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ISL6566 Fiches technique(PDF) 14 Page - Intersil Corporation

No de pièce ISL6566
Description  Three-Phase Buck PWM Controller with Integrated MOSFET Drivers for VRM9, VRM10, and AMD Hammer Applications
Download  28 Pages
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Fabricant  INTERSIL [Intersil Corporation]
Site Internet  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL6566 Fiches technique(HTML) 14 Page - Intersil Corporation

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14
FN9178.3
July 25, 2005
As shown in Figure 6, a voltage, VDROOP, proportional to the
total current in all active channels, IOUT, 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, VDROOP, 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,
IL, flowing through the inductor, passes through the DCR.
Equation 5 shows the s-domain equivalent voltage, VL,
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,
VDROOP, can be shown to be proportional to the channel
currents IL1, IL2, and IL3, 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 VDROOP is equal to the sum of the voltage
drops across the individual DCRs, multiplied by a gain. As
Equation 7 shows, VDROOP is therefore proportional to the
total output current, IOUT.
By simply adjusting the value of RS, 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
RCOMP 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 ISL6566 allows the designer to accurately adjust the
offset voltage by connecting a resistor, ROFS, from the OFS
pin to VCC or GND. When ROFS is connected between OFS
and VCC, the voltage across it is regulated to 1.5V. This
causes a proportional current (IOFS) to flow into the OFS pin
and out of the FB pin. If ROFS is connected to ground, the
voltage across it is regulated to 0.5V, and IOFS 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 (IOFS x
RFB). These functions are shown in Figures 8 and 9.
VL s
()
IL sL DCR
+
()
=
(EQ. 5)
VDROOP s
()
sL
DCR
------------- 1
+


sRCOMP CCOMP
⋅⋅
1
+
()
--------------------------------------------------------------------------
RCOMP
RS
-----------------------
IL1 IL2 IL3
++
() DCR
⋅⋅
=
(EQ. 6)
VDROOP
RCOMP
RS
--------------------- IOUT DCR
⋅⋅
=
(EQ. 7)
FIGURE 7. DCR SENSING CONFIGURATION
ICOMP
RS
DCR
L
INDUCTOR
VOUT
COUT
IL1
VL(s)
DCR
L
INDUCTOR
DCR
L
INDUCTOR
PHASE1
PHASE2
PHASE3
IL2
IL3
RS
RS
RCOMP
CCOMP
ISUM
IREF
ISL6566
-
+
VDROOP
IOUT
(optional)
ISL6566


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