L6756D datasheet(24/36 Pages) STMICROELECTRONICS

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Output voltage positioning

L6756D

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5.6

Soft-start

L6756D implements a soft-start to smoothly charge the output filter avoiding high in-rush

currents to be required to the input power supply. During this phase, the device increases

the reference from zero up to the programmed reference in closed loop regulation. Soft-start

is implemented only when all the power supplies are above their own turn-on thresholds and

the EN pin is set free. At the end of the digital soft-start, VR_RDY signal is set free.

Soft-start phase is initiated with a T1 = 2 ms (min) delay. After that, the reference ramps up

to VBOOT = 1.081 V in T2 according to the RSS settings and waits for T3 = 200 μsec (typ)

during which the device reads the VID lines. Output voltage will then ramps up to the

programmed value in T4 with the same slope as before (See Figure 9).

SS sets the output voltage dv/dt during soft-start according to the resistor RSS connected to

SGND. soft-start time TSS will proportionally change with a gain of 18.52 [µs / kΩ]. Connect

27 k

Ω resistor to program T2 = 500 µs.

Protections are active during soft-start, UVP is enabled after the reference reaches 0.5 V

while OVP is always active with a fixed 1.24 V threshold before VBOOT and with the

threshold coming from the VID after VBOOT (See red-dashed line in Figure 9).

Note:

If during T3 the programmed VID selects an output voltage lower than VBOOT, the output

voltage will ramp to the programmed voltage starting from VBOOT.

Note:

The VID code is checked at the end of T3. In case the device is enabled over an OFF code,

the output voltage will rise up to VBOOT and then device will latch. VID lines does not show

any pull-up and/or pull-down before T3.

Figure 9.

Soft-start timings

L6756D 데이터시트(PDF) 24 Page - STMicroelectronics