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LT3466

Dual Full Function White LED Step-Up Converter with Built-In Schottky Diodes

Linear Technology 

LT3466

APPLICATIO S I FOR ATIO

DUTY CYCLE

The duty cycle for a step-up converter is given by:

D = VOUT + VD – VIN

VOUT + VD – VCESAT

where:

VOUT = Output voltage

VD = Schottky forward voltage drop

VCESAT = Saturation voltage of the switch

VIN = Input battery voltage

The maximum duty cycle achievable for LT3466 is 96%

(typ) when running at 1MHz switching frequency. It in-

creases to 99% (typ) when run at 200kHz and drops to

92% (typ) at 2MHz. Always ensure that the converter is not

duty-cycle limited when powering the LEDs at a given

switching frequency.

SETTING THE SWITCHING FREQUENCY

The LT3466 uses a constant frequency architecture that

can be programmed over a 200KHz to 2MHz range with a

single external timing resistor from the RT pin to ground.

The nominal voltage on the RT pin is 0.54V, and the

1000

100

10

200

600 1000 1400 1800

OSCILLATOR FREQUENCY (kHz)

3466 F06

Figure 6. Timing Resistor (RT) Value

current that flows into the timing resistor is used to

charge and discharge an internal oscillator capacitor. A

graph for selecting the value of RT for a given operating

frequency is shown in Figure 6.

OPERATING FREQUENCY SELECTION

The choice of operating frequency is determined by sev-

eral factors. There is a tradeoff between efficiency and

component size. Higher switching frequency allows the

use of smaller inductors albeit at the cost of increased

switching losses and decreased efficiency.

Another consideration is the maximum duty cycle achiev-

able. In certain applications, the converter needs to oper-

ate at the maximum duty cycle in order to light up the

maximum number of LEDs. The LT3466 has a fixed

oscillator off-time and a variable on-time. As a result, the

maximum duty cycle increases as the switching frequency

is decreased.

The circuit of Figure 1 is operated with different values of

timing resistor (RT). RT is chosen so as to run the

converters at 800kHz (RT = 63.4k), 1.25MHz (RT = 39.1k)

and 2MHz (RT = 20.5k). The efficiency comparison for

different RT values is shown in Figure 7.

85

CIRCUIT OF FIGURE 1

80

VIN = 3.6V

8/8 LEDs

RT = 63.4k

RT = 39.1k

75

70

RT = 20.5k

65

60

55

50

0

5

10

15

LED CURRENT (mA)

20

3466 F07

Figure 7. Efficiency Comparison for Different RT Resistors

3466fa

10

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