Micrel, Inc.

MIC2289C

September 2006

8

M9999-090706

Start-Up and Inrush Current

During start-up, inrush current of approximately double

the nominal current flows to set up the inductor current

and the voltage on the output capacitor. If the inrush

current needs to be limited, a soft-start circuit similar to

Figure 5 could be implemented. The soft-start capacitor,

CSS, provides over-drive to the FB pin at start-up,

resulting in gradual increase of switch duty cycle and

limited inrush current.

VIN

EN

10k

2200pF

SW

OUT

FB

R

GND

Figure 5. One of Soft-Start Circuit

6-Series LED Circuit without External Soft-Start

TIME (100µs/div.)

L = 10µH

C

6 LEDs

6-Series LED Circuit with External Soft-Start

TIME (100µs/div.)

L = 10µH

C

6 LEDs

C

Figure 6. 6-Series LED Circuit

without External Soft Start

Figure 7. 6-Series LED Circuit

with External Soft Start