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LT3745
APPLICATIONS INFORMATION
I LED (NOM) =
V ISET
inverselyproportionaltoR ISET .ThenominalLEDcurrent,
I LED(NOM) , can be calculated as:
? 2500
R ISET
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
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 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:
20
30
40
150
100
75
GS n % =
GS n
4096
? 100%
50
* Recommend 1% Standard Values
60.4
where GS n is the nth programmed grayscale setting
(GS n = 0 to 4095).
I LEDn = I LED(NOM) ? ?
?
? I LED 1
SettingDotCorrection
The LT3745 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:
? 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:
= ≈ 1.56%
I LED (NON) 64
Open/Short LED Fault
The LT3745 has individual LED fault diagnostic circuitry that
detects both open and short LED faults for each channel.
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.
which enhances the relative LED current match accuracy
if used as calibration.
3745f
24
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