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LT3754 Datasheet(PDF) 16 Page - Linear Technology

Part No. LT3754
Description  16-Channel × 50mA LED Driver
Download  28 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT3754 Datasheet(HTML) 16 Page - Linear Technology

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LT3754
16
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)
10
29.4
20
14.7
30
9.76
40
7.32
50
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
R
X
ISET
()
()
>
()
295
A CTRL 1.1V
See Table 6 for resistor values and corresponding
programmed LED.


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