L6711 datasheet(24/50 Pages) STMICROELECTRONICS

24 / 50 page

Voltage positioning

L6711

24/50

Voltage positioning

Output voltage positioning is performed by selecting the reference DAC and by

programming the different contributors to the IFB current (see Figure 9). This current,

sourced from the FB pin, causes the output voltage to vary according to the external RFB

resistor: this allows programming precise output voltage variations depending on the

sensed current (Droop Function) as well as offsets for the regulation.

The three contributors to the IFB current value are:

●

Droop Function (green);

●

Offset (red);

●

Integrated Temperature Compensation (fuchsia).

Moreover, the embedded Remote Buffer allows to precisely programming the output voltage

offsets and variations by recovering the voltage drops across distribution lines.

The output voltage is then driven by the following relationship (IOFFSET sign depends on TC

setting):

Figure 9.

Voltage positioning and droop function

10.1

Droop function

Droop function allows the device to satisfy the requirements of high performance

microprocessors, reducing the size and the cost of the output capacitor. This method

"recovers" part of the drop due to the output capacitor ESR in the load transient, introducing

a dependence of the output voltage on the load current: a static error proportional to the

output current causes the output voltage to vary according to the sensed current. As shown

in figure 4-right, the ESR drop is present in any case, but using the droop function the total

deviation of the output voltage is minimized.

The information about the total current delivered (IDROOP) is sourced from the FB pin (see

Figure 9): connecting 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 9). 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).

VID

⋅

–

VID

D ROOP

OFFSET

–

()

⋅

–

VMAX

VMIN

VNOM

ESRDROP

RESPONSE WITHOUT DROOP

RESPONSE WITH DROOP

IDROOP

VID

COMP

VSEN

64k

FBR

FBG

64k

RFB

To Vcore

(Remote Sense)

IOFFSET

OUT

VID

DR OOP

⋅

–

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