MIC2103 datasheet(24/36 Pages) MICREL

24 / 36 page

Micrel, Inc.

MIC2103/04

August 2012

M9999-080712-A

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  ΔIL(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)

2  RWINDING

(Eq. 17)

The resistance of the copper wire, RWINDING, increases

with the temperature. The value of 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

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

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated in

Equation 20:

2

L(PP)

OUT

L(PP)

OUT(pp)

OUT

ESR

ΔI

ΔV





















(Eq. 20)

where:

D = duty cycle

COUT = output capacitance value

fsw = switching frequency

As described in the “Theory of Operation” subsection in

Functional Description

, the MIC2103/04 requires at least

20mV peak-to-peak ripple at the FB pin to make the gm

amplifier and the error comparator behave properly.

Also, the output voltage ripple should be in phase with

the inductor current. Therefore, the output voltage ripple

caused by the output capacitors value should be much

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