B130 datasheet(15/20 Pages) LINER

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LT3474/LT3474-1

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-

ﬁciency. 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 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 VADJ pin. For a 1A LED current, either tie VADJ to REF

or to a 1.25V source. For lower output currents, program

the VADJ using the following formula:

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

VADJ

GND

LT3474

3474 F04

Figure 5. Setting VADJ with a Resistor Divider

In order to have accurate LED current, precision resistors

are preferred (1% or better is recommended). Note that

the VADJ pin sources a small amount of bias current, so

use the following formula to choose resistors:

ADJ

125

.–

BIAS

VIN

3474 F04c

BOOST

VIN

VIN2 > 3V

GND

LT3474

(4c)

VBOOST – VSW ≈ VIN2

MAX VBOOST ≈ VIN2 + VIN

MINIMUM VALUE FOR VIN2 = 3V

VOUT

BIAS

VIN

3474 F04b

BOOST

VIN

GND

LT3474

(4b)

VBOOST – VSW ≈ VIN

MAX VBOOST ≈ 2VIN

VOUT

BIAS

VIN

3474 F04a

BOOST

VIN

GND

LT3474

(4a)

VBOOST – VSW ≈ VOUT

MAX VBOOST ≈ VIN + VOUT

VOUT

BIAS

VIN

3474 F04d

BOOST

VIN

GND

LT3474

(4d)

VBOOST – VSW ≈ VOUT –VZ

MAX VBOOST ≈ VIN + VOUT –VZ

VOUT

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