MIC2133 datasheet(33/50 Pages) MICROCHIP

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DS20006653B-page 33

MIC2133

5.2

Output Capacitor Selection

The output capacitor is usually determined by its capac-

itance 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 electro-

lytic, OS-CON and POSCAP. The output capacitor’s

ESR is usually the main cause of the output ripple volt-

age in the steady state, while the total output capaci-

tance must be large enough to sustain and maintain the

output voltage during the load transient to meet the

desired load transient output voltage requirement.

To determine the required output capacitance for a

two-phase

buck

converter

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 both a single-phase and

two-phase buck converter is shown in the figure below.

The graph shows that peak-to-peak output ripple cur-

rent, normalized by the maximum value, is a function of

the duty cycle. Each channel is 180 degrees out of

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

therefore, 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 reduc-

tion effect of the 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.

FIGURE 5-1:

Normalized Peak-to-Peak

Output Ripple Current vs. Duty Cycle.

The peak-to-peak output ripple current, shown in the

figure above, is normalized by the maximum value,

which is used as the normalizing factor for simplifying

the calculation of the output ripple current.

The peak-to-peak output ripple current maximum value

and normalizing factor is calculated by the equation

below.

EQUATION 5-7:

The approximate peak-to-peak output ripple current of

a two-phase buck converter at a given duty cycle can

be determined from the corresponding normalized

value for the two-phase buck converter in Figure 5-1,

multiplied by the normalizing factor, as shown in the

equation below.

EQUATION 5-8:

The total output ripple voltage is a combination of the

ripple voltages caused by the ESR and output capaci-

tance. Then, the output ripple voltage of the two-phase

buck converter in the steady state can be determined

from the equation below.

EQUATION 5-9:

The minimum output capacitance required for the

two-phase buck converter in the steady state can be

estimated by the equation below.

EQUATION 5-10:

To meet the load transient requirement, the output

capacitance must also fulfill the criteria in the equation

below. The output capacitance value chosen must

meet the criteria in both equations.

EQUATION 5-11:

I

OPP MAX



VOUT

LfSW



------------------

I

OPP

I

OPP NORMALIZED



I

OPP MAX





Where:

ΔIOPP(NORMALIZED) = Normalized Peak-to-Peak

Output Ripple Current Value for

Two-Phase Buck Converter at

Given Duty Cycle in Figure 5-1

V

OUT PP



I

OPP

16 COUT



fSW



----------------------------------------------





 2

I

OPP ESRCOUT





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

COUT

I

OPP

V

OUT PP





fSW



-------------------------------------------------------



COUT

I

LOAD

V

OUT TRANS







fCO



-------------------------------------------------------------



Where:

ΔILOAD = Output Load Current Step in Load

Transient

ΔVOUT(TRANS) = Output Voltage Change in Load

Transient

fCO = Crossover Frequency, Equal to

About fSW/10

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