L6751B Datasheet, PDF (31/58 Pages) STMicroelectronics – Digitally controlled dual PWM for Intel VR12 and AMD SVI

L6751B
Output voltage positioning
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. R_G
programs a trans-conductance gain and generates a current I_CSx proportional to the current
of the phase. The sum of the I_CSx current, with proper gain eventually adjusted by the
PMBus commands, is then sourced by the FB pin (I_DROOP). R_FB 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
V_OUT = VID – R_FB ⋅
I_DROOP = VID – R_FB ⋅
D_-----C_-----R_---
R_G
⋅
I_OUT = VID – R_LL ⋅
I_OUT
where R_LL is the resulting load-line resistance implemented by the multi-phase section.
The R_FB resistor can be then designed according to the R_LL specifications as follows:
Equation 5
R_FB = R_LL ⋅
---^R-----^G-----
DCR
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
I_SCSN
=
D_-----C_----R_---
R_SG
⋅
I_SOUT = I_SDROOP
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-
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