L6728AH

Soft-start

Doc ID 15726 Rev 1

11/33

6

Soft-start

L6728AH implements a soft-start to smoothly charge the output filter avoiding high in-rush

currents to be required from the input power supply. The device gradually increases the

internal reference from 0 V to 0.8 V in 4.5 msec (typ.), in closed loop regulation, linearly

charging the output capacitors to the final regulation voltage. A pre-charged output voltage

will affect the soft-start duration, resulting in a reduction of this period of time (< 4 msec).

During the soft-start process all the protections but the UVP are active: the UVP becomes

active as soon as the soft-start ends up.

and over-current threshold setting phase has been completed.

6.1

Low-side-less start up (LSLess)

In order to avoid any kind of negative undershoot and dangerous return from the load during

start-up, L6728AH 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.

If the output voltage is pre-biased to a voltage higher than the final one, the HS would never

start to switch. In this case, at the end of soft-start time, LS is enabled and discharge the

output to the final regulation value.

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

view: protections by-pass this turning ON the LS MOSFET in case of need.

Figure 4.

LSLess startup (left) vs. non-LSLess startup (right)