MIC2199BML datasheet(10/14 Pages) MICREL

10 / 14 page

January 2010

M9999-011310

Micrel, Inc.

MIC2199

where:

IG[high-side](avg) =

averagehigh-sideMOSFETgatecurrent

QG=totalgatechargeforthehigh-sideMOSFET

taken from manufacturer’s data sheet

with VGS = 5V.

fs = 300kHz

Thelow-sideMOSFETisturnedonandoffatVDS=0because

the freewheeling diode is conducting during this time. The

switchinglossesforthelow-sideMOSFETisusuallynegli-

gible.Also,thegatedrivecurrentforthelow-sideMOSFET

is more accurately calculated using CISS at VDS = 0 instead

of gate charge.

Forthelow-sideMOSFET:

G[low-side](avg)

ISS

=×

Sincethecurrentfromthegatedrivecomesfromtheinput

voltage, the power dissipated in the MIC2199 due to gate

drive is:

PV II

GATEDRIVE

IN G[high-side](avg)

G[low-side](avg)

()

AconvenientfigureofmeritforswitchingMOSFETsisthe

on-resistance times the total gate charge (RDS(on) × QG).

Lowernumberstranslateintohigherefficiency.Lowgate-

chargelogic-levelMOSFETsareagoodchoiceforusewith

the MIC2199. Power dissipation in the MIC2199 package

limits the maximum gate drive current.

ParametersthatareimportanttoMOSFETswitchselection

are:

• Voltagerating

• On-resistance

• Totalgatecharge

ThevoltageratingoftheMOSFETsareessentiallyequalto

theinputvoltage.Asafetyfactorof20%shouldbeaddedto

the VDS(max)oftheMOSFETstoaccountforvoltagespikes

due to circuit parasitics.

The power dissipated in the switching transistor is the sum

of the conduction losses during the on-time (PCONDUCTION)

and the switching losses that occur during the period of time

whentheMOSFETsturnonandoff(PAC).

CONDUCTION

where:

CONDUCTION

SW(rms)

=×

AC(off)

AC(on)

RSW=on-resistanceoftheMOSFETswitch.

Makingtheassumptiontheturn-onandturnofftransitiontimes

are equal, the transition time can be approximated by:

ISS

OSS

×+

where:

CISS and COSS are measured at VDS = 0.

IG = gate drive current (1A for the MIC2199)

Thetotalhigh-sideMOSFETswitchinglossis:

(VV )I

DPKT

××

where:

tT = switching transition time (typically 20ns to 50ns)

VD = freewheeling diode drop, typically 0.5V.

fS it the switching frequency, nominally 300kHz

Thelow-sideMOSFETswitchinglossesarenegligibleand

can be ignored for these calculations.

RMS Current and MOSFET Power Dissipation

Calculation

Undernormaloperation,thehigh-sideMOSFETsRMScur-

rent is greatest when VIN is low (maximum duty cycle). The

low-sideMOSFETsRMScurrentisgreatestwhenVIN is high

(minimum duty cycle). However, the maximum stress the

MOSFETsseeoccursduringshortcircuitconditions,where

the output current is equal to IOVERCURRENT(max).(Seethe

“Sense Resistor” section). The calculations below are for

normal operation. To calculate the stress under short circuit

conditions, substitute IOVERCURRENT(max) for IOUT(max). Use

the formula below to calculate D under short circuit condi-

tions.

D0.063 1.810V

SHORTCIRCUIT

=−

××

−

TheRMSvalueofthehigh-sideswitchcurrentis:

ID I

SW(high side)(rms)

OUT(max)

−

=×













I1 DI

SW(low side)(rms)

OUT(max)

−

=−

()













where:

D = duty cycle of the converter

OUT

=efficiencyoftheconverter.

Converter efficiency depends on component parameters,

which have not yet been selected. For design purposes, an

efficiencyof90%canbeusedforVINlessthan10Vand85%

can be used for VINgreaterthan10V.Theefficiencycanbe

more accurately calculated once the design is complete. If the

assumedefficiencyisgrosslyinaccurate,aseconditeration

through the design procedure can be made.

For the high-side switch, the maximum DC power dissipa-

tion is:

SWITCH1(dc)

DS(on)1

SW1(rms) 2

=×

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