Micrel, Inc.

MIC2199.

January 2010

11

M9999-011310

Forthelow-sideswitch(N-ChannelMOSFET),theDCpower

dissipation is:

PR

I

SWITCH2(dc)

DS(on)2

=×

SincetheACswitchinglossesforthelowsideMOSFETis

near zero, the total power dissipation is:

PP

low-sideMOSFET(max)

SWITCH2(dc)

=

ThetotalpowerdissipationforthehighsideMOSFETis:

PP

P

high sideMOSFET(max)

SWITCH 1(dc)

AC

−

=+

External Schottky Diode

An external freewheeling diode is used to keep the inductor

currentflowcontinuouswhilebothMOSFETsareturnedoff.

This dead time prevents current from flowing unimpeded

through both MOSFETs and is typically 80ns The diode

conducts twice during each switching cycle. Although the

average current through this diode is small, the diode must

be able to handle the peak current.

II

2 80ns f

D(avg)

OUT

S

=× ××

The reverse voltage requirement of the diode is:

VV

DIODE(rrm)

IN

=

ThepowerdissipatedbytheSchottkydiodeis:

PI

V

DIODE

D(avg)

F

=×

where:

TheexternalSchottkydiode,D2,isnotnecessaryforcircuit

operationsincethelow-sideMOSFETcontainsaparasitic

bodydiode.Theexternaldiodewillimproveefficiencyand

decreasehighfrequencynoise.IftheMOSFETbodydiode

is used, it must be rated to handle the peak and average cur-

rent. The body diode has a relatively slow reverse recovery

time and a relatively high forward voltage drop. The power

lost in the diode is proportional to the forward voltage drop

ofthediode.Asthehigh-sideMOSFETstartstoturnon,the

body diode becomes a short circuit for the reverse recovery

period, dissipating additional power. The diode recovery and

the circuit inductance will cause ringing during the high-side

MOSFETturn-on.

AnexternalSchottkydiodeconductsatalowerforwardvoltage

preventingthebodydiodeintheMOSFETfromturningon.

The lower forward voltage drop dissipates less power than

the body diode. The lack of a reverse recovery mechanism

inaSchottkydiodecauseslessringingandlesspowerloss.

Depending on the circuit components and operating condi-

tions, an external Schottky diode will give a 1/2% to 1%

improvementinefficiency.

Output Capacitor Selection

The output capacitor values are usually determined by the

capacitorsESR(equivalentseriesresistance).Voltagerating

andRMScurrentcapabilityaretwootherimportantfactorsin

selecting the output capacitor. Recommended capacitors are

tantalum,low-ESRaluminumelectrolytics,andOS-CON.

Theoutputcapacitor’sESRisusuallythemaincauseofoutput

ripple.ThemaximumvalueofESRiscalculatedby:

R

V

I

ESR

OUT

PP

≤

∆

where:

ThetotaloutputrippleisacombinationoftheESRandthe

output capacitance. The total ripple is calculated below:

∆V

I(1D)

Cf

IR

OUT

PP

OUT

S

2

PP

ESR

2

=

×−

×











 +×

()

where:

D = duty cycle

The voltage rating of capacitor should be twice the output

voltage for a tantalum and 20% greater for an aluminum

electrolyticorOS-CON.

TheoutputcapacitorRMScurrentiscalculatedbelow:

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

polymerfilmcapacitorscanhandlethehigherinrushcurrents

without voltage derating.

The input voltage ripple will primarily depend on the input

capacitorsESR.Thepeakinputcurrentisequaltothepeak

inductor current, so:

∆VI

R

IN

INDUCTOR(peak)

ESR(C )

IN

=×

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

TheRMSvalueofinputcapacitorcurrentisdeterminedat

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

inductor ripple current is low:

II

D(1D)

C (rms) OUT(max)

IN

≈×

×−

The power dissipated in the input capacitor is:

PI

R

DISS(C )

C (rms)

ESR(C )

IN

IN

2

IN

=×