9

LT1613

START-UP/SOFT-START

When the LT1613 SHDN pin voltage goes high, the device

rapidly increases the switch current until internal current

limit is reached. Input current stays at this level until the

output capacitor is charged to final output voltage. Switch

current can exceed 1A. Figure 16’s oscillograph details

start-up waveforms of Figure 17’s SEPIC into a 50

Ω load

without any soft-start. The output voltage reaches final

value in approximately 200

µs, while input current reaches

400mA. Switch current in a SEPIC is 2x the input current,

so the switch is conducting approximately 800mA peak.

Soft-start reduces the inrush current by taking more time

to reach final output voltage. A soft-start circuit consisting

to limit inrush current to a lower value. Figure 18 pictures

Input current is limited to a peak value of 200mA as the

OPERATIO

5V

1613 F17

SW

L1

22

µH

L2

22

µH

330pF

D1

GND

LT1613

C1: AVX TAJB226M006

C2: TAIYO YUDEN LMK325BJ106MN

C3: TAIYO YUDEN LMK212BJ105MG

R2

12.1k

Q1

2N3904

R1

37.4k

R3

10k

33k

C3

1

µF

FB

SOFT-START COMPONENTS

C1

22

µF

4V

C2

10

µF

SHDN

+

33k

10nF/

33nF

D1: MOTOROLA MBR0520

L1, L2: MURATA LQH3C220

time required to reach final value increases to 1.7ms. In

exceed the steady-state current the device uses to supply

power to the 50

Ω load. Start-up time increases to 4.3ms.

desired.

2V/DIV

200

µs/DIV

1613 F16

Figure 16. Start-Up Waveforms of

Figure 17’s SEPIC Into 50

Ω Load

200mA/DIV

5V/DIV

2V/DIV

500

µs/DIV

1613 F18

Figure 18. Soft-Start Components in Figure 17’s SEPIC

200mA/DIV

5V/DIV

2V/DIV

1ms/DIV

1613 F18

200mA/DIV

5V/DIV

Figure 17. 5V SEPIC with Soft-Start Components