Datasheet

LT3746
19
3746fa
For more information www.linear.com/3746
applicaTions inForMaTion
Globally, the LT3746 converts a higher input voltage to a
single lower LED bus voltage (V
OUT
) supplying 32 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 0.5V
during the tracking phase. Therefore, the maximum V
OUT
should be programmed high enough to keep all the LED
pin voltages higher than 0.5V to maintain LED current
regulation across temperature, current, and manufactur
-
ing variation. As a starting point, the maximum LED bus
vo
ltage, V
OUT(MAX)
, can be calculated as:
V
OUT(MAX)
=0.5V + 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:
R
FB2
=R
FB1
V
OUT(MAX)
1.205V
-1
Ê
Ë
Á
ˆ
¯
˜
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 LT3746 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 2V 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)
+ 2V
Choosing Switching Frequency
Selection of the switching frequency is a tradeoff 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
D
MIN
t
ON(MIN)
,
1– D
MAX
t
OFF(MIN)








where the minimum duty cycle D
MIN
and the maximum
duty cycle D
MAX
are determined by:
D
MIN
=
V
OUT(MIN)
+ V
D
V
IN(MAX)
+ V
D
and D
MAX
=
V
OUT(MAX)
+ V
D
V
IN(MIN)
+ V
D
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 maxi-
mum input voltage, and V
D
is the catch diode forward volt-
age (~0.5V). The calculation of f
SW(MAX)
simplifies to:
f
SW(MAX)
=
MIN 5 •
V
OUT(MIN)
+ V
D
V
IN(MAX)
+ V
D
, 8.33 •
V
IN(MIN)
– V
OUT(MAX)
V
IN(MIN)
+ V
D








MHz
Obviously, lower frequency operation accommodates both
extremely high and low V
OUT
to V
IN
ratios.
Besides these common considerations, the specific
application also plays an important role in switching fre
-
quency choice. In a noise-sensitive system, the switching