LT3592

15

3592fa

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

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

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.

Figure 6. Input Voltage Needed to Start at 500mA Output Current vs LED Voltage

3592 F06a

INPUT VOLTAGE (V)

2

12

11

10

4

7

6

5

8

9

3

2

8

12

10

46

400kHz, L = 22μH

3592 F06b

INPUT VOLTAGE (V)

2

12

11

10

4

7

6

5

8

9

3

2

8

12

10

46

900kHz, L = 6.8μH

INPUT VOLTAGE (V)

2

12

11

10

4

7

6

5

8

9

3

2

8

3592 F06c

16

10

12

14

46

400kHz, L = 22μH