LX1676(2005) Просмотр технического описания (PDF) - Microsemi Corporation
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LX1676 Datasheet PDF : 16 Pages
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LX1676
TM
®
Mobile AMD Athlon™ VRM Controller
PRODUCTION DATA SHEET
THEORY OF OPERATION (CONTINUED)
THEORY OF OPERATION FOR A BI-PHASE,
LOADSHARETM CONFIGURATION
The basic principle used in LoadSHARETM in a multiple
phase buck converter topology is that if multiple, identical,
inductors have the same identical voltage impressed across
their leads, they must then have the same identical current
passing through them. The current that we would like to
balance between inductors is mainly the DC component
along with as much as possible the transient current. All
inductors in a multiphase buck converter topology have
their output side tied together at the output filter capacitors.
Therefore this side of all the inductors has the same
identical voltage.
If the input side of the inductors can be forced to have
the same equivalent DC potential on this lead, then they
will have the same DC current flowing. To achieve this
requirement, phase 1 will be the control phase that sets the
output operating voltage, under normal PWM operation.
To force the current of phase 2 to be equal to the current of
phase 1; a second feedback loop is used. Phase 2 has a low
pass filter connected from the input side of each inductor.
This side of the inductors has a square wave signal that is
proportional to its duty cycle. The output of each LPF is a
DC (+ some AC) signal that is proportional to the
magnitude and duty cycle of its respective inductor signal.
The second feedback loop will use the output of the
phase 1 LPF as a reference signal for an error amplifier
that will compare this reference to the output of the phase 2
LPF. This error signal will be amplified and used to
control the PWM circuit of phase 2. Therefore, the duty
cycle of phase 2 will be set so that the equivalent voltage
potential will be forced across the phase 2 inductor as
compared to the phase 1 inductor. This will force the
current in the phase 2 inductor to follow and equal the
phase 1 inductor current.
With the LoadSHARETM topology it is possible to
imbalance the phases so that one phase will supply more
current than the other under unique situations. The LX1676
will normally be used with the same supply voltages on
phase 1 and 2 PWM inputs and will have equal currents in
both phases.
LP1
DC Bias
LP2
Phase 1
Low Pass Filter
Differential Feedbach From Vout
DC Bias
+
-
Differential
Amp
EA -
Phase 1 Diff
Amp
DACOUT
DC Bias
Phase 2
Integrating
Amp
Phase 1 Error
Amp
0°
-
+
+
_
Phase 1
Comparator
_
gm
180°
+
Phase 2
Ramp .75 V to 3 V Comparator
Both PWMs
EO2
Phase 2
Low Pass Filter
HO1
LO1
HO2
LO2
Vout
Figure 5 – LoadSHARE Control Loop
Copyright © 2000
Revision: 1.0, 4/12/2005
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 11
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