MAX5038/MAX5041
Dual-Phase, Parallelable, Average Current-Mode
Controllers
18 ______________________________________________________________________________________
Overload Conditions
Average current-mode control has the ability to limit the
average current sourced by the converter during a fault
condition. When a fault condition occurs, the VEA out-
put clamps to +0.9V with respect to the common-mode
voltage (V
CM
= +0.6V) and is compared with the output
of the current-sense amplifiers (CA1 and CA2) (see
Figures 3a and 3b). The current-sense amplifier’s gain
of 18 limits the maximum current in the inductor or
sense resistor to I
LIMIT
= 50mV/R
S
.
Parallel Operation
For applications requiring large output current, parallel
up to three MAX5038/MAX5041s (six phases) to triple
the available output current. The paralleled converters
operate at the same switching frequency but different
phases keep the capacitor ripple RMS currents to a mini-
mum. Three parallel MAX5038/MAX5041 converters
deliver up to 180A of output current. To set the phase
shift of the on-board PLL, leave PHASE unconnected for
90° of phase shift (2 paralleled converters), or connect
PHASE to SGND for 60° of phase shift (3 converters in
parallel). Designate one converter as master and the
remaining converters as slaves. Connect the master and
slave controllers in a daisy-chain configuration as shown
in Figure 6. Connect CLKOUT from the master controller
to CLKIN of the first slaved controller, and CLKOUT from
the first slaved controller to CLKIN of the second slaved
controller. Choose the appropriate phase shift for mini-
mum ripple currents at the input and output capacitors.
The master controller senses the output differential volt-
age through SENSE+ and SENSE- and generates the
DIFF voltage. Disable the voltage sensing of the slaved
controllers by leaving DIFF unconnected (floating).
Figure 7 shows a detailed typical parallel application cir-
cuit using two MAX5038s. This circuit provides four
phases at an input voltage of +12V and an output volt-
age range of +1V to +3.3V at 104A.
Applications Information
Each MAX5038/MAX5041 circuit drives two 180° out-of-
phase channels. Parallel two or three MAX5038/
MAX5041 circuits to achieve four- or six-phase opera-
tion, respectively. Figure 1 shows the typical application
circuit for a two-phase operation. The design criteria for
a two-phase converter includes frequency selection,
inductor value, input/output capacitance, switching
MOSFETs, sense resistors, and the compensation net-
work. Follow the same procedure for the four- and six-
phase converter design, except for the input and output
capacitance. The input and output capacitance require-
ments vary depending on the operating duty cycle.
The examples discussed in this data sheet pertain to a
typical application with the following specifications:
V
IN
= +12V
V
OUT
= +1.8V
I
OUT(MAX)
= 52A
f
SW
= 250kHz
Peak-to-Peak Inductor Current (∆I
L
) = 10A
Table 1 shows a list of recommended external compo-
nents (Figure 1) and Table 2 provides component sup-
plier information.
Number of Phases
Selecting the number of phases for a voltage regulator
depends mainly on the ratio of input-to-output voltage
(operating duty cycle). Optimum output-ripple cancella-
tion depends on the right combination of operating duty
cycle and the number of phases. Use the following
equation as a starting point to choose the number of
phases:
N
PH
≈ K/D
where K = 1, 2, or 3 and the duty cycle is D = V
OUT
/V
IN.
Choose K to make N
PH
an integer number. For exam-
ple, converting V
IN
= +12V to V
OUT
= +1.8V yields
better ripple cancellation in the six-phase converter
than in the four-phase converter. Ensure that the output
load justifies the greater number of components for
multiphase conversion. Generally limiting the maximum
output current to 25A per phase yields the most cost-
effective solution. The maximum ripple cancellation
occurs when N
PH
= K/D.
Single-phase conversion requires greater size and power
dissipation for external components such as the switch-
ing MOSFETs and the inductor. Multiphase conversion
eliminates the heatsink by distributing the power dissipa-
tion in the external components. The multiple phases
operating at given phase shifts effectively increase the
switching frequency seen by the input/output capacitors,
thereby reducing the input/output capacitance require-
ment for the same ripple performance. The lower induc-
tance value improves the large-signal response of the
converter during a transient load at the output. Consider
all these issues when determining the number of phases
necessary for the voltage regulator application.
Inductor Selection
The switching frequency per phase, peak-to-peak rip-
ple current in each phase, and allowable ripple at the
output determine the inductance value.
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