 2016 Microchip Technology Inc.

DS20005620A-page 21

MIC2128

EQUATION 5-8:

EQUATION 5-9:

EQUATION 5-10:

5.4.2

LOW-SIDE MOSFET POWER

LOSSES

is the sum of the power losses because of conduction

EQUATION 5-11:

EQUATION 5-12:

cycle.

5.5

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.

The lower the inductance value, the higher the

peak-to-peak ripple current through the inductor, which

increases the core losses in the inductor. Higher

inductor ripple current also requires more output

capacitance to smooth out the ripple current. The

greater

the

inductance

value,

the

lower

the

peak-to-peak ripple current, which results in 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 30% of the

maximum output current.

The inductance value is calculated by Equation 5-13:

EQUATION 5-13:

For a selected Inductor, the peak-to-peak inductor

ripple current ripple can be calculated using

Equation 5-14.

EQUATION 5-14:

The peak inductor current is equal to the load current

plus one half of the peak-to-peak inductor current ripple

which is shown in Equation 5-15.

EQUATION 5-15:

t

R

Q

SW HS



R

DH PULL_UP



R

HS GATE



+





V

DD

V

TH

–

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

=

t

F

Q

SW HS



R

DH P ULL_DOWN



R

HS GATE



+





V

TH

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

=

Where:

= High-side gate driver pull-up

resistance

resistance

= High-side MOSFET gate resistance

= Gate threshold voltage of the

high-side MOSFET

= Switching gate charge of the

high-side MOSFET which can be

approximated by Equation 5-10.

Q

SW HS



Q

GS HS



2

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

Q

GD HS



+

=

Where:

= High-side MOSFET gate to source

charge

P

LSFET

P

C ONDUC TION LS



P

DT

+

=

P

CONDUCTION LS



I

RMS LS



DS ON _LS





=

P

DT

2

V



F

I

LOAD



t

DT



f

SW



=

Where:

= Low-side MOSFET body diode forward

voltage drop

= Dead time which is approximately 20 ns

= Switching Frequency

= RMS current of the low-side MOSFET

which can be calculated using

Equation 5-12

I

RMS LS



I

LOAD

1

D

–



=

L

V

OUT

V

IN

V

OUT

–





V

IN

f

SW

0.3

I

FL







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

=

Where:

= Input voltage

= Switching frequency

= Full load current

= Output voltage

L_PP

V

OUT

V

IN

V

OUT

–





V

IN

f

SW

L





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

=

I

L_PK

I

LOAD

L_PP

2

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

+

=