Figure 6. Feed-forward

Vs

Vcomp

Ton

a*Vs

Ton/a

VIN

a*VIN

Ramp slope = Vs/T

T

Ramp slope = a*Vs/T

T

Vs

Vcomp

Ton

Ton/a

Fsw

a*Fsw

Ramp slope = Vs/T

T

Ramp slope = a*Vs/T

T/a

Vs

Vs

Vcomp

Ton

Ton/a

Fsw

a*Fsw

Ramp slope = Vs/T

T

Ramp slope = Vs/T

T/a

Vs

Vcomp

a) Voltage feed forward

b) Fsw adjusted by external resistor

c) Synchronized by external clock signal

5.2

Soft-start

The soft-start is essential to assure a correct and safe start-up of the step-down converter. It avoids inrush current

surge and makes the output voltage increase monotonically.

The soft-start is performed as soon as EN and DIM pin are asserted high; when this occurs, an external capacitor,

connected between SS pin and ground, is charged by a constant current (5 µA typ.). The SS voltage is used as

reference of the switching regulator and the output voltage of the converter tracks the ramp of the SS voltage.

When the SS pin voltage reaches 0.25 V level, the error amplifier switches to the internal 0.25 V reference to

regulate the output voltage.

ALED6000

Soft-start

DS13018 - Rev 4

page 11/45