Datasheet

LT3745-1
24
37451f
I
LED(NOM)
must be set between 10mA and 50mA. Typical
R
ISET
resistor values for various nominal LED currents
are listed in Table 3.
Table 3. Nominal LED Current I
LED(NOM)
vs. R
ISET
Value
I
LED(NOM)
(mA) R
ISET
* (kΩ)
10 301
20 150
30 100
40 75
50 60.4
* Recommend 1% Standard Values
Setting Dot Correction
The LT3745-1 can adjust the LED current for each channel
independently. This fine current adjustment, also called
dot correction, is mainly used to calibrate the brightness
deviation between LED channels. The 6-bit (64 steps) dot
correction setting adjusts each LED current
from 0.5X to
1.5X of the nominal LED current according to:
I
LEDn
= I
LED(NOM)
•
DC
n
+ 32
64






where I
LEDn
is the nth LED current and DC
n
is the nth
programmed dot correction setting (DC
n
= 0 to 63). The
fine current step over the nominal LED current gives an
excellent resolution:
∆I
LED
I
LED(NON)
=
1
64
≈ 1.56%
which enhances the relative LED current match accuracy
if used as calibration.
Setting Grayscale
Although adjusting the LED current changes its luminous
intensity, or brightness, it will also affect the color match-
ing between LED channels by shifting the chromaticity
coordinate. The best way to adjust the brightness is to
control the amount of LED on-/off-time by pulse width
modulation (PWM).
The LT3745-1 can adjust
the brightness for each channel
independently. The 12-bit grayscale PWM dimming results
in 4096 linear brightness steps from 0% to 99.98%. The
brightness level GS
n
% for channel n can be calculated as:
GS
n
% =
GS
n
4096
• 100%
where GS
n
is the nth programmed grayscale setting
(GS
n
= 0 to 4095).
Open/Short LED Fault
The LT3745-1 has individual LED fault diagnostic circuitry
that detects both open and short LED faults for each chan-
nel. The open LED fault is defined as any LED string is
open or disconnected from the circuit; and the short LED
fault is defined as any LED string is shorted across
itself.
The open LED flag is set if the LED pin voltage is lower
than 0.35V (typical) during on status with initial 500ns
blanking. The short LED flag is set if the LED pin voltage
is higher than 75% of the LED bus voltage V
OUT
any time.
If one LED channel is shorted across itself, the channel will
be turned off to eliminate unnecessary power dissipation.
The function
can also be used to disable LED channels by
connecting their LED pins to the output directly. Both the
open and short LED flags are combined to set the LED
fault bits (S0 to S15) in the status frame to 1.
Thermal Protection
The LT3745-1 has two overtemperature thresholds: one
is the fixed internal thermal shutdown and the other one
is programmed by a resistor, R
TSET
, between the T
SET
pin and ground. When the junction temperature exceeds
165°C, the part will enter thermal shutdown mode, shut
down serial data interface, turn off LED channels, and stop
switching. After the junction temperature drops below
155°C, the part will initiate a new soft-start.
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