MIC2104 datasheet(28/42 Pages) MICROCHIP

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MIC2103/4

DS20005899A-page 28

 2017 Microchip Technology Inc.

Making the assumption that the turn-on and turn-off

transition times are equal; the transition times can be

approximated by:

EQUATION 5-7:

The total high-side MOSFET switching loss is:

EQUATION 5-8:

The high-side MOSFET switching losses increase with

the switching frequency and the power stage input

voltage VHSD. The low-side MOSFET switching losses

are negligible and can be ignored for these

calculations.

5.3

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 among 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 5-9:

EQUATION 5-9:

The peak-to-peak inductor current ripple is:

EQUATION 5-10:

The peak inductor current is equal to the average

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

inductor current ripple.

EQUATION 5-11:

The RMS inductor current is used to calculate the I2R

losses in the inductor.

EQUATION 5-12:

Maximizing efficiency requires the proper selection of

core material and minimizing the winding resistance.

The high frequency operation of the MIC2103/4

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 buck converter. 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

ISS



OSS

HSD



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

Where:

CISS and COSS are measured at VDS = 0.

IG = Gate drive current.

HSD

 I

LPK





Where:

tT = Switching transition time.

VD = Body diode drop (0.5V).

fSW = Switching frequency.

OUT

IN MAX



OUT

–





IN MAX





20%



OUT MAX





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

Where:

fSW = Switching frequency.

20% = Ratio of AC ripple current to DC output

current.

VIN(MAX) = Max. power stage input voltage.

LPP





OUT

IN MAX



OUT

–





IN MAX





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

LPK



OUT MAX



0.5

LPP







LRMS



OUT MAX



LPP





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

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