Datasheet

LT3760
16
3760fc
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 LT3760 LED driver for a given
application, the duty cycle requirements should be
considered and compared to the minimum/maximum
achievable duty cycles for the LT3760 GATE pin. If required,
the LT3760 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 f
OSC
MAX Duty Cycle = 1 – (GATE Minimum Off-Time •
Switching Frequency f
OSC
)
The typical values for LT3760 GATE pin minimum on
and off times versus temperature are shown in the Typi-
cal Performance Characteristics. The range of GATE pin
minimum on time and off times are given in the electrical
specifications.
Table 6. LED Current vs. R
ISET
(1% resistors)
LED CURRENT PER CHANNEL
(mA)
R
ISET
(kΩ)
20 29.4
40 14.7
60 9.76
80 7.32
100 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 LT3760 oscillator to be synchro-
nized 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 synchroniza-
tion, the free running oscillator frequency (f
OSC
) of the
LT3760 should be programmed to 80% of the external
clock frequency.
Programming LED Current
The current source to ground at each LED pin is programmed
using a single resistor R
ISET
connected from the I
SET
pin
to ground according to the following equation:
I LED
X
( )
≈
590
R
ISET
A
( )
CTRL > 1.1V
( )
See Table 6 for resistor values and corresponding pro-
grammed LED.