November 2003

9

M9999-111803

MIC2168

Micrel

The peak-to-peak inductor current (AC ripple current) is:

I

V

(V max

V

)

V max

f

L

PP

OUT

IN

OUT

IN

S

=

×−

××

()

()

The peak inductor current is equal to the average output

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

current.

I

I

max

0.5 I

PK

OUT

PP

=+

×

()

× R

losses in the inductor.

I

I

max

1

1

3

I

I

max

INDUCTOR(rms)

OUT

P

OUT

2

=×

+









()

()

Maximizing efficiency requires the proper selection of core

material and minimizing the winding resistance. The high

frequency operation of the MIC2168 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 the

equation below:

PI

R

INDUCTORCu

INDUCTOR(rms)

WINDING

2

=×

The resistance of the copper wire, R

temperature. The value of the winding resistance used should

be at the operating temperature.

R

R

1 0.0042 (T

T

)

WINDING(hot)

WINDING(20 C)

HOT

20 C

=× +

×

−

()

°°

where:

T

T

20

R

WINDING(20

ally specified by the manufacturer)

Output Capacitor Selection

The output capacitor values are usually determined capaci-

tors ESR (equivalent series resistance). Voltage and RMS

current capability are two other important factors selecting

the output capacitor. Recommended capacitors tantalum,

low-ESR aluminum electrolytics, and POSCAPS. The output

capacitor’s ESR is usually the main cause of output ripple.

The output capacitor ESR also affects the overall voltage

feedback loop from stability point of view. See

“Feedback

maximum value of ESR is calculated:

R

V

I

ESR

OUT

PP

≤

∆

where:

V

I

The total output ripple is a combination of the ESR output

capacitance. The total ripple is calculated below:

∆V

I(1 D)

Cf

IR

OUT

PP

OUT

S

2

PP

ESR

2

=

×−

×











 +×

()

where:

D = duty cycle

C

f

The voltage rating of capacitor should be twice the voltage for

a tantalum and 20% greater for an aluminum electrolytic.

The output capacitor RMS current is calculated below:

I

I

12

C

PP

OUT(rms)

=

The power dissipated in the output capacitor is:

PI

R

DISS(C

C

ESR(C

)

OUT

OUT(rms)2

OUT

Input Capacitor Selection

The input capacitor should be selected for ripple current

rating and voltage rating. Tantalum input capacitors may fail

when subjected to high inrush currents, caused by turning the

input supply on. Tantalum input capacitor voltage rating

should be at least 2 times the maximum input voltage to

maximize reliability. Aluminum electrolytic, OS-CON, and

multilayer polymer film capacitors can handle the higher

inrush currents without voltage derating. The input voltage

ripple will primarily depend on the input capacitor’s ESR. The

peak input current is equal to the peak inductor current, so:

∆VI

R

IN

INDUCTOR(peak)

ESR(C )

IN

=×

The input capacitor must be rated for the input current ripple.

The RMS value of input capacitor current is determined at the

maximum output current. Assuming the peak-to-peak induc-

tor ripple current is low:

I

I

max

D (1 D)

C (rms)

OUT

IN

≈×

× −

()

The power dissipated in the input capacitor is:

PI

R

DISS(C )

C (rms)

ESR(C )

IN

IN

2

IN

=×