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Soft-Start Circuit for

Step-Down Switching Regulator

020

R

U IN

U

U

IN

OUT

L1

SW

IN

T1

R1

IC1

FB

t

ON/OFF

GND

R2

D1

C1

C

C2

U IN

014079 - 11

U ref

There are very many switching regulator ICs for reducing a

dc voltage. In the ‘step-down’ operating mode, an internal

switch connects the input voltage to the charging coil L1.

The increasing magnetic field stores energy. When the

switch opens, coil L1 attempts to force the current to con-

tinue to flow. For this purpose, the flyback diode D1 must

be present to allow continued current flow. Output capaci-

tor C1 smoothes the output voltage, while the voltage

divider R1/R2 feeds back a portion of the output voltage to

the sense input (FB) of the IC. The predefined internal ref-

erence voltage U ref causes the indicated output voltage to

be achieved. One limitation is that the output voltage must

be less than the input voltage (step-down principle). The

t

014079 - 12

RC

t

R

R

1

2

UU

=

1

+

out

ref

reference voltage usually lies at 1.2 V.

If it is necessary to have the output voltage rise slowly after

the power has been switched on, the circuit trick shown

here can be used. Transistor T1 initially shorts out the upper

resistor of the output voltage divider, since capacitor C is at

first uncharged. This means that the output voltage is equal

12/2001

Elektor Electronics

45

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SMALL CIRCUITS COLLECTION

to the reference voltage. Capacitor C is slowly charged by

the current passing through R, which is connected to the

input voltage. Transistor T1 thus conducts increasingly less,

so the voltage divider R1/R2 slowly becomes effective.

Once the voltage across C is greater than the output volt-

age, the base-emitter junction of T1 is reverse biased and

the transistor remains completely cut off.

Attention should be given to the base-emitter breakdown

voltage of the transistor selected for T1. The difference

between the input voltage and the output voltage must not

exceed this limit.

(014079-1)

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