MIC2104 datasheet(25/38 Pages) MICREL

25 / 38 page

Micrel, Inc.

MIC2103/04

November 26, 2013

Revision 2.0

A good compromise between 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 13:

OUT(max)

IN(max)

OUT

IN(max)

OUT

20%

)

−

Eq. 13

where:

fSW = Switching frequency

20% = Ratio of AC ripple current to DC output current

VIN(max) = Maximum power stage input voltage

The peak-to-peak inductor current ripple is:

)

IN(max)

OUT

IN(max)

OUT

L(pp)

−

∆

Eq. 14

The peak inductor current is equal to the average output

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

ripple.

IL(pk) =IOUT(max) + 0.5 ×

ΔI

L(pp)

Eq. 15

The RMS inductor current is used to calculate the I

losses in the inductor.

ΔI

L(PP)

OUT(max)

L(RMS)

Eq. 16

Maximizing efficiency requires the proper selection of

core material and minimizing the winding resistance. The

high frequency operation of the MIC2103/04 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 magnetic vendor.

Copper loss in the inductor is calculated by Equation 17:

PINDUCTOR(Cu) = IL(RMS)

× R

WINDING

Eq. 17

The resistance of the copper wire, RWINDING, increases

with

the

temperature.

The

value

the

winding

resistance used should be at the operating temperature.

PWINDING(Ht) = RWINDING(20°C) × (1 + 0.0042 × (TH – T20°C))

Eq. 18

where:

TH = temperature of wire under full load

T20°C = ambient temperature

RWINDING(20°C) = room temperature winding resistance

(usually specified by the manufacturer)

Output Capacitor Selection

The type of the output capacitor is usually determined by

its ESR (equivalent series resistance). Voltage and RMS

current capability are two other important factors for

selecting the output capacitor. Recommended capacitor

types are tantalum, low-ESR aluminum electrolytic, OS-

CON and POSCAP. The output capacitor’s ESR is

usually the main cause of the output ripple. The output

capacitor ESR also affects the control loop from a

stability point of view. The maximum value of ESR is

calculated:

L(PP)

OUT(pp)

ΔI

ΔV

ESR

OUT

≤

Eq. 19

where:

ΔVOUT(pp) = peak-to-peak output voltage ripple

ΔI

L(PP) = peak-to-peak inductor current ripple

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