L6756D datasheet(27/36 Pages) STMICROELECTRONICS

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L6756D

Output voltage monitoring and protections

27/36

6.3

VR_RDY

It is an open-drain signal set free after the soft-start sequence has finished.

6.4

Over current

The over current threshold has to be programmed to a safe value, in order to be sure that

the system doesn't enter OC during normal operation. This value must take into

consideration also the extra current needed during the DVID transition (IDVID) and the

process spread and temperature variations of the sensing elements (inductor DCR).

Moreover, since also the internal threshold spreads, the design has to consider the

minimum/maximum value of the thresholds.

L6756D monitors the average current and allows to set the OC threshold by programming

RILIM. ILIM pin allows to define a maximum average output current for the system (IMAX). A

copy of the DROOP current is sourced from the ILIM pin. By connecting a resistor RILIM to

SGND, a load indicator with 1.7 V (VOC) end-of-scale can be implemented. This means that

when the voltage present at the ILIM pin crosses 1.7 V, the device detects an OC and

immediately latches with all the mosfets OFF (HiZ).

Typical design flow is the following:

–

Define the maximum average output current (IMAX) according to system

requirements

–Design RG resistor in order to have IINFOx = 35 μA when IOUT is about 10 % higher

than the IMAX current. It results:

where N is the number of phases and DCR the DC resistance of the inductors. RG

design must be typically performed in worst-case conditions.

–Design RILIM in order to have the ILIM pin to VOC at the desired IMAX current. It

results:

where IMAX is the OC threshold desired.

–

Adjust the defined values according to bench-test of the application.

Note:

OC intervention can be delayed by adding a capacitor in parallel to the above defined RILIM.

1.1 I

MAX

⋅

() DCR

⋅

OCTH

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

ILIM

OCAVG

⋅

MAX

DCR

⋅

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

1.7V R

⋅

MAX

DCR

⋅

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

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