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MIC2199 Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC2199 Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 14 page 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: fS= switching frequency 0.2 = ratio of AC ripple current to DC output current VIN(max) = maximum input voltage 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 =+ × TheRMSinductorcurrentisusedtocalculatetheI2×R losses 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 =× The resistance of the copper wire, RWINDING, increases with 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: THOT = temperature of the wire under operating load T20°C = ambient temperature RWINDING(20°C) is room temperature winding 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- tor is selected. The value of RSENSE is chosen based on the 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 an internal 5V VDDregulator.Logic-levelMOSFETs,whose operationisspecifiedatVGS = 4.5V must be used. 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 onandoffunderspecifiedoperatingconditions(VDS and VGS). The gate charge is supplied by the MIC2199 gate drive 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 =× |
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