L6711 datasheet(24/38 Pages) STMICROELECTRONICS

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L6711

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device turn-on threshold and the PreOVP turn on threshold and depends also on the OUTEN pin status

as detailed in Figure 17 - left.

A simple way to provide protection to the output in all conditions when the device is OFF (then avoiding

the unprotected red region in Figure 17) consists in supplying the controller through the 5VSB bus as

shown in Figure 17 - right.

Both Over Voltage and Under Voltage are active also during soft start (see the relevant section).

Figure 17. Output Voltage Protections and typical principle connections to assure complete

protection.

13 OVERCURRENT PROTECTION

Depending on the current reading method selected, the device limits the peak or the bottom of the inductor

current entering in constant current until setting UVP as below explained.

The Over Current threshold has to be programmed, by designing the Rg resistors, to a safe value, in order

to be sure that the device doesn't enter OCP during normal operation of the device. This value must take

into consideration also the extra current needed during the Dynamic VID Transition ID-VID and, since the

device reads across mosfets RdsON or inductor DCR, the process spread and temperature variations of

these sensing elements.

Moreover, since also the internal threshold spreads, the Rg design must consider its minimum value

IOCTH(min) as follow:

where IOCPx is the current measured by the current reading circuitry when the device enters Quasi Con-

stant Current (LS Mosfet Sense) or Constant Current (Inductor Sense), IOCPx must be calculated starting

from the corresponding output current value IOUT(OCP) as follow (ID-VID must also be considered when

D-VID are implemented):

where IOUT(OCP) is still the output current value at which the device enters Quasi Constant Current (LS

Mosfet Sense) or Constant Current (Inductor Sense),

∆IPP is the inductor current ripple in each phase and

ID-VID is the additional current required by D-VID (when applicable). In particular, since the device limits

the peak or the valley of the inductor current (according to CS_SEL status), the ripple entity, when not

negligible, impacts on the real OC threshold value and must be considered.

Vcc

PreOVP TurnON

Vcc TurnON

Preliminary OVP Enabled

FBR Monitored

No Protection

Provided

(OUTEN = 1)

Programmable OVP

VSEN Monitored

(OUTEN = 0)

Preliminary OVP

FBR Monitored

VCC

VCCDR1

VCCDR2

VCCDR3

+12V

+5V

OCPx max

() RSENSE max

()

⋅

OCTH min

()

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

OCPx =

OUT OCP

()

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

∆

-----------

D-VID

--------------- Low Side Mosfet Sense

–

OUT OCP

()

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

∆

-----------

D-VID

--------------- Inductor DCR Sense

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