L6714 datasheet(37/70 Pages) STMICROELECTRONICS

37 / 70 page

L6714

Voltage positioning

37/70

12.1

Droop function (Optional)

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 11, the ESR drop is present in any case, but using the droop function

the total deviation of the output voltage is minimized. Moreover, more and more high-

performance CPUs require precise load-line regulation to perform in the proper way.

DROOP function is not then required only to optimize the output filter, but also beacomes a

requirement of the load.

Connecting DROOP pin and FB pin together, the device forces a current IDROOP,

proportional to the read current, into the feedback resistor RFB implementing the load

regulation dependence. Since IDROOP depends on the current information about the three

phases, the output characteristic vs. load current is then given by:

Where RSENSE is the chosen sensing element resistance (Inductor DCR or LS RdsON) and

IOUT is the output current of the system.

The whole power supply can be then represented by a "real" voltage generator with an

equivalent output resistance RDROOP and a voltage value of VREF. RFB resistor can be also

designed according to the RDROOP specifications as follow:

Droop function is optional, in case it is not desired, the DROOP pin can be disconnected

from the FB and an information about the total delivered current becomes available for

debugging, and/or current monitoring. When not used, the pin can be shorted to SGND.

Figure 11. Voltage positioning (left) and droop function (right)

OUT

REF

DROOP

⋅

–

REF

SENSE

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

OUT

⋅⋅

–

REF

DROOP

OUT

⋅

–

DROOP

SENSE

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

⋅

VREF

COMP

VSEN

64k

FBR

FBG

64k

RFB

To Vcore

(Remote Sense)

ESR Drop

VMAX

VMIN

VNOM

RESPONSE WITHOUT DROOP

RESPONSE WITH DROOP

DROOP

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