L6701TR datasheet(28/44 Pages) STMICROELECTRONICS

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12 Soft Start

L6701

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Soft Start

L6701 implements a soft-start to smoothly charge the output filter avoiding high in-rush currents

to be required to the input power supply. The device increases the reference from zero up to

the programmed value in 2048 clock periods and the output voltage increases accordingly with

closed loop regulation. At the end of the digital Soft-Start, SSEND/PGOOD signal is set free.

Protections are active during this phase; Under Voltage is enabled when the reference voltage

reaches 0.6V while Over Voltage is always enabled with a threshold dependent on the selected

Operative Mode. DAC table information is frozen just before initializing the Soft-Start.

The device implements Soft-Start only when all the power supplies are above their own turn-on

thresholds and the EN pin is set free.

12.1

Low-Side-Less Startup (LSLess)

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

start-up, L6701 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 (See Figure 12).

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 12. LSLess Startup (left) vs. Non-LSLess Startup (right)

L6701TR Datasheet(PDF) 28 Page - STMicroelectronics