L6713A Datasheet, PDF (36/64 Pages) Holtek Semiconductor Inc – 2/3 Phase controller with embedded drivers for Intel VR10, VR11 and AMD 6 bit CPUs

Voltage positioning
L6713A
12.2
Droop function (Optional)
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 11, 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 beacomes a
requirement of the load.
Connecting DROOP pin and FB pin together, the device forces a current I_DROOP,
proportional to the read current, into the feedback resistor (R_FB+R_OFFSET) implementing the
load regulation dependence. Since I_DROOP depends on the current information about the N
phases, the output characteristic vs. load current is then given by (neglecting the OFFSET
voltage term):
V_OUT = V_REF – (R_FB + R_OFFSET) ⋅ I_DROOP
V_REF – (R_FB + R_OFFSET) ⋅
D_-----C_-----R_---
Rg
⋅
I_OUT = V_REF – R_DROOP ⋅
I_OUT
Where DCR is the inductor parasite resistance (or sense resistor when used) and I_OUT is the
output current of the system. The whole power supply can be then represented by a "real"
voltage generator with an equivalent output resistance R_DROOP and a voltage value of V_REF.
R_FB resistor can be also designed according to the R_DROOP specifications as follow:
R_FB = R_DROOP ⋅
---^R-----^g-----
DCR
–
R_OFFSET
Droop function is optional, in case it is not desired, the DROOP pin can be disconnected
from the FB and an information about the total delivered current becomes available for
debugging, and/or current monitoring. When not used, the pin can be shorted to SGND.
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