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 I
)
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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:
V
= V - R · I
ID FB FB
OUT
Since I depends on the current information about the two phases, the output characteristic vs. load current is
FB
given by:
R
SENSE
=
–
---------------------
V
VID
R
I
OUT
OUT
FB
Rg
Figure 6. Output transient response without (a) and with (b) the droop function
ESR DROP
ESR DROP
VMAX
VDROOP
VNOM
VMIN
(a)
(b)
Figure 7. Active Droop Function Circuit
RFB
To VOUT
COMP
FB
IFB
VPROG
µ
µ
) and 70 A at the OC threshold,
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The feedback current is equal to 50 A at nominal full load (I = I
+ I
FB
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so the maximum output voltage deviation is equal to:
∆
V
µ
∆
µ
V = +R · 70 A
POSITIVE_OC_THRESHOLD FB
= +R · 50 A
FULL_POSITIVE_LOAD
FB
Droop function is provided only for positive load; if negative load is applied, and then I
sunk from the FB pin. The device regulates at the voltage programmed by the VID.
< 0, no current is
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STMICROELECTRONICS [ ST ]
