MIC2132YML datasheet(31/48 Pages) MICROCHIP

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DS20006654B-page 31

MIC2132

5.2

Output Capacitor Selection

The output capacitor is usually determined by its

capacitance and Equivalent Series Resistance (ESR).

Voltage and RMS current capability are two other

important factors in selecting the output capacitor.

Recommended capacitor types are ceramic, low-ESR

aluminum electrolytic, OS-CON and POSCAP. The

output capacitor’s ESR is usually the main cause of the

output ripple voltage in steady state, while the total

output capacitance must be large enough to sustain and

maintain the output voltage during load transient to meet

the desired load transient output voltage requirement.

To determine the required output capacitance for a

two-phase buck converter in steady state, peak-to-peak

output ripple current as seen by the output capacitors

must be known. The peak-to-peak output ripple current

for single-phase, two-phase, four-phase, six-phase and

eight-phase buck converters is shown in Figure 5-1. The

graph shows that peak-to-peak output ripple current, nor-

malized by the maximum value, is a function of the duty

cycle. Since each channel is 180 degrees out of phase

with the other for a two-phase buck converter, the

two-phase peak-to-peak output ripple current is less than

that for a single-phase converter and the ripple current

effective frequency is doubled, as seen by the output

capacitor. This is the ripple reduction effect of two-phase

operation. In addition, at 50% duty cycle, the inductor

ripple currents from each channel cancel each other and

the output ripple current is close to zero.

More ripple reduction can be achieved similarly for multi-

phase operation by stacking MIC2132 devices, up to four

devices together, for multiphase operation from

four-phase up to eight-phase.

FIGURE 5-1:

Normalized Peak-to-Peak

Output Ripple Current vs. Duty Cycle.

The peak-to-peak output ripple current shown in

Figure 5-1 is normalized by the maximum value, which

is used as the normalizing factor for simplifying the

calculation of output ripple current.

The peak-to-peak output ripple current maximum value

and normalizing factor is calculated by Equation 5-7.

EQUATION 5-7:

The approximate peak-to-peak output ripple current of

a given multiphase buck converter at a given duty cycle

can be determined from the corresponding normalized

value for the multiphase buck converter in Figure 5-1,

multiplied by the normalizing factor, as shown in

Equation 5-8.

EQUATION 5-8:

The total output ripple voltage is a combination of the

ripple voltages caused by the ESR and output capaci-

tance. The output ripple voltage of the two-phase buck

converter in steady state can then be determined from

Equation 5-9.

EQUATION 5-9:

The minimum output capacitance required for a

two-phase buck converter in steady state can be

estimated by Equation 5-10.

EQUATION 5-10:

To meet the load transient requirement, the output

capacitance

must

also

fulfill

the

criteria

Equation 5-11. The output capacitance value chosen

must meet the criteria in both equations.

IOPP(MAX) =

VOUT

L × fSW

Where:

ΔIOPP(NORMALIZED) = Normalized Peak-to-Peak

Output Ripple Current Value for

Given Multiphase Buck

Converter at Given Duty Cycle

in Figure 5-1

IOPP = IOPP(NORMALIZED) × IOPP(MAX)

Where:

ΔVOUT(PP) = Peak-to-Peak Output Ripple Voltage

ΔIOPP = Peak-to-Peak Output Ripple Current

COUT = Output Capacitance

fSW = Switching Frequency per Phase

ESRCOUT = ESR of Output Capacitor

VOUT(PP) =

IOPP

16 × COUT × fSW

2 + (IOPP × ESRCOUT)2

√





COUT ≥

IOPP

16 ×

VOUT(PP) × fSW

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