LT3466EDD-1 Ver la hoja de datos (PDF) - Linear Technology

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LT3466EDD-1

White LED Driver and Boost Converter in 3mm × 3mm DFN Package

Linear Technology 

LT3466-1

APPLICATIO S I FOR ATIO

Typically peak inrush current will be less than the value

calculated above. This is due to the fact that the DC

resistance in the inductor provides some damping result-

ing in a lower peak inrush current.

SETTING THE LED CURRENT

The current in the LED string can be set by the choice of the

resistor RFB1 (Figure 1). The feedback reference is 200mV.

In order to have accurate LED current, precision resistors

are preferred (1% is recommended).

RFB1

=

200mV

ILED1

Table 4. RFB1 Value Selection

ILED1 (mA)

5

10

15

20

25

RFB1 (Ω)

40.2

20.0

13.3

10.0

8.06

Most White LEDs are driven at maximum currents of

15mA to 20mA.

Feedback voltage variation versus control voltage is given

in the Typical Performance Characteristics graphs.

Using a Filtered PWM Signal

A variable duty cycle PWM can be used to control the

brightness of the LED string. The PWM signal is filtered

(Figure 8) by an RC network and fed to the CTRL1 pin.

The corner frequency of R1, C1 should be much lower than

the frequency of the PWM signal. R1 needs to be much

smaller than the internal impedance in the CTRL pin, which

is 100kΩ.

R1

PWM 10k

10kHz TYP

LT3466-1

CTRL1

C1

1µF

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Figure 8. Dimming Control Using a Filtered PWM Signal

SETTING THE BOOST OUTPUT VOLTAGE

The LT3466-1 regulates the voltage at the FB2 pin to 0.8V.

The output voltage of the boost converter (VOUT2) is set by

a resistor divider according to the formula:

DIMMING WHITE LEDS

The LED current in the driver can be set by modulating the

CTRL1 pin. There are two different ways to control the

intensity of white LEDs.

Using a DC Voltage

For some applications, the preferred method of brightness

control is a variable DC voltage to adjust the LED current.

The CTRL1 pin voltage can be modulated to set the

dimming of the LED string. As the voltage on the CTRL1

pin increases from 0V to 1.8V, the LED current increases

from 0 to ILED1. As the CTRL1 pin voltage increases

beyond 1.8V, it has no effect on the LED current.

The LED current can be set by:

ILED1 ≈ (VCTRL1/5 • RFB1), when VCTRL1 < 1V

ILED1 ≈ (200mV/RFB1), when VCTRL1 > 1.8V

VOUT2 = 0.8V ⎛⎝⎜1+ RR21⎞⎠⎟

Choose 1% resistors for better accuracy. The FB2 input

bias current is quite low, on the order of 10nA (typ). Large

resistor values (R1 ~ 1MΩ) can be used in the divider

network maximizing efficiency.

PROGRAMMING THE BOOST OUTPUT VOLTAGE

The output voltage of the boost converter can be modu-

lated by applying a variable DC voltage at the CTRL2 pin

The nominal voltage at the FB2 pin is 800mV. As the

voltage on the CTRL2 pin is ramped from 0V to 1V, the FB2

pin voltage ramps up to 0.8V. The feedback voltage can be

programmed as:

VFB2 ≈ VCTRL2, when VCTRL2 < 0.8V

VFB2 ≈ 0.8V, when VCTRL2 > 1V

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