DC1205A Datasheet, PDF (15/24 Pages) Linear Integrated Systems – 500mA Wide Input Voltage Range Step-Down LED Driver with 10:1 Dimming

LT3592
APPLICATIONS INFORMATION
The minimum operating voltage of an LT3592 application
is limited by the undervoltage lockout (UVLO, ~3.25V) and
by the maximum duty cycle as outlined above. For proper
startup, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT3592 is turned on with its SHDN pin when the output is
already in regulation, then the boost capacitor might not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load generally goes to
zero once the circuit has started. Figure 6 shows a plot
of minimum input voltage needed to start with a 500mA
output current versus output voltage with LED loads. For
LED applications, the output voltage will typically drop
rapidly after start due to diode heating, but this is not
a concern because the voltage to run is lower than the
voltage to start. The plots show the worst case situation
when V_IN is ramping very slowly. For a lower startup
voltage, the boost diode’s anode can be tied to V_IN, but
this restricts the input range to one-half of the absolute
maximum rating of the BOOST pin.
At light loads, the inductor current becomes discontinuous
and the effective duty cycle can be very high. This reduces
the minimum input voltage to about 400mV above V_CAP.
At higher load currents, the inductor current is continu-
ous and the duty cycle is limited by the maximum duty
cycle of the LT3592, requiring a higher input voltage to
maintain regulation.
Soft-Start
The SHDN pin can be used to soft-start the LT3592, reducing
the maximum input current during startup. The SHDN pin
is driven through an external RC filter to create a voltage
ramp at this pin. Figure 7 shows the startup waveforms
with and without the soft-start circuit. By choosing a large
RC time constant, the peak startup current can be reduced
to programmed LED current, with no overshoot. Choose
the value of the resistor so that it can supply 20μA when
the SHDN pin reaches 2.3V.
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INPUT VOLTAGE (V)
400kHz, L = 22μH
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3592 F06a
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INPUT VOLTAGE (V)
900kHz, L = 6.8μH
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3592 F06b
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INPUT VOLTAGE (V)
2.2MHz, L = 4.7μH
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3592 F06c
Figure 6. Input Voltage Needed to Start at 500mA Output Current vs LED Voltage
3592fc
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