Soft-start

L6728D

12/33

Doc ID 16498 Rev 1

7

Soft-start

The L6728D implements a soft-start to smoothly charge the output filter, avoiding the 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 ms (typ.), in closed-loop regulation, linearly

charging the output capacitors to the final regulation voltage.

In the event of overcurrent triggering during soft-start, the overcurrent logic overrides the

soft-start sequence and shuts down the PWM logic and both the high side and low side

gates. This condition is latched. Cycle the VCC to recover.

The device begins the soft-start phase only when the VCC power supply is above the UVLO

threshold and the overcurrent threshold setting phase has been completed.

7.1

Low-side-less startup (LSLess)

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

startup, the L6728D performs a special sequence in enabling the LS driver to switch: during

the soft-start phase, the LS driver is disabled (LS = OFF) until the HS starts to switch. This

prevents the dangerous negative spike on the output voltage which 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 discharges 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: protection features bypass this turning on of the LS MOSFET if needed.

Figure 4.

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