HCF1305-2R2-R datasheet(19/39 Pages) MICROCHIP

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DS20007043B-page 19

MIC24054

5.0

APPLICATION INFORMATION

5.1

Inductor Selection

Values for inductance, peak, and RMS currents are

required to select the output inductor. The input and

output voltages and 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 will

increase the power dissipation in the inductor and

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 5-1:

EQUATION 5-1:

The peak-to-peak inductor current ripple is:

EQUATION 5-2:

The peak inductor current is equal to the average

output current plus one half of the peak-to-peak

inductor current ripple.

EQUATION 5-3:

The RMS inductor current is used to calculate the I2R

losses in the inductor.

EQUATION 5-4:

Maximizing efficiency requires the proper selection of

core material and minimizing the winding resistance.

The high frequency operation of the MIC24054

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 5-5:

EQUATION 5-5:

The resistance of the copper wire, RWIND, increases

with the temperature. The value of the winding

resistance used should be at the operating

temperature.

EQUATION 5-6:

VOUT VIN MAX





VOUT

–







VIN MAX





fSW



20%



IOUT MAX







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

Where:

fSW = Switching frequency, 600 kHz

20% = Ratio of AC ripple current to DC output current

VIN(MAX) = Maximum power stage input voltage

IL PP







VOUT VIN MAX





VOUT

–







VIN MAX





fSW



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

IL PK





IOUT MAX





0.5

IL PP









IL RMS





IOUT MAX





IL PP







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

PIND CU





IL RMS





2 R

WIND



PWIND HT





RWIND 20C





1 0.0042

TH T20C

–













Where:

TH = Temperature of wire under full load

T20C = Ambient temperature

RWIND(20C) = Room temperature winding resistance

(usually specified by the manufacturer)

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