AN3050

STBP120 highlights

Doc ID 16207 Rev 1

13/18

The soft-start circuit is very useful for USB-powered applications due to USB power

limitations. The USB specification allows a maximum inrush current equivalent to a value

STBP120 softstart feature allows an even larger capacitor value to be connected to the

STBP120 output, providing better decoupling and higher reservoir of energy to cover

possible current peaks, without affecting this requirement. Maximum capacitance proven

with a standard 2 m USB cable is 100 µF. Higher capacitance can lead to a oscillation due to

USB cable impedance and the capacitor inrush current, leading the input voltage to drop

below the UVLO threshold during the start-up. The input capacitor value can be 1 - 10 µF.