_ADJ
GND
R2
3474 F04
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:
R2 =
V_ADJ
1.25V – V_{ADJ + 50nA}
R1
3474fd
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