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LT3746
APPLICATIONS INFORMATION
R UV2 = R UV1 ? á
– 1 ?
pin can also be connected to a resistor divider between V IN
and ground to program a power input V IN UVLO threshold.
After R UV1 is selected, R UV2 can be calculated by:
ê V IN(ON) ?
? 1.3V ˉ
FROM μCONTROLLER
V IN
V CC
EN/UVLO
V IN
R UV2
R UV1
V IN
V CC
EN/UVLO
3746 F06
R UV1
R UV1 + R UV2
t SS = SS
? DC n + 32 ?
I LEDn = I LED(NOM) ? ? ?
Δ I LED 1
I LED(NON)
= ≈ 1.56%
where V IN(ON) is the power input voltage above which the
part goes into normal operation. It is important to check
the EN/UVLO pin voltage not to exceed its 6V absolute
maximum rating:
V IN(MAX) ? < 6V
Soft-Start
During soft-start, the SS pin voltage smoothly ramps up
inductor current and output voltage. The effective voltage
range of SS pin is from 0V to 1V. Therefore, the typical
soft-start period is:
C  ? 1V
12μA
where C SS is the capacitor connected at SS pin and
12μA is the soft-start charge current. Whenever a UVLO
or thermal shutdown occurs, the SS pin will be discharged
and the part will stop switching until the UVLO event has
disappeared and the SS pin has reached it reset threshold,
0.35V. The part then initiates a new soft-start cycle.
Setting Nominal LED Current
The nominal LED current is defined as the average LED
current across 32-channel when all the individual dot cor-
rection registers are set to 0x20. The nominal LED current
is programmed by a single resistor, R ISET , between the
I SET pin and ground. The voltage at the I SET pin, V ISET ,
is trimmed to an accurate 1.205V, generating a current
inversely proportional to R ISET . The nominal LED current,
I LED(NOM) , can be calculated as:
(A) (B)
Figure 6. Methods of Controlling the EN/UVLO Pin
I LED(NOM) must be set between 4mA and 20mA. 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 * (k Ω)
4 301
10 121
15 80.6
20 60.4
* Recommend 1% Standard Values
Setting Dot Correction
The LT3746 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:
? 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:
64
I LED(NOM) =
V ISET
R ISET
? 1000
which enhances the relative LED current match accuracy
if used as calibration.
3746fa
23
For more information www.linear.com/3746
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