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by Wei Chen and Craig Varga
Introduction
As more functions are integrated into one IC, the power drawn by a single
IC can easily exceed the capability of a single input power source.
Redesigning the front-end power supply to increase the supply's capability
will take time and money. Another solution is to use several available
power sources to obtain the required output power, drawing some percentage
of the total power from each source. The LTC1929 PolyPhaseTM regulator
provides a simple solution to this problem.
Design Details
The LTC1929 is a PolyPhase dual, current mode controller. It is
capable of driving two synchronous buck channels 180 degrees out of phase
to reduce output switching ripple current and
voltage. One buck stage receives its input power from the
12V input and the other receives its power from the 5V input. In a 2-phase
design, as the inductor current in the 5V circuit increases, the inductor
current in the 12V circuit decreases. This results in a smaller net
ripple current flowing into the output capacitor. Since there are two
intervals in one switching period where ripple cancellation takes place,
the output ripple voltage of the 2-phase design is much smaller than that of
a single-phase design, and fewer output capacitors can be used.
A Typical Application
The currents available from a PCI connector are limited to 2A for the
5V supply and 1A for the 12V supply. In the example shown here, the load
can be as high as 6A or 16.8W at 2.8V. Neither the 5V nor the 12V source
is capable of providing this power. Hence, it is desirable to design
a power supply that can draw currents from two power sources and
whose maximum input currents from each source will not exceed the
corresponding limit. This design shows how to easily accomplish this using
the LTC1929 PolyPhase controller. With only one IC, two MOSFETs and
two small inductors, a high efficiency, low noise power supply can be obtained.
Figure 1 shows the schematic diagram of the complete power
supply. Since each buck circuit only supplies about 3.5A maximum, dual
MOSFETs such as the Fairchild FDS6990A can be used. A total of two
SO-8 MOSFETs are needed. The switching frequency is about 300kHz
per-channel for an effective output ripple frequency of 600kHz. The
inductors in both stages are 7µH. The design uses Sumida CEE125-7R0
inductors, but any inductor with a similar inductance value and 4A or greater current rating should do the job.
The current sense resistor is 0.007W for each channel.
Figure 1. LTC1929 PCI-bus powered, dual-input PolyPhase supply
Test Results
Figure 2 shows the overall efficiency vs load currents. For most of the
load range, the efficiency is above 90%. Figure 3 shows the distribution of
two input currents as the load current varies. The maximum input
currents for the 5V and 12V sources are 1.66A and 0.84A, respectively, which
are well below the PCI connector's current limits. Figure 4 shows
the waveforms of the inductor ripple currents and output ripple voltages.
Note the ripple cancellation phenomenon. The peak-to-peak switching
ripple voltage at the output terminal is only
50mVP-P with one 1500µF/6.3V aluminum electrolytic capacitor. If
two buck circuits are synchronized in phase, the ripple voltage will
be 70mVP-P, almost a 50% increase.
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