MIC2104 datasheet(29/42 Pages) MICROCHIP

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DS20005899A-page 29

MIC2103/4

losses. At higher output loads, the core losses are

usually insignificant and can be ignored. At lower

output currents, the core losses can be a significant

contributor. Core loss information is usually available

from the magnetic vendor.

Copper loss in the inductor is calculated by

Equation 5-13:

EQUATION 5-13:

The resistance of the copper wire, RWINDING, increases

with the temperature. The value of the winding

resistance

used

should

the

operating

temperature.

EQUATION 5-14:

5.4

Output Capacitor Selection

The type of the output capacitor is usually determined

by its ESR (equivalent series resistance). Voltage and

RMS current capability are two other important factors

for selecting the output capacitor. Recommended

capacitor types are tantalum, low-ESR aluminum

electrolytic, OS-CON and POSCAP. The output

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

ripple. The output capacitor ESR also affects the

control loop from a stability point of view. The maximum

value of ESR is calculated:

EQUATION 5-15:

The total output ripple is a combination of voltage

ripples caused by the ESR and output capacitance.

The total ripple is calculated in Equation 5-16:

EQUATION 5-16:

As described in the Theory of Operation subsection in

Functional Description, the MIC2103/4 requires at least

20 mV 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 5-17:

EQUATION 5-17:

The power dissipated in the output capacitor is:

EQUATION 5-18:

5.5

Input Capacitor Selection

The input capacitor for the power stage input VIN

should be selected for ripple current rating and voltage

rating. Tantalum input capacitors may fail when

INDUCTOR Cu



LRMS



WINDING



WINDING Ht



WINDING 20

C



1 0.0042

T20C

–









Where:

TH = Temp. of wire under full load.

T20°C = Ambient temperature.

RWINDING(20°C) = Room temperature winding

resistance (usually specified by

the manufacturer).

ESR

COUT

OUT PP





LPP





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



Where:

∆VOUT(PP) = Peak-to-peak output voltage ripple.

∆IL(PP) = Peak-to-peak inductor current ripple.

OUT PP





LPP





OUT



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





LPP





ESR

COUT





Where:

COUT = Output capacitance value.

fSW = Switching frequency.

COUT RMS



LPP





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

DISS COUT



COUT RMS



ESR

COUT



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