LT3754 datasheet(16/28 Pages) LINER

16 / 28 page

LT3754

3754f

APPLICATIONS INFORMATION

Selecting the optimum frequency depends on several

factors. Higher frequency allows reduction of inductor

size but efﬁciency drops due to higher switching losses.

Lower frequency allows higher operational duty cycles to

drive a larger number of LEDs per string from a low input

supply but require larger magnetics. In each application

the switching frequency can be tailored to provide the

optimum solution.

Table 5. Switching Frequency vs. RT (1% resistors)

SWITCHING FREQUENCY (kHz)

RT (kΩ)

100

523

200

249

300

158

400

115

500

90.9

600

73.2

700

60.4

800

51.1

900

44.2

1000

39.2

Duty Cycle Considerations

When designing the LT3754 LED driver for a given

application, the duty cycle requirements should be

considered and compared to the minimum/maximum

achievable duty cycles for the LT3754 GATE pin. If required,

the LT3754 switching frequency can be programmed to a

lower value to meet the duty cycle requirements. In general,

the minimum/maximum GATE duty cycles required for a

particular application are given by:

MIN Duty Cycle = GATE Minimum On-Time • Switching

Frequency fOSC

MAX Duty Cycle = 1 – (GATE Minimum Off-Time •

Switching Frequency fOSC)

The typical values for LT3754 GATE pin minimum on and

off times versus temperature are shown in the Typical

Performance Characteristics. The range of GATE pin

minimum on time and off times are given in the electrical

speciﬁcations.

Table 6. LED Current vs. RISET (1% resistors)

LED Current per CHANNEL (mA)

RISET (kΩ)

29.4

14.7

9.76

7.32

5.76

An extra 50ns should be added to these tested timings to

account for errors in the rise/fall times of the GATE and

DRAIN of the external MOSFET and the d.c. resistance of

the external MOSFET and inductor.

Synchronizing to an external clock

The SYNC pin allows the LT3754 oscillator to be

synchronized to an external clock. The SYNC pin can be

driven from a logic level output, requiring less than 0.6V

for a logic level low and greater than 2.2V for a logic level

high. SYNC pin high or low periods should exists for at

least 100ns. If unused, the SYNC pin should be tied to

ground. To avoid loss of slope compensation during

synchronization, the free running oscillator frequency

(fOSC) of the LT3754 should be programmed to 80% of

the external clock frequency.

Programming LED Current

ThecurrentsourcetogroundateachLEDpinisprogrammed

using a single resistor RISET connected from the ISET pin

to ground according to the following equation:

I LED

ISET

()≈

()

295

A CTRL 1.1V

See Table 6 for resistor values and corresponding

programmed LED.

LT3754 Datasheet(PDF) 16 Page - Linear Technology

LT3754 Datasheet(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL