L6728

Soft start

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7

Soft start

L6728 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 0V to 0.8V in 4.5ms (typ.), in closed loop regulation, linearly charging

the output capacitors to the final regulation voltage.

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

the soft start sequence and will shut down the PWM logic and both the high side and low

side gates. This condition is latched, cycle VCC to recover.

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

and over current threshold setting phase has been completed.

7.1

Low-Side-Less Start up (LSLess)

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

start-up, L6728 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)