 2015 Microchip Technology Inc.

DS20005459B-page 21

MIC2125/6

EQUATION 5-6:

The high-side MOSFET switching losses increase with

the switching frequency and the input voltage. The

low-side MOSFET switching losses are negligible and

can be ignored for these calculations.

5.3

Inductor Selection

Inductance value, saturation, 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. Larger

peak-to-peak ripple current increases the power

dissipation in the inductor and MOSFETs. Larger

output ripple current also requires more output

capacitance to smooth out the larger ripple current.

Smaller peak-to-peak ripple current requires 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 40% of the

maximum output current.

The inductance value is calculated by Equation 5-7.

EQUATION 5-7:

The peak-to-peak inductor current ripple is:

EQUATION 5-8:

The peak inductor current is equal to the average

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

inductor current ripple.

EQUATION 5-9:

The saturation current rating is given by:

EQUATION 5-10:

losses in the inductor.

EQUATION 5-11:

Maximizing efficiency requires the proper selection of

core material and minimizing the winding resistance.

The high-frequency operation of the MIC2125/6

requires the use of ferrite materials. Lower cost iron

powder cores may be used, but the increase in core

loss reduces 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 significant.

Core loss information is usually available from the

magnetics vendor.

The amount of copper loss in the inductor is calculated

by Equation 5-12:

EQUATION 5-12:

t

F

Q

SW HS



R

HSD PULL

UP

–



R

HS GATE



+





V

TH

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

=

Where:

High-Side Gate Driver Pull-Up

Resistance

Resistance

High-Side MOSFET Gate

Resistance

Switching Gate Charge of the

High-Side MOSFET

Gate Threshold Voltage

L

V

OUT

V

IN MAX



V

OUT

–





V

IN MAX



f

SW



0.4



I

OUT MAX





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

=

Where:

Switching Frequency

0.4

Ratio of AC Ripple Current to DC Output

Current

Maximum Power Stage Input Voltage

I

LPP





V

OUT

V

IN MAX



V

OUT

–





V

IN MAX



f

SW



L



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

=

I

LPK



I

OUT MAX



0.5

I

LPP







+

=

I

LSAT



R

CL

I

CL



 V

OFFSET

–

R

DS ON



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

=

Where:

Current-Limit Resistor

Current-Limit Source Current

Current-Limit Comparator Offset

On-Resistance of Low-Side Power

MOSFET

I

LRMS



I

OUT MAX



2

I

LPP



2



12

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

+

=

P

INDUCTOR CU



I

LRMS



2

R

WINDING



=