參數(shù)資料
型號(hào): ISL6568CRZA-T
廠商: INTERSIL CORP
元件分類(lèi): 穩(wěn)壓器
英文描述: VP Fixed Series, 500 Ohms, 50.0 W, Fixed, Wirewound Resistor
中文描述: SWITCHING CONTROLLER, 1500 kHz SWITCHING FREQ-MAX, PQCC32
封裝: 5 X 5 MM, LEAD FREE, PLASTIC, MO-220-VHHD, MLPL, QFN-32
文件頁(yè)數(shù): 14/28頁(yè)
文件大小: 716K
代理商: ISL6568CRZA-T
14
FN9187.3
July 25, 2005
Load-Line (Droop) Regulation
Some microprocessor manufacturers require a precisely-
controlled output impedance. This dependence of output
voltage on load current is often termed “droop” or “l(fā)oad line”
regulation.
As shown in Figure 6, a voltage, V
DROOP
, proportional to the
total current in all active channels, I
OUT
, feeds into the
differential remote-sense amplifier. The resulting voltage at
the output of the remote-sense amplifier is the sum of the
output voltage and the droop voltage. As Equation 4 shows,
feeding this voltage into the compensation network causes
the regulator to adjust the output voltage so that it’s equal to
the reference voltage minus the droop voltage.
The droop voltage, V
DROOP
, is created by sensing the
current through the output inductors. This is accomplished
by using a continuous DCR current sensing method.
Inductor windings have a characteristic distributed
resistance or DCR (Direct Current Resistance). For
simplicity, the inductor DCR is considered as a separate
lumped quantity, as shown in Figure 7. The channel current,
I
L
, flowing through the inductor, passes through the DCR.
Equation 5 shows the s-domain equivalent voltage, V
L
,
across the inductor.
The inductor DCR is important because the voltage dropped
across it is proportional to the channel current. By using a
simple R-C network and a current sense amplifier, as shown
in Figure 7, the voltage drop across all of the inductors’
DCRs can be extracted. The output of the current sense
amplifier, V
DROOP
, can be shown to be proportional to the
channel currents I
L1
and I
L2
, shown in Equation 6.
If the R-C network components are selected such that the
R-C time constant matches the inductor L/DCR time
constant, then V
DROOP
is equal to the sum of the voltage
drops across the individual DCRs, multiplied by a gain. As
Equation 7 shows, V
DROOP
is therefore proportional to the
total output current, I
OUT
.
By simply adjusting the value of R
S
, the load line can be set
to any level, giving the converter the right amount of droop at
all load currents. It may also be necessary to compensate for
any changes in DCR due to temperature. These changes
cause the load line to be skewed, and cause the R-C time
constant to not match the L/DCR time constant. If this
becomes a problem a simple negative temperature
coefficient resistor network can be used in the place of
R
COMP
to compensate for the rise in DCR due to
temperature.
Note:
An optional 10nF ceramic capacitor from the ISUM pin
to the IREF pin is recommended to help reduce any noise
affects on the current sense amplifier due to layout.
Output-Voltage Offset Programming
The ISL6568 allows the designer to accurately adjust the
offset voltage by connecting a resistor, R
OFS
, from the OFS
pin to VCC or GND. When R
OFS
is connected between OFS
and VCC, the voltage across it is regulated to 1.5V. This
causes a proportional current (I
OFS
) to flow into the OFS pin
and out of the FB pin. If R
OFS
is connected to ground, the
voltage across it is regulated to 0.5V, and I
OFS
flows into the
FB pin and out of the OFS pin. The offset current flowing
through the resistor between VDIFF and FB will generate the
desired offset voltage which is equal to the product (I
OFS
x
R
FB
). These functions are shown in Figures 8 and 9.
V
L
s
( )
I
L
s L
DCR
+
(
)
=
(EQ. 5)
VDROOPs
( )
-------------
1
+
1
+
)
-------------------------------------------------------------------------
-----------------------
IL1
IL2
+
(
)
DCR
=
(EQ. 6)
V
DROOP
R
S
--------------------
I
OUT
DCR
=
(EQ. 7)
FIGURE 7. DCR SENSING CONFIGURATION
-
+
ICOMP
DCR
L
INDUCTOR
IL
1
V
OUT
C
OUT
-
+V
L
(s)
DCR
L
INDUCTOR
IL
2
PHASE1
PHASE2
R
S
R
S
R
COMP
C
COMP
ISUM
IREF
ISL6568
-
+
V
DROOP
I
OUT
(optional)
ISL6568
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