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LT3745
APPLICATIONS INFORMATION
R FB2 = R FB1 ?
? 1 ?
? D MIN 1– D MAX ?
? t ON(MIN) t OFF(MIN) ?
V OUT(MIN) + V D V OUT(MAX) + V D
V IN(MAX) + V D
V IN(MIN) + V D
?
?
Globally,theLT3745convertsahigherinputvoltagetoa
single lower LED bus voltage (V OUT ) supplying 16 parallel
LED strings with the adaptive-tracking-plus-precharging
technique. Locally, the part regulates and modulates the
current of each string to an independent dot correction and
grayscale PWM dimming setting sent by TTL/CMOS logic
serial data interface. This Application Information section
serves as a guideline of selecting external components
(refer to the Block Diagram) and avoiding common pitfalls
for the typical application.
Programming Maximum V OUT
The adaptive-tracking-plus-precharging technique regu-
lates V OUT to its maximum value during the startup and
precharging phases, and adaptively lowers the voltage to
keep the minimum active LED pin voltage around 0.7V
during the tracking phase. Therefore, the maximum V OUT
should be programmed high enough to keep all the LED
pin voltages higher than 0.8V to maintain LED current
regulation across temperature, current, and manufactur-
ing variation. As a starting point, the maximum LED bus
voltage, V OUT(MAX) , can be calculated as:
V OUT(MAX) = 0.8V + n ? V F(MAX)
where n is the number of LED per string and V F(MAX) is
the maximum LED forward voltage rated at the highest
operating current and the lowest operating temperature.
The V OUT(MAX) is programmed with a resistor divider
between the output and the FB pin. The resistor values
are calculated as:
? V OUT(MAX) ?
? 1.205V ?
Tolerance of the feedback resistors will add additional
errors to the output voltage, so 1% resistor values should
be used. The FB pin output bias current is typically
120nA, so use of extremely high value feedback resistors
could also cause bias current errors. A typical value for
R FB1 is 10k.
V IN Power Input Supply Range
The power input supply for the LT3745 can range from 6V
to 55V, covering a wide variety of industrial power supplies.
Another restriction on the minimum input voltage V IN(MIN)
is the 2.1V minimum dropout voltage between the V IN and
ISN pins, and thus the V IN(MIN) is calculated as:
V IN(MIN) = V OUT(MAX) + 2.1V
Choosing Switching Frequency
Selection of the switching frequency is a trade-off between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
and gate charge losses. However, lower frequency opera-
tion requires larger inductor and capacitor values.
Another restriction on the switching frequency comes
from the input and output voltage range caused by the
minimum switch on and switch off-time. The highest
switching frequency f SW(MAX) for a given application can
be calculated as:
f SW(MAX) = MIN ? , ?
? ?
where the minimum duty cycle D MIN and the maximum
duty cycle D MAX are determined by:
D MIN = and D MAX =
t ON(MIN) is the minimum switch on-time (~200ns), t OFF(MIN)
is the minimum switch off-time (~120ns), V OUT(MIN) is the
minimum adaptive output voltage, V IN(MAX) is the maximum
input voltage, and V D is the catch diode forward voltage
(~0.5V). The calculation of f SW(MAX) simplifies to:
f SW(MAX) =
? V OUT(MIN) + V D V IN(MIN) – V OUT(MAX) ?
MIN ? 5 ? , 8.33 ? ? MHz
? V IN(MAX) + V D V IN(MIN) + V D ?
Obviously, lower frequency operation accommodates both
extremely high and low V OUT to V IN ratios.
3745f
19
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