L6758ATR datasheet(28/42 Pages) STMICROELECTRONICS

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Output voltage monitoring and protection

L6758A

28/42

DocID023298 Rev 2

7.2

Overcurrent

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 extra current needed during the DVID transition (I

DVID

) and the

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

7.2.1

Multi-phase section

The L6758A performs two different types of OC protection for the multi-phase section: it

monitors both the total current and the per-phase current and allows an OC threshold to be

set for both.

•

Per-phase OC

–

Maximum information current per-phase (I

INFOx

) is internally limited to 35

μA. This

end-of-scale current (I

OC_TH

) is compared with the information current generated

for each phase (I

INFOx

). If the current information for the single-phase exceeds the

end-of-scale current (i.e. if I

INFOx

> I

OC_TH

), the device turns on the LS MOSFET

until the threshold is re-crossed (i.e. until I

INFOx

< I

OC_TH

•

Total current OC

–

The IMON pin allows a maximum total output current to be defined for the system

OC_TOT

). I

MON

current is sourced from the IMON pin. By connecting a resistor

IMON

to SGND, a load indicator with 1.55 V (V

OC_TOT

) end-of-scale can be

implemented. When the voltage present at the ILIM pin crosses V

OC_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. Total current OC is dependant on the IMON design and on the application

TDC and MAX current supported. The typical design flow is the following:

–

Define the maximum total output current (I

OC_TOT

) according to system

requirements (I

MAX

, I

TDC

). Considering I

MON

design, I

MAX

must correspond to 1.24

V (for correct IMAX detection) so, I

OC_TOT

results as defined, as a consequence:

–

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

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

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

should be designed in worst-case conditions.

–

Design the total current OC and R

IMON

in order to have the IMON pin voltage to

1.24 V at the I

MAX

current specified by the design. It results:

OC_TOT

MA X

1.55

⋅

1.24

⁄

1.1

OC_TOT

⋅

() DCR

⋅

OCT H

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

IMON

1.24 V

⋅

MA X

DC R

⋅

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

MON

DC R

------------- I

OU T

⋅





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