L6751 datasheet(39/59 Pages) STMICROELECTRONICS

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L6751

Output voltage monitoring and protection

Doc ID 023992 Rev 1

39/59

7.2.3

Single-phase section

The L6751 performs two different kinds of OC protection for the single-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 (ISINFOx) is internally

limited to 35

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

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

for the single-phase exceeds the end-of-scale current (i.e. if ISINFOx > ISOC_TH),

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

ISINFOx < ISOC_TH).

–

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

system (ISOC_TOT) to be defined. ISMON current is sourced from the SIMON pin. By

connecting a resistor RSIMON to GND, a load indicator with 1.55 V (VSOC_TOT) end-

of-scale can be implemented. When the voltage present at the SIMON pin crosses

VSOC_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, moreover, dependant on the SIMON design and on the

application TDC and MAX current supported. Typical design flow is the following:

–

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

requirements (ISMAX, ISTDC). Considering ISMON design, ISMAX must correspond to

1.24 V (for correct SIMAX detection) so ISOC_TOT results defined, as a

consequence, as

–

Design per-phase OC and RSG resistor in order to have ISINFOx = ISOC_TH (35 μA)

when ISOUT is about 10% higher than the ISOC_TOT current. It results:

Equation 15

where DCR is the DC resistance of the inductors. RSG should be designed in

worst-case conditions.

–

Design the total current OC and RSIMON in order to have the SIMON pin voltage to

1.24 V at the ISMAX current specified by the design. It results:

Table 14.

Multi-phase section OC scaling and power states

Power state [Hex]

OC level (VOC_TOT)

00h

3 to 6

2.500 V

01h, 02h

1.650 V

1.250 V

1.000 V

0.830 V

SOC_TOT

SMAX

1.55

⋅

1.24

⁄

1.1

SOC_TOT

⋅

() DCR

⋅

SOCTH

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