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LT3745
APPLICATIONS INFORMATION
R UV2 = R UV1 ?
V IN(ON)
1.31V
t SS =
R UV1
UndervoltageLockout(UVLO)andShutdown
LT3745 has three UVLO thresholds with hysteresis for
the EN/UVLO, V CC , and CAP pins. The part will remain in
UVLO mode not switching until all the EN/UVLO, V CC , and
(V IN - V CAP ) voltages pass their respective typical thresh-
olds (1.31V, 2.89V, and 4.9V). As shown in Figure 6, the
EN/UVLO pin can be controlled in two different ways. The
EN/UVLO pin can accept a digital input signal to enable or
disable the chip. Tie to 0.35V or lower to shut down the
chip or tie to 1.34V or higher for normal operation. This
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:
–1
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
R UV1 + R UV2
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 SS ? 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 16-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
V IN
V CC
V IN
R UV2
V IN
V CC
FROM μCONTROLLER
EN/UVLO
EN/UVLO
(A)
R UV1
(B)
3745 F06
Figure 6. Methods of Controlling the EN/UVLO Pin
3745f
23
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