L6727

Soft-start and disable

Doc ID 12933 Rev 4

11/34

6

Soft-start and disable

L6727 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 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 the event of an overcurrent triggering during soft start, the over current logic will override

the soft start sequence and will shut 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 will

begin a new soft start.

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

and overcurrent threshold setting phase has been completed.

6.1

Low-side-less start up (LSLess)

In order to manage start up over pre-biased output, L6727 performs a special sequence in

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

(LS = OFF) until HS starts to switch. This avoids 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 lower than the programmed one, neither HS

will ramp 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 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: protections by-pass LSLESS, turning ON the LS MOSFET in case of need.

Figure 4.

LSless startup (left) vs non-lsless startup (right)