L6722 datasheet(21/34 Pages) STMICROELECTRONICS

21 / 34 page

L6722

8 Soft start

21/34

Soft start

L6722 implements a soft-start to smoothly charge the output filter avoiding high in-rush currents

to be required to the input power supply. The device increases the reference from zero up to

the programmed value in 2048 clock periods and the output voltage increases accordingly with

closed loop regulation. At the end of the digital Soft-Start, PGOOD signal is set free.

Protections are active during this phase; Under Voltage is enabled when the reference voltage

reaches 0.6V while Over Voltage is always enabled with a threshold dependent on the selected

Operative Mode

The device implements Soft-Start only when all the power supplies are above their own turn-on

thresholds and the INH pin is set free.

8.1

Low-side-less startup for pre-bias output

To manage pre-biased output start-up and in order to avoid any kind of negative undershoot

and dangerous return from the load, L6722 performs a special sequence in enabling LS driver

to switch: during the soft-start phase, the LS driver results disabled (LS=OFF) until the HS

starts to switch. This avoid the dangerous negative spike on the output voltage that can happen

if starting over a pre-biased output (See Figure 11).

This particular feature of the device masks the LS turn-on only from the control-loop point of

view: protections are still allowed to turn-on the ls mosfet in case of over-voltage if needed.

Figure 11. low-side-less startup

L6722 Datenblatt(PDF) 21 Page - STMicroelectronics