L6718 datasheet(52/71 Pages) STMICROELECTRONICS

52 / 71 page

Output voltage positioning

L6718

52/71

DocID023399 Rev 3

Time constant matching between the inductor (L/DCR) and the current reading filter (RC) is

required to implement a real equivalent output impedance of the system, therefore avoiding

over and/or undershoot of the output voltage as a consequence of a load transient. The

output voltage characteristic vs. load current is then given by:

Equation 3

where RLL is the resulting load-line resistance implemented by the multi-phase section.

The RFB resistor can then be designed according to the RLL specifications, as follows:

Equation 4

8.3

Single-phase section - current reading

The single-phase section performs the same differential current reading across DCR as the

multi-phase section. According to Section 8.1, the current that flows from the SCSN pin is

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

Equation 5

8.4

Single-phase section - defining load-line

This method introduces a dependence of the output voltage on the load current recovering

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.

Figure 11 shows the current sense circuit used to implement the load-line. The current

flowing across the inductor DCR is read through RSG. This resistor programs a trans-

conductance gain and generates a current ISDROOP proportional to the current delivered by

the single-phase section that is then sourced from the SFB pin. RSFB gives the final gain to

program the desired load-line slope (Figure 10).

The output characteristic vs. load current is then given by:

Equation 6

where RSLL is the resulting load-line resistance implemented by the single-phase section.

OUT

VID

DROOP

⋅

–

VID

DCR

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

OUT

⋅⋅

–

VID

OUT

⋅

–

DCR

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

⋅

SCSN

DCR

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

SOUT

⋅

SDROOP

SOUT

VID

SFB

SDROOP

⋅

–

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