MIC2133 datasheet(32/50 Pages) MICROCHIP

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DS20006653B-page 32

MIC2133

5.0

APPLICATION INFORMATION

5.1

Inductor Selection

Certain values for inductance, peak and RMS currents

are required to select the output inductor. The input and

output voltages, as well as the inductance value,

determine the peak-to-peak inductor ripple current.

Generally, higher inductance values are used with

higher input voltages. Larger peak-to-peak ripple

currents increase the power dissipation in the inductor

and MOSFETs. Larger output ripple currents also

require more output capacitance to smooth out the

larger ripple current. Smaller peak-to-peak ripple

currents require a larger inductance value, and there-

fore, a larger and more expensive inductor. Higher

switching frequencies allow the use of a small

inductance, but increase power dissipation in the

inductor core and MOSFET switching loss. A good

compromise between size, loss and cost is to set the

inductor ripple current to be equal to 20% of the

maximum DC output current contributed per phase.

The inductance value of the inductor in each phase

channel is calculated by the equation below.

EQUATION 5-1:

The peak-to-peak inductor current ripple in each phase is:

EQUATION 5-2:

The peak inductor current per phase is equal to the

maximum average output current per phase, plus one

half of the peak-to-peak inductor current ripple, as

given in the following equation.

EQUATION 5-3:

The RMS inductor current in each phase is used to

calculate the I2R losses in the inductor per phase.

EQUATION 5-4:

Maximizing efficiency requires selecting the proper core

material and minimizing the winding resistance. The

high-frequency operation of the MIC2133 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 reduces the efficiency

of the buck converter. This is especially noticeable at low

output power. The winding resistance decreases

efficiency at the higher output current levels. The wind-

ing resistance must be minimized, although this usually

comes at the expense of a larger inductor size. 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 the equation below.

EQUATION 5-5:

The resistance of the copper wire, RWINDING, increases

with the temperature.The value of the winding resis-

tance used must be at the operating temperature for

accurate power dissipation estimation, as calculated in

the equation below:

EQUATION 5-6:

VOUT

Eff VIN MAX





VOUT

–





NPH



Eff VIN MAX



fSW



0.2



IOUT MAX





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

Where:

fSW = Switching Frequency, 500 kHz

0.2 = Ratio of AC Ripple Current to

Maximum DC Output Current

Contributed per Phase

VIN(MAX) = Maximum Power Stage Input Voltage

NPH = Total Number of Phases

Eff = Efficiency of the Buck Converter

IOUT(MAX) = Maximum DC Output Current

I

LPP



VOUT

Eff VIN MAX



VOUT

–







Eff VIN MAX





fSW



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

IL_PH PK



IOUTPH MAX



0.5

I

LPP





Where:

IOUTPH(MAX) = Maximum Average DC Output Current

Contributed per Phase

IOUT(MAX) = Maximum Output Current

n = Total Number of Phases

IOUTPH MAX



IOUT MAX



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

IL_PH RMS



IOUTPH MAX



I

LPP



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

PINDUCTOR Cu



IL_PH RMS



RWINDING



RWINDING HT



RWINDING 20C



10.0042

TH T20C

–









Where:

TH = Temperature of Wire Under Full Load

T20°C = Ambient Room Temperature

RWINDING(20°C) = Room Temperature Winding

Resistance (usually specified by the

manufacturer)

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