MIC28513 datasheet(22/34 Pages) MICROCHIP

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MIC28513

DS20005522A-page 22

 2016 Microchip Technology Inc.

MOSFETs. Larger output ripple currents will also

require more output capacitance to smooth out the

larger ripple current. Smaller peak-to-peak ripple

currents require a larger inductance value and

therefore a larger and more expensive inductor. A good

compromise between size, loss and cost is to set the

inductor ripple current to be equal to 20% of the

maximum output current. The inductance value is

calculated by:

EQUATION 5-4:

In continuous conduction mode, the peak inductor

current is equal to the average output current plus one

half of the peak-to-peak inductor current ripple.

EQUATION 5-5:

The RMS inductor current is used to calculate the I2R

losses in the inductor.

EQUATION 5-6:

Maximizing efficiency requires the proper selection of

core material and minimizing the winding resistance.

The high frequency operation of the MIC28513

requires the use of ferrite materials for all but the most

cost sensitive applications. Lower cost iron powder

cores may be used but the increase in core loss will

reduce the efficiency of the power supply. This is

especially noticeable at low output power. The winding

resistance decreases efficiency at the higher output

current levels.

The winding resistance must be minimized although

this usually comes at the expense of a larger inductor.

The power dissipated in the inductor is equal to the sum

of the core and copper 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 magnetics vendor. Copper

loss in the inductor is calculated by Equation 5-7:

EQUATION 5-7:

The resistance of the copper wire, DCR, increases with

the temperature. The value of the winding resistance

used should be at the operating temperature.

EQUATION 5-8:

5.4

Output Capacitor Selection

The type of the output capacitor is usually determined

by its equivalent series resistance (ESR). Voltage and

RMS current capability are also important factors in

selecting an output capacitor. Recommended capacitor

types are ceramic, tantalum, low-ESR aluminum

electrolytic, OS-CON and POSCAP. For high ESR

electrolytic capacitors, ESR is the main cause of the

output ripple. The output capacitor ESR also affects the

control loop from a stability point of view. For a low ESR

ceramic output capacitor, ripple is dominated by the

reactive impedance. The maximum value of ESR is

calculated by Equation 5-9.

EQUATION 5-9:

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated by

Equation 5-10.

EQUATION 5-10:

OUT

IN MAX



OUT

–





IN MAX



LPP







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

Where:

fSW

Switching Frequency

∆IL(PP)

The peak-to-peak inductor current

ripple; typically 20% of the maximum

output current

LPK



OUT

0.5

LPP







LRMS



OUT MAX



LPP





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

LCU



LRMS



DCR



DCR



DCR

20C



10.0042

20C

–









Where:

Temperature of wire under full load

T20C

Ambient temperature

DCR(20C)

Room temperature winding resistance

(usually specified by the manufacturer)

ESR

OUT

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:

Duty Cycle

COUT

Output Capacitance Value

fSW

Switching Frequency

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