Application ideas

AN1723

22/24

Doc ID 9553 Rev 4

7.6

External soft-start network

At startup, the device can quickly increase the current up to the current limit in order to

charge the output capacitor. If a soft ramp-up of the output voltage is required, an external

soft-start network can be implemented as shown in Figure 24.

The capacitor C is charged up to an external reference (through R), and the BJT clamps the

COMP pin. This clamps the duty cycle, limiting the slew rate of the output voltage.

Figure 24.

Soft-start network example

AM11331v1

VIN

Vcc

COMP

GND

OUT

FB

INH

SYNC

VREF

1

3

7

5

6

4

8

2

L5973AD

VOUT

VREF

R=4K7

C=2.7nF

BC327

VIN = 4.4V to 25V

Vcc

COMP

GND

OUT

FB

INH

SYNC

VREF

1

3

7

5

6

4

8

2

L5973AD

VOUT = 3.3V

VREF

R=4K7

VREF

R=4K7

C=2.7nF

BC327

C1

10uF

25

Ceramic

C4

22nF

C3

220pF

R3

4K7

R1

5K6

R2

4K7

L1

15uH

STPS3400

C2

330uF

6.3V