Soft-start and disable

A6727

12/29

DocID025003 Rev 1

6

Soft-start and disable

The A6727 implements a soft-start to smoothly charge the output filter avoiding the high in-

rush currents to the input power supply. The device progressively increases the internal

reference from 0 V to 0.8 V in about 5.1 ms, in closed loop regulation, gradually charging the

output capacitors to the final regulation voltage.

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

soft-start sequence and shuts down both the high-side and low-side gates for the internal

soft-start residual time (up to 2048 clock cycles) plus 2048 clock cycles, then it begins a new

soft-start.

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

and the overcurrent threshold setting phase has been completed.

6.1

Low-side-less startup (LS-less)

In order to manage the startup over pre-biased output, the A6727 performs a special

sequence enabling LS driver to switch: during the soft-start phase, LS driver is disabled

(LS = OFF) until HS starts switching. In this manner, the dangerous negative spike on the

output voltage is avoided.

If the output voltage is pre-biased to a voltage lower than the programmed one, neither HS

ramps up from there, without any drop or current return.

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

starts switching. 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 masks the LS turn-on only from the control loop point of view:

protections by-pass LS-less, turning on the LS MOSFET if needed.

Figure 4. LS-less startup

Figure 5. Non-LS-less startup