L6740L datasheet(30/44 Pages) STMICROELECTRONICS

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Output voltage positioning

L6740L

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6.10

Soft-start

L6740L implements a soft-start to smoothly charge the output filter avoiding high in-rush

currents to be required to the input power supply. In SVI mode, Soft-Start Time is intended

as the time required by the device to set the output voltages to the Pre-PWROK Metal VID.

During this phase, the device increases the reference of the enabled section(s) from zero up

to the programmed reference in closed loop regulation. Soft-Start is implemented only when

VCC is above UVLO Threshold and the EN pin is set free. See Section 5 for details about

the SVI interface and how SVC/SVD are interpreted in this phase.

At the end of the digital Soft-Start, PWRGOOD signal is set free.

Protections are active during this phase as follow:

–

Under Voltage is enabled when the reference voltage reaches 0.5V.

–

Over Voltage is always enabled according to the programmed threshold (by ROVP).

–

FBDisconnection is enabled.

Reference is increased with fixed dV/dt; Soft-Start time depends on the programmed

voltage as follow:

Figure 12.

System Start-up: SVI (left) and PVI (right)

6.10.1

LS-Less Start-up

In order to avoid any kind of negative undershoot on the load side during start-up, L6740L

performs a special sequence in enabling the drivers for both sections: during the soft-start

phase, the LS MOSFET is kept OFF (PWMx set to HiZ and ENDRVx = 0) until the first PWM

pulse. After the first PWM pulse, the PWMx outputs switches between logic “0” and logic “1”

and ENDRVx are set to logic “1”.

This particular sequence avoids the dangerous negative spike on the output voltage that

can happen if starting over a pre-biased output especially when exiting from a CORE-OFF

state.

Low-Side MOSFET turn-on is masked only from the control loop point of view: protections

are still allowed to turn-ON the Low-Side MOSFET in case of over voltage if needed.

SS ms

[]

Target_VID 2.56

⋅

VDD_CORE

PWRGOOD

VDD_NB

VDD_CORE

PWRGOOD

VDD_NB

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