L6751 datasheet(37/59 Pages) STMICROELECTRONICS

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L6751

Output voltage monitoring and protection

Doc ID 023992 Rev 1

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7.2

Overcurrent and current monitor

The overcurrent threshold must be programmed to a safe value, in order to be sure that

each section does not enter OC during normal operation of the device. This value must take

into consideration also the process spread and temperature variations of the sensing ele-

ments (inductor DCR).

Furthermore, since also the internal threshold spreads, the design must consider the mini-

mum/maximum values of the threshold.

7.2.1

Multi-phase section

The L6751 features two independent load indicator signals, IMON and ILIM, to properly

manage OC protection, current monitoring and DPM. Both IMON and ILIM source a current

proportional to the current delivered by the regulator, as follows:

Equation 11

The IMON and ILIM pins are connected to GND through a resistor (RIMON and RILIM respec-

tively), implementing a load indicator with different targets.

●

IMON is used for current reporting purposes and for the DPM phase shedding. RIMON

must be designed considering that IMAX must correspond to 1.24 V (for correct IMAX

detection).

●

ILIM is used for the overcurrent protection only. RILIM must be designed considering

that the OC protection is triggered when V(ILIM)=2.5 V.

In addition, the L6751 also performs per-phase OC protection.

–

Per-phase OC. Maximum information current per-phase (IINFOx) is internally

limited to 35

μA. This end-of-scale current (IOC_TH) is compared with the

information current generated for each phase (IINFOx). If the current information for

the single-phase exceeds the end-of-scale current (i.e. if IINFOx > IOC_TH), the

device turns on the LS MOSFET until the threshold is re-crossed (i.e. until IINFOx <

IOC_TH).

–

Total current OC. the ILIM pin allows a maximum total output current for the

system (IOC_TOT) to be defined. ILIM current is sourced from the ILIM pin. By

connecting a resistor RILIM to GND, a load indicator with 2.5 V (VOC_TOT) end-of-

scale can be implemented. When the voltage present at the ILIM pin crosses

VOC_TOT, the device detects an OC and immediately latches with all the MOSFETs

of all the sections OFF (HiZ).

Typical design considers the intervention of the total current OC before the per-phase OC,

leaving this last one as an extreme-protection in case of hardware failures in the external

components. Per-phase OC depends on the RG design while total OC is dependant on the

ILIM design and on the application TDC and max. current supported. Typical design flow is

the following:

–

Define the maximum total output current (IOC_TOT) according to system

requirements (IMAX, ITDC). Considering IMON design, IMAX must correspond to 1.24

V (for correct IMAX detection) while considering ILIM design IOC_TOT must

correspond to 2.5 V.

–

Design per-phase OC and RG resistor in order to have IINFOx = IOC_TH (35 μA)

when IOUT is about 10% higher than the IOC_TOT current. It results:

MON

LIM

DCR

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

OUT

⋅

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