MIC2132 datasheet(30/48 Pages) MICROCHIP

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MIC2132

DS20006654B-page 30

 2022 Microchip Technology Inc. and its subsidiaries

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 cur-

rents require a larger inductance value, and therefore,

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

Equation 5-1:

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 Equation 5-3.

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 MIC2132 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 Equation 5-5:

EQUATION 5-5:

The resistance of the copper wire, RWINDING, increases

with the temperature. The value of the winding

resistance used must be at the operating temperature

for accurate power dissipation estimation:

EQUATION 5-6:

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

L =

VOUT × (Eff × VIN(MAX) – VOUT) × NPH

Eff × VIN(MAX) × fSW × 0.2 × IOUT(MAX)

IL(PP) =

VOUT × (Eff × VIN(MAX) – VOUT)

Eff × VIN(MAX) × fSW × L

Where:

IOUTPH(MAX) = Maximum Average DC Output Current

Contributed per Phase

IOUT(MAX) = Maximum Output Current

n = Total Number of Phases

IL_PH(PK) = IOUTPH(MAX) + 0.5 × IL(PP)

IOUTPH(MAX) =

IOUT(MAX)

IL_PH(RMS) =

IL(PP)2

IOUTPH(MAX)2 +

√

PINDUCTOR(Cu) = IL_PH(RMS)2 × RWINDING

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)

RWINDING(HT) = RWINDING(20ºC) × [1 + 0.0042 × (TH – T20ºC)]

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