MIC2124YMM datasheet(13/24 Pages) MICREL

13 / 24 page

Micrel, Inc.

MIC2124

June 2010

M9999-060810-D

where:

tT = Switching transition time

VD = Body diode drop (0.5V)

fSW = Switching Frequency (300kHz)

The low-side MOSFET switching losses are negligible

and can be ignored for these calculations.

Inductor Selection

Values for inductance, peak, 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. Generally, higher

inductance values are used with higher input voltages.

Larger peak-to-peak ripple currents will increase the

power dissipation in the inductor and MOSFETs. Larger

output ripple currents will also require more output

capacitance to smooth out the larger ripple current.

Smaller peak-to-peak ripple currents require 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 20% of the maximum output current. The

inductance value is calculated by the equation below.

(

)

OUT(max)

HSD

OUT

HSD(max)

OUT

20%

⋅

−

⋅

(12)

where:

fSW = switching frequency, 300 kHz

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

VHSD(max) = maximum power stage input voltage

The peak-to-peak inductor current ripple is:

)

ΔI

HSD(max)

OUT

HSD(max)

OUT

L(PP)

⋅

−

⋅

(13)

The peak inductor current is equal to the average output

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

ripple.

L(PP)

OUT(max)

L(pk)

ΔI

0.5

(14)

The RMS inductor current is used to calculate the I

losses in the inductor.

ΔI

L(PP)

OUT(max)

L(RMS)

(15)

Maximizing efficiency requires the proper selection of

core material and minimizing the winding resistance. The

high frequency operation of the MIC2124 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:

WINDING

L(RMS)

INDUCTOR

⋅

(16)

The resistance of the copper wire, RWINDING, increases

with the temperature. The value of the winding

resistance used should be at the operating temperature.

())

0.0042

WINDING(20

)

WINDING(Ht

−

⋅

(17)

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 capacitors

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

and POSCAPS. 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. See “Feedback Loop Compensation” section for

more information. The maximum value of ESR is

calculated:

L(PP)

OUT(PP)

ΔI

ΔV

ESR

OUT

≤

(18)

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 below:

()2

L(PP)

OUT

L(PP)

OUT(PP)

OUT

ESR

ΔI

ΔV

⋅

⎟

⎠

⎞

⎜

⎝

⎛

⋅

(19)

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