L6706 datasheet(21/47 Pages) STMICROELECTRONICS

21 / 47 page

L6706

Voltage positioning

Doc ID 15698 Rev 2

21/47

where:

Offset resistor can be designed by considering the following relationship (RFB is fixed by the

Droop effect):

Offset automatically given by the DAC selection differs from the offset implemented through

the OFFSET pin: the built-in feature is trimmed in production and assures ±0.5% error over

load and line variations

Figure 7.

Voltage positioning with positive offset

8.2

Droop function

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 6, 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 becomes a

requirement of the load.

The device forces a current IDROOP, proportional to the read current, into the feedback RFB

resistor implementing the load regulation dependence. Since IDROOP depends on the

current information, the output characteristic vs. load current is then given by (neglecting the

OFFSET voltage term):

Where DCR is the inductor parasitic resistance (or sense resistor when used) and IOUT is

the output current of the system. The whole power supply can be then represented by a

OUT IOUT

()

PROG

DROOP IOUT

()

1.240V

OFFSET

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

–

⋅

–

1.240V

OFFSET

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

⋅

OFFSET

1.240V

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

⋅

COMP

To GND_core

(Remote Sense)

ERROR AMPLIFIER

FBG

VSEN

DROOP

To VCC_core

(Remote Sense)

RFB

VPROG

VREF

FBG

GND DROP

RECOVERY

OFFSET

1.240V

ROFFSET

OFFSET

OUT

PROG

DROOP

⋅

–

REF

DCR

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

OUT

⋅⋅

–

PROG

DROOP

OUT

⋅

–

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