L6706 datasheet(33/47 Pages) STMICROELECTRONICS

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L6706

Soft-start

Doc ID 15698 Rev 2

33/47

Note:

Connecting SSOSC pin to SGND through only the RFLIM_SS resistor (blue one network in

Figure 14), the soft-start time depends on the FLIMIT selected.

In this case use the following relationship to select FLIMIT and as a consequence the soft-

start time:

Figure 16.

Soft-start time (TSS) vs FLIMIT when using RFLIM_SS resistor versus SGND

14.1

Low-side-less startup

In order to avoid any kind of negative undershoot on the load side during startup, L6706

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 17).

This particular feature of the device masks the LS turn-on only from the control loop point of

view: protections are still allowed to turn-ON the LS MOSFET in case of over voltage if

needed.

Figure 17.

Low-side-less startup comparison.WITH LS-Less startup

FLIM

–

[]

2.11 10

⋅

LIMIT kHz

[]

---------------------------------

D2 μs

[]

5.275 10

⋅

LIMIT kHz

[]

---------------------------------

VOUT

LGATE

WITHOUT LS-Less startup

WITH LS-Less startup

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