Dual-Phase, Parallelable, Average-Current-Mode

Controllers

18

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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

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

Parallel Operation

For applications requiring large output current, parallel

up to three MAX5038A/MAX5041As (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 MAX5038A/MAX5041A 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 (two paralleled converters), or connect

PHASE to SGND for 60° of phase shift (three 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 MAX5038As. 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 MAX5038A/MAX5041A circuit drives two 180° out-

of-phase channels. Parallel two or three MAX5038A/

MAX5041A 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:

Peak-to-Peak Inductor Current (

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:

≈ K/D

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

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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