L6751B datasheet(31/58 Pages) STMICROELECTRONICS

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L6751B

Output voltage positioning

Doc ID 024028 Rev 1

31/58

Figure 7 shows the current sense circuit used to implement the load-line. The current flow-

ing across the inductor(s) is read through the R-C filter across the CSxP and CSxN pins. RG

programs a trans-conductance gain and generates a current ICSx proportional to the current

of the phase. The sum of the ICSx current, with proper gain eventually adjusted by the

PMBus commands, is then sourced by the FB pin (IDROOP). RFB gives the final gain to pro-

gram the desired load-line slope (Figure 6).

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 avoid-

ing 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 4

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

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

Equation 5

Caution:

When in DDR mode, and enabled, droop current has a scaling factor equal to 1/4. All the

above equations must be scaled accordingly.

6.4

Single-phase section - disable

The single-phase section can be disabled by pulling high the SPWM pin. The related

command is rejected.

6.5

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 6.2, the current that flows from the SCSN pin is

then given by the following equation (see Figure 7):

Equation 6

6.6

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 depen-

OUT

VID

DROOP

⋅

–

VID

DCR

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

OUT

⋅⋅

–

VID

OUT

⋅

–

DCR

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

⋅

SCSN

DCR

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

SOUT

⋅

SDROOP

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