
Keywords: MAX5038,asica,microprocessors,highperformance
Related Parts


Powering HighPerformance ASICs and Microprocessors

Abstract: Today's highest performing ASICs and microprocessors can consume greater than 150W. With supply voltages of 1V to 1.5V, the required current for these devices can easily exceed 100A. By using multiphase DCDC converters, the task of providing power to these devices is made more manageable.
Currently, scalable powersupply controllers are available that allow the designer to choose the number of phases for specific DCDC converters. Scalability allows several controllers to be paralleled and synchronized. Onboard PLLbased clock generators allow the controllers to be synchronized.
Multiphase Topologies
While no hardandfast power limit exists for a singlephase
buck regulator, the advantages of designing with
multiphase converters become apparent as load currents
rise above 20A to 30A. These advantages include:
reduced inputripple current, substantially decreasing the
number of input capacitors; reduced outputripple voltage
due to an effective multiplication of the ripple frequency;
reduced component temperature achieved by distributing
the losses over more components; and reducedheight
external components.
Multiphase converters are essentially multiple buck regulators
operated in parallel with their switching frequencies
synchronized and phase shifted by 360/n degrees, where n
identifies each phase. Paralleling converters makes output
regulation slightly more complex. This problem is easily
solved with a currentmode control IC that regulates each
inductor current in addition to the output voltage.
InputRipple Current
The key issue designers face when selecting input capacitors
is inputripplecurrent handling. Inputripple current is
substantially reduced by using a multiphase topology—the
input capacitor of each phase conducts a lower amplitude
inputcurrent pulse. Also, phase shifting increases the
effective duty cycle of the current waveform, which results
in a lower RMS ripple current. The ripplecurrent levels
shown in Table 1 demonstrate the ripplecurrent reduction
and the inputcapacitor savings.
Highk dielectric ceramic capacitors provide the best
ripplecurrent handling and the smallest PCB footprint.
Ceramic devices housed in an 1812 form factor exhibit
ripplecurrent ratings of 2A to 3A per capacitor. Electrolytic
capacitors are a good choice for costsensitive designs.
OutputRippleVoltage Reduction
Accuracy requirements of <2% are commonly required for
core voltage supplies. For a 1.2V supply, this translates
to a ±25mV outputvoltage window. A technique for using
the outputvoltage window more effectively is called active
voltage positioning. At light loads, the converter regulates
the output voltage above the midpoint of the outputvoltage
window and, at heavy loads, regulates the output
voltage below the midpoint of the outputvoltage window.
In the case of a ±25mV window, regulating at the high
end (low end) of the range during light loads (heavy
loads) allows the entire outputvoltage window to be
used for a stepload increase (decrease).
Large loadcurrent steps require both extremely lowESR
capacitors to minimize transients and large enough capacitance
to absorb the stored energy of the main inductor
during a stepload decrease. Organic polymer chemistries
have improved lowESR tantalum capacitors. Polymer
capacitors provide the most capacitance with the lowest
ESR. Ceramic capacitors have excellent highfrequency
characteristics, but the total capacitance per device is onehalf
to onequarter that of tantalum and polymer capacitors.
Typically, ceramic capacitors are, therefore, not the
best choice as output capacitors.
LowSide MOSFETs
A 12V to 1.2V converter requires 90% ontime from a
lowside MOSFET; conduction losses dominate switching
losses in this case. For this reason, two or three MOSFETs
are often paralleled. Operating several MOSFETs in
parallel effectively reduces R
_{DS(ON)} and thus lowers
conduction losses. When the MOSFET is turned off,
inductor current continues to flow through the MOSFET's
body diode. Under this condition, the MOSFET drain
voltage is essentially zero, reducing switching losses
substantially. Table 1 shows the losses for several multiphase
configurations. Note that the lowside MOSFET's
total losses decrease as the number of phases increases,
thus reducing the MOSFET's temperature rise.
HighSide MOSFETs
With a duty cycle of 10 percent, highside MOSFETswitching
losses dominate conduction losses. Because the
highside MOSFET conducts for a small percentage of
time, conduction losses are less significant. Thus, low onresistance
is not as important as low switching losses.
During the switching intervals (both t
_{ON} and t
_{OFF}), the
MOSFET has to withstand voltage and conduct current.
The product of this voltage and current determines the
MOSFET peakpower dissipation; therefore, the shorter
the switching interval, the lower the power dissipation.
When selecting a highside MOSFET, choose a MOSFET
with low gate charge and gatedrain capacitance, both of
which are more important than low onresistance. Table 1
illustrates how the total MOSFET losses decrease as the
number of phases increases.
Inductor Selection
The inductor value determines the peaktopeak ripple
current. Allowable ripple current is typically calculated as
a percentage of maximum DCoutput current. In most
applications, an optional ripple current is 20% to 40% of
the maximum DCoutput current.
At low core voltages, the inductor current cannot decrease
as quickly as it can increase. During a load decrease, the
output capacitor can be overcharged, causing an overvoltage
condition. By using an inductor of smaller value
(allowing higher ripple current—closer to 40%), a lower
amount of stored energy is transferred to the output
capacitor, which minimizes voltage surge.
Thermal Design
Table 1 provides estimates of heatsinking requirements
for the number of phases used. In a forcedconvection
cooling system that can provide 100LFM to 200LFM, a
singlephase design would require a fairly large heatsink
to achieve a 0.6°C/W thermal resistance. In the fourphase
design, the thermal resistance can increase to 2°C/W. This
thermal resistance is easily achieved without a heatsink
and 100LFM to 200LFM airflow.
Table 1. Comparison of critical parameters and the number of phases used for the design of
synchronous buck regulators. Example is a 12V to 1.2V, 100A buck regulator.

Number of Phases 

1 
2 
4 
8 
Current per phase
 100A 
50A 
25A 
12.5A 
Input capacitor, 3A rated 
Ripple current 
31.6A 
22A 
15.8A 
11.2A 
Number required 
11 
8 
6 
4 
H/S MOSFET 
RMS ripple current 
31.6A 
15.8A 
7.9A 
3.9A 
Package size 
DPAK 
DPAK 
SO8 
SO8 
Number required 
2 
2 (1/ph) 
8 (2/ph) 
8 (1/ph) 
Power dissipation (each) 
22W 
1.8W 
0.32W 
0.22W 
Total power dissipation 
4.4W 
3.6W 
2.5W 
1.76W 
L/S MOSFET 
RMS ripple current (each) 
94.8A 
47.4A 
23.7A 
11.9A 
Package Size 
DPAK 
DPAK 
SO8 
SO8 
Number required 
3 
2 (1/ph) 
8 (2/ph) 
8 (1/ph) 
Power dissipation (each) 
6W 
12W 
1.4W 
1W 
Total power dissipation 
18W 
24W 
11.2W 
8W 
C_{OUT} 470µF, 10m 
Number required 
7 
7 
7 
7 
V_{SS} ripple 
22mV 
11mV 
5mV 
1mV 
Heatsink capacity 
0.6°C/W 
1°C/W 
2°C/W 
4°C/W 
Estimated efficiency 
69 
77 
85 
89 
Design Example
Figure 1 illustrates the MAX5038 configured as a fourphase
DCDC converter. The MAX5038 master remotevoltage
sense input (VSP to VSN pins) provides a signal
(DIFF) to both the master and the slave EAN inputs,
enabling parallel operation. The MAX5038 master also
provides a clock (CLKOUT) to the MAX5038 slave
controller. By floating the PHASE pin, the slave locks on
to the CLKIN signal with a 90° phase shift. The error
amplifier also performs the active voltagepositioning
function by setting the gain of the voltageerror amplifier.
Using precision gainsetting resistors ensures accurate
load sharing. The output of the voltageerror amplifier
(EAOUT) programs the load current of each phase.
Compensation (not shown) is provided for each current
loop at the CLP1 and CLP2 pins, providing a very stable
output for most line and load conditions.
Figure 1. A fourphase example using two MAX5038s. The master performs the remote voltagesense function and the clockgeneration function, which the slave controller uses to increase output current and synchronize the operating frequency.
Conclusion
Multiphase synchronous DCDC converters effectively
power ASICs and processors that require 1V to 1.5V at
100A or more. They solve basic problems involving
capacitor ripple current, MOSFET power dissipation,
transient response, and allowable outputripple voltage.
Related Parts 
MAX5038 
DualPhase, Parallelable, Average CurrentMode Controllers 

© Jul 08, 2004, Maxim Integrated Products, Inc.

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