MIC2133 datasheet(34/50 Pages) MICROCHIP

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DS20006653B-page 34

MIC2133

The maximum value of the overall ESR of the output

capacitor in steady state is calculated in the equation

below.

EQUATION 5-12:

The maximum overall ESR value of the output

capacitor must also meet the load transient

requirement and is calculated in the equation below.

Then, the lower value must be chosen for the output

capacitor ESR.

EQUATION 5-13:

As described in Section 4.1 “Control Architecture”,

the MIC2133 requires at least 20 mV peak-to-peak rip-

ple at the FBS pin to make the gm amplifier and the error

comparator behave properly.

Also, it is recommended that the output voltage ripple

be in phase with the inductor current.

Therefore, it is recommended that the output voltage

ripple caused by the output capacitor’s value be much

smaller than the ripple caused by the output capacitor’s

ESR. If low-ESR capacitors, such as ceramic capaci-

tors, are selected as the output capacitors, a ripple

injection method must be applied to provide enough

feedback voltage ripple. Refer to Section 4.4 “Ripple

Injection Circuit Components Selection” for more

details.

It is recommended that the voltage rating of the output

capacitor be 25% greater than the maximum output

voltage. The output capacitor RMS current is calcu-

lated in the equation below.

EQUATION 5-14:

The power dissipated in the output capacitor is

calculated in the equation below.

EQUATION 5-15:

5.3

Input Capacitor Selection

In addition to high-frequency ceramic capacitors, it is

recommended that a larger bulk capacitance, either

ceramic or aluminum electrolytic, be used to help

attenuate ripple on the input and to supply current to

the input during large output current transients. It is

recommended that the input capacitor for the power

stage input, VIN, be selected for the ripple voltage at

VIN, capacitance, ESR, ripple current rating and

voltage rating. Tantalum input capacitors may fail when

subjected to high inrush currents caused by turning the

input supply on. A tantalum input capacitor’s voltage

rating must be at least two times the maximum input

voltage to maximize reliability. Aluminum electrolytic,

OS-CON and multilayer polymer film capacitors can

handle the higher inrush currents without voltage

derating. Due to the ripple cancellation effect of the

two-phase buck converter, the input ripple voltage and

ripple current are smaller than those of the

single-phase converter, and the effective ripple

frequency seen by the input capacitor is twice the

switching frequency. The input ripple voltage depends

on the IOUT and input capacitor’s capacitance and

ESR. The steady state input voltage ripple can be

estimated by the equation below.

EQUATION 5-16:

The capacitance of the input capacitor can be

determined in the equation below.

EQUATION 5-17:

The ESR of the total input capacitance can be

determined in the equation below.

EQUATION 5-18:

The input capacitor must be rated for the input current

ripple. The rated RMS value of the input capacitor cur-

rent is determined at the maximum output current.

ESRCOUT

V

OUT PP



I

OPP

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



ESRCOUT

V

OUT TRANS



I

LOAD

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



ICOUT RMS



I

OPP

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

PDISS COUT



ICOUT RMS



ESR



COUT

Where:

IOUT = Total Output Current

D = Duty Cycle per Phase

n = Total Number of Phases

k = 0,1 for D > k/n and k < n

fSW

Switching Frequency per Phase

CIN = Total Input Capacitance

ESRCIN = Equivalent Series Resistance of Input

Capacitor

CIN

IOUT D

---

–





---

–





–



n fSW



V



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



Where:

k = 0,1 for D > k/n and k < n

n = Total Number of Phases

ESRCIN VIN

IOUT

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





VIN ≈

n × fSW × CIN









IOUT × D –×

[]









– D –

× ESRCIN

IOUT

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