L6711 datasheet(18/38 Pages) STMICROELECTRONICS

18 / 38 page

L6711

18/38

The information about the total current delivered (IDROOP) is sourced from the FB pin (see Figure 10): con-

necting a resistor between this pin and VSEN (i.e. the output voltage), the total current information flows

only in this resistor because the compensation network between FB and COMP has always a capacitor in

series (CF, see Figure 10). The voltage regulated is then equal to:

Where VID is the reference programmed through VIDx and VID_SEL (Only the IDROOP contribute to IFB

has been considered).

Since IDROOP depends on the current information about the three phases, the output characteristic vs. load

current is given by:

Where RSENSE is the chosen sensing element resistance (Inductor DCR or LS RdsON), IOUT is the output

current of the system and RDROOP is its equivalent output resistance (The whole power supply can be then

represented by a "real" voltage generator with a voltage value of VID and an equivalent series resistance

RDROOP).

RFB resistor can be also designed according to the RDROOP specifications as follow:

8.2

OFFSET

The OFFSET pin allows programming a positive or a negative offset (VOS) for the output voltage.

When the Integrated Thermal Sensor is disabled (TC = SGND) a resistor ROFFSET connected vs. SGND

increases the output voltage: since the pin is internally fixed at 1.240V, the current programmed by the

resistor ROFFSET is mirrored and then properly subtracted from the IFB current (see Figure 11) as follow

(Only the IOFFSET contribute to IFB has been considered):

Figure 11. Voltage Positioning with Offset

The device will add the programmed offset VOS to the output programmed voltage (considering now also

the droop effect) subtracting the relative offset current from the feedback current IFB:

Offset resistor can be designed by considering the following relationship (RFB is fixed by the Droop effect):

OUT

VID

FB IDROOP

⋅

–

OUT

VID

FB IDROOP

⋅

–

VID

SENSE

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

OUT

⋅⋅

–

VID

DROOP IOUT

⋅

–

DROOP

SENSE

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

⋅

OUT

VID

FB IOFFSET

⋅

VID

1.240V

OFFSET

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

⎝⎠

⎛⎞

⋅

VID

1.240V

1:1

ROFFSET

OFFSET

IDROOP

VID

COMP

VSEN

64k

FBR

FBG

64k

RFB

To Vcore

(Remote Sense)

IOFFSET

OUT

VID

FB IFB

⋅

–

VID

DROOP

OFFSET

–

() = VID R

FB IOFFSET

DROOP IOUT

⋅

–

⋅

–

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