MIC28304 datasheet(27/39 Pages) MICREL

27 / 39 page

Micrel, Inc.

MIC28304

The maximum value of ESR is calculated as in Equation

L(PP)

OUT(pp)

ΔI

ΔV

ESR

OUT

≤

Eq. 6

Where:

ΔVOUT(pp) = Peak-to-peak output voltage ripple

ΔI

L(PP) = Peak-to-peak inductor current ripple

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated in

Equation 7:

(

L(PP)

OUT

L(PP)

OUT(pp)

OUT

ESR

ΔI

ΔV













Eq. 7

Where:

D = Duty cycle

COUT = Output capacitance value

fsw = Switching frequency

As described in the “Theory of Operation” subsection in

Functional Description, the MIC28304 requires at least

20mV peak-to-peak ripple at the FB pin to make the gm

amplifier and the error comparator behave properly. Also,

the output voltage ripple should be in phase with the

inductor current. Therefore, the output voltage ripple

caused by the output capacitors value should be much

smaller than the ripple caused by the output capacitor

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

are selected as the output capacitors, a ripple injection

method should be applied to provide enough feedback

voltage ripple. Please refer to the “Ripple Injection”

subsection for more details.

The voltage rating of the capacitor should be twice the

output voltage for a tantalum and 20% greater for

aluminum electrolytic or OS-CON.

The output capacitor RMS current is calculated in

Equation 8:

ΔI

L(PP)

(RMS)

COUT

Eq. 8

The power dissipated in the output capacitor is:

OUT

(RMS)

)

DISS(C

ESR

Eq. 9

Input Capacitor Selection

The input capacitor for the power stage input PVIN

should be selected for 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

should 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 de-rating. The

input voltage ripple will primarily depend on the input

capacitor’s ESR. The peak input current is equal to the

peak inductor current, so:

ΔV

IN = IL(pk) × ESRCIN

Eq. 10

The input capacitor must be rated for the input current

ripple. The RMS value of input capacitor current is

determined at the maximum output current. Assuming the

peak-to-peak inductor current ripple is low:

OUT(max)

CIN(RMS)

−

≈

Eq.11

The power dissipated in the input capacitor is:

PDISS(CIN) = ICIN(RMS)

2 × ESR

CIN

Eq. 12

The general rule is to pick the capacitor with a ripple

current rating equal to or greater than the calculated

worst (VIN_MAX) case RMS capacitor current. Its voltage

rating should be 20% to 50% higher than the maximum

input voltage. Typically the input ripple (dV) needs to be

kept down to less than ±10% of input voltage. The ESR

also increases the input ripple.

March 25, 2014

Revision 1.1

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