L6917 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6917' PDF : 33 Pages View PDF
L6917B
Integrated Droop Function
The device uses a droop function to satisfy the requirements of high performance microprocessors, reducing
the size and the cost of the output capacitor.
This method "recovers" part of the drop due to the output capacitor ESR in the load transient, introducing a de-
pendence of the output voltage on the load current
As shown in figure 6, the ESR drop is present in any case, but using the droop function the total deviation of the
output voltage is minimized. In practice the droop function introduces a static error (Vdroop in figure 6) propor-
tional to the output current. Since the device has an average current mode regulation, the information about the
total current delivered is used to implement the Droop Function. This current (equal to the sum of both IINFOx)
is sourced from the FB pin. Connecting a resistor between this pin and Vout, the total current information flows
only in this resistor because the compensation network between FB and COMP has always a capacitor in series
(See fig. 7). The voltage regulated is then equal to:
VOUT = VID - RFB · IFB
Since IFB depends on the current information about the two phases, the output characteristic vs. load current is
given by:
VOUT = VID – RFB ⋅ -R----S---R-E---Ng----S----E- ⋅ IOUT
Figure 6. Output transient response without (a) and with (b) the droop function
ESR DROP
ESR DROP
VNOM
VMAX
VMIN
VDROOP
(a)
(b)
Figure 7. Active Droop Function Circuit
COMP
FB
RFB
To VOUT
I FB
VPROG
The feedback current is equal to 50µA at nominal full load (IFB = IINFO1 + IINFO2) and 70µA at the OC threshold,
so the maximum output voltage deviation is equal to:
∆VFULL_POSITIVE_LOAD = +RFB · 50µA
∆VPOSITIVE_OC_THRESHOLD = +RFB · 70µA
Droop function is provided only for positive load; if negative load is applied, and then IINFOx < 0, no current is
sunk from the FB pin. The device regulates at the voltage programmed by the VID.
13/33
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