L6718 datasheet(50/71 Pages) STMICROELECTRONICS

50 / 71 page

Output voltage positioning

L6718

50/71

DocID023399 Rev 3

Output voltage positioning

Output voltage positioning is performed by selecting the controller operative-mode (CPU,

DDR, GPU, Jmode, see Section 7 for details) for the two sections and by programming the

droop function effect (see Figure 10). The controller reads the current delivered by each

section by monitoring the voltage drop across the DCR inductors. The current (IDROOP /

ISDROOP) sourced from the FB/SFB pins, directly proportional to the read current, causes

the related section output voltage to vary according to the external RFB / RSFB resistor,

therefore implementing the desired load-line effect.

In DDR mode it is possible to disable or to decrease the droop effect by using CONFIG1

pinstrapping (see Section 6.6.4 for details).

The L6718 embeds a dual remote-sense buffer to sense remotely the regulated voltage of

each section without any additional external components. In this way, the output voltage

programmed is regulated compensating for board and socket losses. Keeping the sense

traces parallel and guarded by a power plane results in common mode coupling for any

picked-up noise.

Figure 10. Voltage positioning

8.1

Multi-phase section - current reading and current sharing

loop

The L6718 embeds a flexible, fully-differential current sense circuitry that is able to read

across the inductor parasitic resistance or across a sense resistor placed in series to the

inductor element. The fully-differential current reading rejects noise and allows the placing

of sensing elements in different locations without affecting the measurement accuracy. The

trans-conductance ratio is issued by the external resistor RG placed outside the chip

between the CSxN pin toward the reading points. The current sense circuit always tracks

the current information, the pin CSxP is used as a reference keeping the CSxN pin to this

voltage. To correctly reproduce the inductor current, an R-C filtering network must be

introduced in parallel to the sensing element. The current that flows from the CSxN pin is

then given by the following equation (see Figure 11):

REFERENCE

COMP

VSEN

RGND

RFB

To VddCORE

(Remote sense)

Protection

Monitor

from DAC...

ROS

AM12884v1

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