MIC2168 1MHz Synchronous Buck Control General Description
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MIC2168
MIC2168
1MHz Synchronous Buck Control
General Description
MIC2168 high-efficiency, simple 1MHz synchronous buck control housed small MSOP-10 package. MIC2168 allows compact DC/DC solutions with minimal external component count cost. MIC2168 operates from 14.5V input, without need additional bias voltage. output voltage precisely regulated down 0.8V. adaptive N-Channel MOSFET drive scheme allows efficiencies over across wide load range. MIC2168 senses current across high-side N-Channel MOSFET, eliminating need expensive lossy current-sense resistor. Current limit accuracy maintained positive temperature coefficient that tracks increasing RDS(ON) external MOSFET. Further cost space saved internal in-rush-current limiting digital soft-start. MIC2168 available 10-pin MSOP package, with wide junction operating range -40°C +125°C. support documentation found Micrel's site www.micrel.com.
Features
14.5V input voltage range Adjustable output voltage down 0.8V efficiency 1MHz operation Adjustable current-limit senses high-side N-Channel MOSFET current external current sense resistor Adaptive gate drive increases efficiency Ultra-fast response with hysteretic transient recovery mode Overvoltage protection protects load fault conditions Dual mode current limit speeds recovery time Hiccup mode short-circuit protection Internal soft-start Dual function COMP allows low-power shutdown Small size MSOP 10-lead package Point-of-load DC/DC conversion Set-top boxes Graphic cards power supplies Telecom power supplies Networking power supplies Cable modems routers
Applications
Typical Application
SD103BWS 100µF 4.7µF
EFFICIENCY
0.1µF
MIC2168 Efficiency
IRF7821 1.2µH 3.3V 150µF 3.24k
VOUT 3.3V ILOAD
MIC2168
COMP/EN 100pF 100nF
IRF7821
MIC2168 Adjustable Output 1MHz Converter
Micrel, Inc. 1849 Fortune Drive Jose, 95131 (408) 944-0800 (408) 944-0970 http://www.micrel.com
November 2003
M9999-111803
MIC2168
Ordering Information
Part Number MIC2168BMM Frequency 1MHz Junction Temp. Range -40°C +125°C Package 10-lead MSOP
Configuration
COMP/EN
10-Pin MSOP (MM)
Description
Number Name Function Supply Voltage (Input): 14.5V. Internal Linear Regulator (Output): external MOSFET gate drive supply voltage internal supply When <5V, this regulator operates dropout mode. Current Sense Enable (Input): Current-limit comparator noninverting input. current limit sensed across MOSFET during time. current resistor series with pin. Compensation (Input): Dual function pin. external compensation. this pulled below 0.2V, with reference fully device shuts down (50µA typical current draw). Feedback (Input): Input error amplifier. Regulates error amplifier 0.8V. Ground (Return). Low-Side Drive (Output): High-current driver output external synchronous MOSFET. Switch (Return): High-side MOSFET driver return. High-Side Drive (Output): High-current output-driver high-side MOSFET. When between 3.0V 2.5V threshold-rated MOSFETs should used. threshold MOSFETs should used. Boost (Input): Provides drive voltage high-side MOSFET driver. gate-drive voltage higher than source voltage minus diode drop.
COMP/EN
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Absolute Maximum Ratings(1)
Supply Voltage (VIN) 15.5V Booststrapped Voltage (VBST) Junction Temperature (TJ) -40°C +125°C Storage Temperature (TS) -65°C +150°C
Operating Ratings(2)
Supply Voltage (VIN) +14.5V Output Voltage Range 0.8V DMAX Package Thermal Resistance 10-lead MSOP 180°C/W
Electrical Characteristics(3)
25°C, unless otherwise specified. Bold values indicate -40°C +125°C Parameter Feedback Voltage Reference Feedback Voltage Reference Feedback Bias Current Output Voltage Line Regulation Output Voltage Load Regulation Output Voltage Total Regulation Oscillator Section Oscillator Frequency Maximum Duty Cycle Minimum On-Time(4) 1000 1100 14.5V; IOUT 10A; (VOUT 2.5V)(4) Condition over temp) 0.792 0.784 0.03 0.808 0.816 Units
Input Supply Mode Supply Current Shutdown Quiescent Current VCOMP Shutdown Threshold VCOMP Shutdown Blanking Period Digital Supply Voltage (VDD)
Notes: Absolute maximum ratings indicate limits beyond which damage component occur. Electrical specifications apply when operating device outside operating ratings. maximum allowable power dissipation function maximum junction temperature, TJ(max), junction-to-ambient thermal resistance, ambient temperature, maximum allowable power dissipation will result excessive temperature, regulator will into thermal shutdown. Devices sensitive, handling precautions required. Specification packaged product only. Guaranteed design.
-0.25V; 0.7V (output switching excluding external MOSFET gate current.) VCOMP/EN CCOMP 100nF
0.25
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Electrical Characteristics(5)
Parameter Error Amplifier Gain Transconductance Soft-Start Soft-Start Current Current Sense Over Current Trip Point Temperature Coefficient Output Fault Correction Thresholds Upper Threshold, VFB_OVT Lower Threshold, VFB_UVT Gate Drivers Rise/Fall Time Output Driver Impedance Into 3000pF Source, Sink, Source, Sink, Driver Non-Overlap Time
Notes: Specification packaged product only. Guaranteed design.
Condition
Units
ppm/°C
After timeout internal timer. "Soft-Start" section.
-0.25V
+1800
(relative VFB) (relative VFB)
Note
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Typical Characteristics
Mode Supply Current Temperature
Mode Supply Current Supply Voltage
100120140 TEMPERATURE (°C)
QUIESCENT CURRENT (mA)
0.820 0.815 0.810
Line Regulation
(mA)
0.805 0.800 0.795 0.790 0.785
SUPPLY VOLTAGE
0.780
Temperature
0.820 0.815 0.810
Line Regulation
REGULATOR VOLTAGE
5.01 4.99 4.97 4.95 4.93 4.91 4.89 4.87 4.85
Load Regulation
0.800 0.795 0.790 0.785 0.780 TEMPERATURE (°C)
0.805
LOAD CURRENT (mA)
LINE REGULATION
Line Regulation Temperature
1200 1150
FREQUENCY (kHz)
Oscillator Frequency Temperature
FREQUENCY VARIATION
Oscillator Frequency Supply Voltage
TEMPERATURE (°C)
-0.5 -1.0 -1.5 SUPPLY VOLTAGE
1100 1050 1000 TEMPERATURE (°C)
Current Limit Foldback
Overcurrent Trip Point Temperature
VOUT (µA)
MOSFET Si4800
ILOAD
TEMPERATURE (°C)
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Functional Diagram
Current Limit Comparator
High-Side Driver
BOOST CBST VOUT
Bandgap Reference
0.8V Valid
Current Limit Reference
Clamp Startup Current
Driver Logic
COUT
Soft-Start Digital Delay Counter
Ramp Clock
Enable Error Loop 0.8V
Low-Side Driver
Comparator
VREF
Comparator
Error
VREF
MIC2168
COMP
MIC2168 Block Diagram
Functional Description
MIC2168 voltage mode, synchronous step-down switching regulator controller designed high output power without external sense resistor. includes internal soft-start function which reduces power supply input surge current start-up controlling output voltage rise time, generator, reference voltage, MOSFET drivers, short-circuit current limiting circuitry form complete 1MHz switching regulator. Theory Operation MIC2168 voltage mode step-down regulator. figure above illustrates block diagram voltage control loop. output voltage variation load line changes will sensed inverting input transconductance error amplifier feedback resistors compared reference voltage noninverting input. This will cause small change voltage level output error amplifier which input comparator. other input comparator triangular waveform. comparator generates rectangular waveform whose width equal time from start clock cycle until time triangle crosses output waveform error amplifier. illustrate control loop, assume output voltage drops sudden load turn-on, this would cause
inverting input error amplifier which divided down version VOUT slightly less than reference voltage causing output voltage error amplifier high. This will cause comparator increase time side MOSFET, causing output voltage bringing VOUT back regulation. Soft-Start COMP/EN MIC2168 used following three functions: Disables part grounding this External compensation stabilize voltage control loop Soft-start better understanding soft-start feature, let's assume 12V, MIC2168 allowed power-up un-grounding COMP/EN pin. COMP internal 8.5µA current source that charges external compensation capacitor. soon this voltage rises 180mV Cap_COMP 0.18V/8.5µA), MIC2168 allows internal linear regulator power soon crosses undervoltage lockout 2.6V, chip's internal oscillator starts switching. this point time, COMP current source increases 40µA internal 11-bit counter starts counting which takes approximately complete. During counting, COMP voltage clamped November 2003
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0.65V. After this counting cycle COMP current source reduced 8.5µA COMP voltage rises from 0.65V 0.95V, bottom edge saw-tooth oscillator. This beginning duty cycle increases slowly causing output voltage rise slowly. MIC2168 hysteretic comparators that enabled when VOUT within steady state. When output voltage reaches programmed output voltage, then error amplifier enabled along with hysteretic comparator. This point onwards, voltage control loop error amplifier) fully control will regulate output voltage. Soft-start time calculated approximately adding following four time frames: Cap_COMP 0.18V/8.5µA counter, approx Cap_COMP 0.3V/8.5µA
Cap_COMP 8.5µA
current limiting resistor calculated following equation:
RDS(ON) 200µA
Equation
ILOAD where:
2(Inductor Ripple Current)
Inductor Ripple Current VOUT
FSWITCHING
(VIN VOUT
Soft-Start Time(Cap_COMP=100nF) 2.1ms 3.5ms 1.8ms 10ms Current Limit MIC2168 uses RDS(ON) power MOSFET measure output current. Since uses drain source resistance power MOSFET, very accurate. This scheme adequate protect power supply external components during fault condition cutting back time MOSFET feedback voltage greater than 0.67V. case hard short when feedback voltage less than 0.67V, MIC2168 discharges COMP capacitor 0.65V, resets digital counter automatically shuts gate drive, error amplifier hysteretic comparators completely disabled softstart cycles restarts. This mode operation called "hiccup mode" purpose protect down stream load case hard short. circuit Figure illustrates MIC2168 current limiting circuit.
Inductor MOSFET COUT MOSFET VOUT
200µA
Figure MIC2168 Current Limiting Circuit
FSWITCHING 1MHz 200µA internal sink current program MIC2168 current limit. MOSFET RDS(ON) varies with temperature; therefore, recommended margin load current (ILOAD) above equation avoid false current limiting increased MOSFET junction temperature rise. also recommended connect resistor directly drain MOSFET resistor source accurately sense MOSFETs RDS(ON). 0.1µF capacitor parallel with should connected filter some switching noise. Internal Supply MIC2168 controller internally generates self biasing provide power gate drives. This supply generated through low-dropout regulator generates from supply greater than supply voltage less than linear regulator approximately 200mV dropout. Therefore, recommended short supply input supply through resistor input supplies between 2.9V MOSFET Gate Drive MIC2168 high-side drive circuit designed switch N-Channel MOSFET. block diagram Figure shows bootstrap circuit, consisting CBST, supplies energy high-side drive circuit. Capacitor CBST charged while low-side MOSFET voltage approximately When high-side MOSFET driver turned energy from CBST used turn MOSFET MOSFET turns voltage increases approximately VIN. Diode reversed biased CBST floats high while continuing keep high-side MOSFET When low-side switch turned back CBST recharged through drive voltage derived from internal bias supply. nominal low-side gate drive voltage nominal high-side gate drive voltage approximately 4.5V voltage drop across approximate 20ns delay between high- low-side driver transitions used prevent current from simultaneously flowing unimpeded through both MOSFETs. MOSFET Selection MIC2168 controller works from input voltages 13.2V internal regulator provide power turn external N-Channel power MOSFETs high-
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low-side switches. applications where internal regulator operates dropout mode, necessary that power MOSFETs used threshold full conduction mode 2.5V. applications when logic-level MOSFETs, whose operation specified 4.5V must used. important note on-resistance MOSFET increases with increasing temperature. 75°C rise junction temperature will increase channel resistance MOSFET resistance specified 25°C. This change resistance must accounted when calculating MOSFET power dissipation calculating value current-sense (CS) resistor. Total gate charge charge required turn MOSFET under specified operating conditions (VDS VGS). gate charge supplied MIC2168 gate drive circuit. 1MHz switching frequency above, gate charge significant source power dissipation MIC2168. output load, this power dissipation noticeable reduction efficiency. average current required drive high-side MOSFET
IG[high-side](avg)
power dissipated switching transistor conduction losses during on-time (PCONDUCTION) switching losses that occur during period time when MOSFETs turn (PAC).
PCONDUCTION
where:
PCONDUCTION ISW(rms)2
PAC(off) PAC(on) on-resistance MOSFET switch
duty cycle
Making assumption turn-on turn-off transition times equal; transition times approximated CISS COSS
where: IG[high-side](avg) average high-side MOSFET gate current. total gate charge high-side MOSFET taken from manufacturer's data sheet low-side MOSFET turned because freewheeling diode conducting during this time. switching loss low-side MOSFET usually negligible. Also, gate-drive current low-side MOSFET more accurately calculated using CISS instead gate charge. low-side MOSFET:
IG[low-side](avg) CISS
where: CISS COSS measured gate-drive current MIC2168) total high-side MOSFET switching loss
(VIN
where: switching transition time (typically 20ns 50ns) freewheeling diode drop, typically 0.5V switching frequency, nominally 1MHz low-side MOSFET switching losses negligible ignored these calculations. Inductor Selection Values inductance, peak, currents required select output inductor. input output voltages inductance value determine peak-to-peak inductor ripple current. Generally, higher inductance values used with higher input voltages. Larger peak-to-peak ripple currents will increase power dissipation inductor MOSFETs. Larger output ripple currents will also require more output capacitance smooth larger ripple current. Smaller peak-to-peak ripple currents require larger inductance value therefore larger more expensive inductor. good compromise between size, loss cost inductor ripple current equal maximum output current. inductance value calculated equation below.
Since current from gate drive comes from input voltage, power dissipated MIC2168 gate drive PGATEDRIVE IG[high-side](avg) IG[low-side](avg)
convenient figure merit switching MOSFETs resistance times total gate charge RDS(ON) Lower numbers translate into higher efficiency. gate-charge logic-level MOSFETs good choice with MIC2168. Parameters that important MOSFET switch selection are: Voltage rating On-resistance Total gate charge voltage ratings bottom MOSFET essentially equal input voltage. safety factor should added VDS(max) MOSFETs account voltage spikes circuit parasitics.
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VOUT (VIN (max) VOUT (max) IOUT (max)
where: switching frequency, 1MHz ratio ripple current output current VIN(max) maximum input voltage November 2003
MIC2168
peak-to-peak inductor current ripple current)
feedback loop from stability point view. "Feedback Loop Compensation" section more information. maximum value calculated: RESR VOUT
VOUT (VIN (max) VOUT (max)
peak inductor current equal average output current plus half peak-to-peak inductor ripple current.
IOUT (max)
inductor current used calculate losses inductor.
where: VOUT peak-to-peak output voltage ripple peak-to-peak inductor ripple current total output ripple combination output capacitance. total ripple calculated below:
RESR COUT
IINDUCTOR(rms)
IOUT (max) IOUT (max)
VOUT
Maximizing efficiency requires proper selection core material minimizing winding resistance. high frequency operation MIC2168 requires ferrite materials most cost sensitive applications. Lower cost iron powder cores used increase core loss will reduce efficiency power supply. This especially noticeable output power. winding resistance decreases efficiency higher output current levels. winding resistance must minimized although this usually comes expense larger inductor. power dissipated inductor equal core copper losses. higher output loads, core losses usually insignificant ignored. lower output currents, core losses significant contributor. Core loss information usually available from magnetics vendor. Copper loss inductor calculated equation below:
where: duty cycle COUT output capacitance value switching frequency voltage rating capacitor should twice voltage tantalum greater aluminum electrolytic. output capacitor current calculated below:
RESR(C
OUT(rms)
power dissipated output capacitor
PDISS(C
OUT(rms)2
PINDUCTORCu IINDUCTOR(rms)2 WINDING
resistance copper wire, RWINDING, increases with temperature. value winding resistance used should operating temperature.
WINDING(hot) WINDING(20°C) 0.0042 (THOT T20°C
where: THOT temperature wire under operating load T20°C ambient temperature RWINDING(20°C) room temperature winding resistance (usually specified manufacturer) Output Capacitor Selection output capacitor values usually determined capacitors (equivalent series resistance). Voltage current capability other important factors selecting output capacitor. Recommended capacitors tantalum, low-ESR aluminum electrolytics, POSCAPS. output capacitor's usually main cause output ripple. output capacitor also affects overall voltage
Input Capacitor Selection input capacitor should selected ripple current rating voltage rating. Tantalum input capacitors fail when subjected high inrush currents, caused turning input supply Tantalum input capacitor voltage rating should least times maximum input voltage maximize reliability. Aluminum electrolytic, OS-CON, multilayer polymer film capacitors handle higher inrush currents without voltage derating. input voltage ripple will primarily depend input capacitor's ESR. peak input current equal peak inductor current,
IINDUCTOR(peak) RESR(C
input capacitor must rated input current ripple. value input capacitor current determined maximum output current. Assuming peak-to-peak inductor ripple current low:
ICIN (rms) IOUT (max)
power dissipated input capacitor PDISS(C
(rms)
RESR(C
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Voltage Setting Components MIC2168 requires resistors output voltage shown Figure
Error
decrease high frequency noise. MOSFET body diode used, must rated handle peak average current. body diode relatively slow reverse recovery time relatively high forward voltage drop. power lost diode proportional forward voltage drop diode. high-side MOSFET starts turn body diode becomes short circuit reverse recovery period, dissipating additional power. diode recovery circuit inductance will cause ringing during high-side MOSFET turn-on. external Schottky diode conducts lower forward voltage preventing body diode MOSFET from turning lower forward voltage drop dissipates less power than body diode. lack reverse recovery mechanism Schottky diode causes less ringing less power loss. Depending circuit components operating conditions, external Schottky diode will give 1/2% improvement efficiency. Feedback Loop Compensation MIC2168 controller comes with internal transconductance error amplifier used compensating voltage feedback loop placing capacitor (C1) series with resistor (R1) another capacitor parallel from COMP ground. "Functional Block Diagram." Power Stage power stage voltage mode controller inductor, with winding resistance (DCR) connected output capacitor, COUT, with electrical series resistance (ESR) shown Figure transfer function G(s), such system
COUT
VREF 0.8V MIC2168 [adj.]
Figure Voltage-Divider Configuration Where: VREF MIC2168 typically 0.8V output voltage determined equation: VREF typical value between 10k. large, allow noise introduced into voltage feedback loop. small, value, will decrease efficiency power supply, especially light loads. Once selected, calculated using: VREF VREF
External Schottky Diode external freewheeling diode used keep inductor current flow continuous while both MOSFETs turned off. This dead time prevents current from flowing unimpeded through both MOSFETs typically 15ns. diode conducts twice during each switching cycle. Although average current through this diode small, diode must able handle peak current.
ID(avg) IOUT 80ns
reverse voltage requirement diode
VDIODE(rrm)
Figure Output Filter Voltage Mode Buck Converter
G(s)
power dissipated Schottky diode
PDIODE ID(avg)
where: forward voltage peak diode current external Schottky diode, necessary circuit operation since low-side MOSFET contains parasitic body diode. external diode will improve efficiency
Plotting this transfer function with following assumed values (L=2 DCR=0.009, COUT=1000µF, ESR=0.050) gives insight needs compensate loop adding resistor capacitors COMP pin. Figures show gain curve phase curve above transfer function.
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PHASE
GAIN
37.5
1.10
1000000
1.103
.104
.105
.106 1000000
Figure Gain Curve G(s)
Figure Phase Curve with 0.002 seen from Figure that 50kHz, phase approximately -90° versus Figure where number -150°. This means that transconductance error amplifier provide phase boost about achieve closed loop phase margin crossover frequency 50kHz Figure versus 105° Figure simple compensation scheme allows maximum error amplifier phase boost about 90°. Therefore, easier stabilize MIC2168 voltage control loop using high value output capacitors. Error Amplifier
PHASE 1.103 .104 .105 .106 1000000
Figure Phase Curve G(s) seen from transfer function G(s) gain curve that output inductor capacitor create pole system with break frequency COUT
undesirable have high error amplifier gain high frequencies because high frequency noise spikes would picked transmitted large amplitude output, thus, gain should permitted fall high frequencies. frequency, desired have high open-loop gain attenuate power line ripple. Thus, error amplifier gain should allowed increase rapidly frequencies. transfer function with internal error amplifier approximated following equation:
Error Amplifier(z)
Therefore, 3.6kHz looking phase curve, seen that output capacitor (0.050) cancels poles (LCOUT) system introducing zero
fZERO COUT
above equation simplified assuming C2<<C1,
Therefore, FZERO 6.36kHz. From point view compensating voltage loop, recommended higher output capacitors since they provide phase gain power path. comparison purposes, Figure shows same phase curve with value 0.002.
Error Amplifier(z) (C1)(1
From above transfer function, that introduce zero pole following frequencies: Fzero= Fpole Fpole@origin
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Figures show gain phase curves above transfer function with 9.3k, 1000pF, 100pF, .005-1. seen that 50kHz, error amplifier exhibits approximately phase margin.
71.607
OPEN LOOP GAIN MARGIN
ERROR AMPLIFIER GAIN
42.933
1.10
1000000
Figure Open-Loop Gain Margin
.001
269.097
1000
10000000
Figure Error Amplifier Gain Curve
215.856
OPEN LOOP PHASE MARGIN
ERROR AMPLIFIER PHASE
1.10
1000000
Figure Open-Loop Phase Margin
1000000
Figure Error Amplifier Phase Curve Total Open-Loop Response open-loop response MIC2168 controller easily obtained adding power path error amplifier gains together, since they already scale. desirable have gain curve intersect zero tens kilohertz, this commonly called crossover frequency; phase margin crossover frequency should least 45°. Phase margins less cause power supply have substantial ringing when subjected transients, have little tolerance component environmental variations. Figures show open-loop gain phase margin. seen from Figure that gain curve intersects approximately 50kHz, from Figure that 50kHz, phase shows approximately margin.
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Design Example Layout Checklist: Connect current limiting (CS) resistor directly drain MOSFET Connect directly source MOSFET thru resistor. purpose this resistor filter switch node. feedback resistors should placed close pin. side should connect directly output node. this trace away from switch node (junction L1). bottom side should connect MIC2168. compensation resistor capacitors should placed right next COMP/EN other side should connect directly MIC2168 rather than going plane. input bulk capacitors should placed close drain MOSFET. ceramic capacitor should placed right MIC2168. 4.7µF 10µF ceramic capacitor should placed right pin. source bottom MOSFET should connect directly input capacitor with thick trace. output capacitor input capacitor should connect directly plane. Place 0.1µF ceramic capacitor parallel with resistor filter switching noise.
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Package Information
Rev.
10-Pin MSOP (MM)
MICREL, INC. 1849 FORTUNE DRIVE
JOSE, 95131
(408) 944-0800
(408) 944-0970
http://www.micrel.com
information furnished Micrel this datasheet believed accurate reliable. However, responsibility assumed Micrel use. Micrel reserves right change circuitry specifications time without notification customer. Micrel Products designed authorized components life support appliances, devices systems where malfunction product reasonably expected result personal injury. Life support devices systems devices systems that intended surgical implant into body support sustain life, whose failure perform reasonably expected result significant injury user. Purchaser's sale Micrel Products life support appliances, devices systems Purchaser's risk Purchaser agrees fully indemnify Micrel damages resulting from such sale. 2003 Micrel, Incorporated. M9999-111803
November 2003
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