Micrel, Inc.

MIC2199.

January 2010

9

M9999-011310

Applications Information

Followingapplicationsinformationincludescomponentselec-

tion and design guidelines.

Inductor Selection

Valuesforinductance,peak,andRMScurrentsarerequired

to select the output inductor. The input and output voltages

and the inductance value determine the peak-to-peak induc-

tor 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

andMOSFETs.Largeroutputripplecurrentswillalsorequire

more output capacitance to smooth out the larger ripple cur-

rent.Smallerpeak-to-peakripplecurrentsrequirealarger

inductance value and therefore a larger and more expensive

inductor. A good compromise between size, loss and cost is

tosettheinductorripplecurrenttobeequalto20%ofthe

maximum output current.

The inductance value is calculated by the equation below.

L

V

(V

)

V

0.2

OUT

IN(max)

OUT

IN(max)

S

OUT(max)

=

×

×

×

- V

f ×

I

where:

0.2 = ratio of AC ripple current to DC output current

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

I

V(VV

)

Vf

L

PP

OUT

IN(max)

OUT

IN(max)

S

=

×−

××

Thepeakinductorcurrentisequaltotheaverageoutputcurrent

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

II

0.5I

PK

OUT(max)

PP

=+

×

in the inductor.

II

1 1

3

I

I

INDUCTOR(rms)

OUT(max)

P

OUT(max)

2

=×

+













Maximizingefficiencyrequirestheproperselectionofcore

material and minimizing the winding resistance. The high

frequency operation of the MIC2199 requires the use of fer-

rite materials for all but the most cost sensitive applications.

Lower cost iron powder cores may be used but the increase

incorelosswillreducetheefficiencyofthepowersupply.

This is especially noticeable at low output power. The winding

resistancedecreasesefficiencyatthehigheroutputcurrent

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

corelossesareusuallyinsignificantandcanbeignored.At

loweroutputcurrents,thecorelossescanbeasignificant

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

=×

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:

resistance

(usuallyspecifiedbythemanufacturer)

Current-Sense Resistor Selection

Lowinductancepowerresistors,suchasmetalfilmresistors

should be used. Most resistor manufacturers make low induc-

tanceresistorswithlowtemperaturecoefficients,designed

specificallyforcurrent-senseapplications.Bothresistance

and power dissipation must be calculated before the resis-

maximum output current and the maximum threshold level.

The power dissipated is based on the maximum peak output

current at the minimum overcurrent threshold limit.

R

55mV

I

SENSE

OUT(max)

=

The maximum overcurrent threshold is:

I

95mV

R

OVERCURRENT(max)

CS

=

The maximum power dissipated in the sense resistor is:

PI

R

D(R

)

OVERCURRENT(max)

2

CS

SENSE

=×

MOSFET Selection

External N-Channel logic-level power MOSFETs must be

used for the high- and low-side switches. The MOSFET

gate-to-source drive voltage of the MIC2199 is regulated by

Itisimportanttonotetheon-resistanceofaMOSFETin-

creases with increasing temperature. A 75°C rise in junction

temperaturewillincreasethechannelresistanceoftheMOS-

FETby50%to75%oftheresistancespecifiedat25°C.This

change in resistance must be accounted for when calculating

MOSFETpowerdissipation.

TotalgatechargeisthechargerequiredtoturntheMOSFET

circuit. At 500kHz switching frequency, the gate charge can

beasignificantsourceofpowerdissipationintheMIC2199.

At low output load this power dissipation is noticeable as a

reductioninefficiency.Theaveragecurrentrequiredtodrive

thehigh-sideMOSFETis:

IQ

f

G[high-side](avg)

GS

=×