SDYA011 October 1996 Copyright 1996, Texas Instruments Incorporat

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SDYA011 October 1996 Copyright 1996, Texas Instruments Incorporat

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Printed-Circuit-Board Layout Improved Electromagnetic Compatibility
SDYA011 October 1996
Copyright 1996, Texas Instruments Incorporated
Contents
Title Page Abstract Introduction Behavior Digital Circuits Suppression Interference Supply Lines Suppression Interference Signal Lines Oscillator Summary
List Illustrations
Figure Title Page Current Paths Electronic System CMOS Inverter Circuit Supply Current CMOS Circuits Function Input Voltage Circuit With Parasitic Components Currents Supply Lines Currents Supply Lines Using Inductor Placement Arrangement Signal Lines Their Return Lines Currents Signal Supply Lines Layout Signal Ground Lines Crystal Oscillator Circuit Proposal Layout Metallization Oscillator
Abstract
significance electromagnetic compatibility (EMC) electronic circuits systems recently been increasing. This increase more stringent requirements electromagnetic properties equipment. property aspects interest: ability circuit generate lowest zero) interference, immunity circuit effects electromagnetic energy subjected effects electronic circuits systems well documented, little attention been paid circuit behavior interference generates. This application report explains important criteria that determine circuit and, thus, provides development engineer with information design circuits layout circuit boards.
Introduction
electronic circuit mainly determined components laid with respect each other electrical connections made between components. Every current flowing line generates current same magnitude flowing corresponding return line. This line loop creates antenna that radiate electromagnetic energy whose magnitude determined current amplitude, repetition frequency signal, geometrical area current loops. Figure shows current paths typical circuit layout.
Figure Current Paths Electronic System Classes lines that contribute, varying degrees, undesirable radiation that generated are:
supply lines Figure form loops A-C-D-B A-E-F-B. energy system needs operate conducted these lines. Since power consumption circuit constant depends instantaneous state, then frequency components generated individual parts system represented these supply lines. Because relatively high impedance supply lines (usually about fast current changes cannot suppressed while route, therefore, this function must fulfilled blocking capacitor (CB). Additional loops formed signal control lines (L-M-F-D N-Q-P-F). area these lines enclose usually small, those lines outside system considered. These lines often transmit signals high frequencies, signal control lines must considered. oscillator circuit external frequency-determining components form loop G-H-J-K. Since highest frequencies usually found this point, particular care must taken with design circuit avoid unnecessary interference voltages and, with routing connecting lines, minimize effective areas antennas.
Behavior Digital Circuits
Knowing relationships several important properties logic circuits leads specific effective ways improving EMC. These properties demonstrated with CMOS integrated circuits (ICs). example will help explain several improving effects that arise similar with other device technologies. Figure shows circuit simple inverter constructed with N-channel P-channel transistors. voltage, applied input, which less than threshold voltage (VIT-) N-channel transistor, this transistor will nonconducting, whereas, P-channel transistor will conduct. opposite way, N-channel transistor will conduct P-channel transistor will nonconducting voltage VIT+ applied input (VIT+ threshold voltage P-channel transistor). both cases, current, except negligible leakage currents, flows through circuit. This also reason extremely current consumption CMOS circuits quiescent state.
Input
Figure CMOS Inverter Circuit However, voltage between limits (VIT applied input this inverter, both transistors will more less conducting. result with this configuration considerable increase supply current (see Figure such case, HCMOS circuits take current about whereas, with advanced CMOS (AC), supply current increase over
Figure Supply Current CMOS Circuits Function Input Voltage
Because input voltage such circuit cannot traverse critical voltage region when changing from high vice versa) infinitely short time, there flow during this time pulse-shaped current peaks (often known current spikes), such magnitude that cannot neglected. input stage, current amplitudes must expected (see Figure Considerably more critical this phenomenon outputs Since output stages must drive load that connected output, these transistors must made considerably bigger. result, amplitudes current peaks also increase correspondingly values from HCMOS devices devices, with pulse width
Suppression Interference Supply Lines
current peaks mentioned previously most significant causes electromagnetic interference. Every time output switched, corresponding pulse current flows along supply lines. latter connections lead more less direct route from module central power supply. problem will aggravated when outputs switched high repetition rate, such along lines connecting processor with corresponding memory. practice, decoupling supply voltage close with ceramic capacitor recommended. digital systems this technique effective ensuring that, with expected load changes, inadmissible supply voltage changes occur. However, this will result only very limited reduction electromagnetic interference. achieve significant improvement, first necessary analyze complete circuit parasitic components. Figure shows circuit under examination. transistors output stage whose behavior analyzed. Connection surrounding circuitry made with Lp/Rp/Cp network, which represents parasitic components package. following individual values assumed: Inductance package leads Capacitance package leads Ohmic resistance package leads
Output
Figure Circuit With Parasitic Components From supply terminals connection made identified Figure across source. following values components impedance unit length line from source circuit boards terminal assumed: Inductance unit length nH/cm Capacitance unit length pF/cm Resistance unit length 0.01
supply line subsequently reaches first blocking capacitor, (see Figure right-hand totem), whose equivalent circuit made follows: Capacitance (typical value) Inductance leads (SMD package) Resistive losses
From here, long line (length taken next blocking capacitor, (see Figure center totem); this line capacitor also represented same equivalent circuit mentioned above. simplicity, will assumed that subsequent circuit represented well-known vehicle power-supply equivalent circuit components: Inductance Capacitance Resistance
behavior this circuit simulated using SPICE program; assumed that load connected output i.e., circuit left open. Figure shows calculated current waveforms. following definitions apply: ICC: Current connection IC1: Current first blocking capacitor IC2: Current second blocking capacitor
Current
Time
Figure Currents Supply Lines waveform current demonstrates current peaks already mentioned that have amplitude about From previous discussion determined that blocking capacitor scarcely able smooth this pulse current. fact, resonant circuit formed line inductance (principally that package will excited, increase current will take place (current IC1). major part current (IC2) transplanted supply line, flows with scarcely diminished amplitude also into next From point view circuit shown, this form, unable significantly reduce radiated interference. long supply lines which, practice, always present with relatively large areas that these lines surround, form effective antenna. frequencies present, unacceptable level interference radiated.
improve behavior circuit, measures must first taken ensure that spread system currents shown Figure limited. This cannot achieved with alone; improvement properties, relative requirements detailed here, cannot achieved. Because inductance causing interference already been formed, large extent packages connection capacitor, significant improvement achieved simply connecting parallel several capacitors having different capacitance values. greater concern preventing current causing disturbance from reaching other parts circuit. This achieved introducing inductive coil behind first which represents sufficiently high resistance high frequencies. simulated circuit, inductor having inductance assumed, impedance which could limited high frequencies resistor connected parallel. results simulation shown Figure might expected, currents leads first (IC1) have become smaller. However, Figure shows that there reduction current amplitude (ICH) more than after inductor. This method contribute significant reduction radiation.
Current
Time
Figure Currents Supply Lines Using Inductor Next question individual components should arranged circuit board achieve maximum reduction radiation. Figure shows circuit proposal this purpose. grounded area under connected circuit. This ground ensures that major part field lines emanating from concentrated between ground level. result skin effect large surface area, line inductance reduced still further. immaterial whether capacitor situated near positive (VCC) negative (GND) supply connection. only important that parasitic inductances effective areas antennas kept small possible. inductor (LCH) should close possible part circuit where interference suppressed.
Figure Placement
Suppression Interference Signal Lines
Figure shows where signal currents should flow reduce interference radiated from signal lines. this circuit, gate drives line that terminated with impedance impedance made input capacitance (CIN input resistance (RIN) several kilohms megohms. transmission negative signal edge, current flows from output driver drain, from drain, ground line, back signal source. Simply expressed, capacitance connecting line input capacitance receiver discharged output resistance driver. When positive signal edge transmitted, opposite occurs: this capacitance must charged supply voltage source output resistance driver. this case, these signal currents also appear supply lines. This demonstrates that precautions taken reduce interference from supply lines effective.
Figure Arrangement Signal Lines Their Return Lines Figure shows results simulation arrangement just discussed. this example, output drives 5-cm-long line having characteristic impedance that terminated with parallel. result largely capacitive loading, amplitude current peak significantly reduced negative edge output VOUT. capacitance output keeps voltage this point original potential (high) short time prevents current flow through upper transistor output stage (voltage difference positive edge, signal current IOUT added lateral current output ICC.
VOUT
Current
IOUT Time
Figure Currents Signal Supply Lines Currents reduced connecting resistance (RS) series with output. Line-transmission theory shows that this resistance negative influence speed circuit, provided output resistance driver (consisting internal resistance series-resistance smaller than, equal characteristic impedance line which connected. practice, resistance values about current amplitude reduced about This solution needs more components should used only when distortion resulting from line reflections must reduced same time. Care should taken make antennas ineffective possible, i.e., make areas enclosed outward return lines small possible. effective method return line parallel signal line (see Figure 10). (This automatically ensured with multilayer circuit boards that have continuous ground level under signal lines.) signals with high frequencies (such clock signals) transmitted lines very long, this method will often used. this case, lines having defined line impedances cautious reflections) result will also provided. With appropriate layout additional ground lines, crosstalk between critical lines reduced.
Ground Matrix
Ground Return
Figure Layout Signal Ground Lines
most cost-effective technically-effective method consists simply keeping critical lines short possible, while observing following priorities: Clock lines Low-order address lines between processor memory Data lines between processor memory
between which information high frequencies exchanged should mounted close another possible keep line lengths short. This applies particularly lines between microprocessor memory. next step keep areas antennas small possible, i.e., provide transmitted signals with return path which, turn, close possible corresponding signal line. reduce effect tangled lines circuit boards fast digital circuits, ground connection circuit board form network effective, mesh area should only square centimeters. this way, inductance connections ground their lengths optimized. This technique results short return lines small-area antennas. With logical reduction mesh area, final arrangement conforms electrically that continuous ground layer multilayer circuit board. Ground lines with spacing horizontally vertically make required network structure. Subsequently, free areas filled with copper, which then must connected shortest possible path ground potential. With large areas, advisable make contacts several ends. With positive supply line connected firmly supply-voltage connections blocking capacitors ground system, network structure connection needed.
Oscillator
highest frequencies digital systems usually found clock generator. From this point, oscillator signal transmitted other subsystems, mostly form divided frequency. customary oscillator amplifier integrated into microcomputer processors that only passive components, such crystal necessary capacitors, need connected externally (see Figure 11).
Figure Crystal Oscillator Circuit
crystal-oscillator circuit needs analyzed with respect flow significant currents determine where interference suppression necessary. parallel resonant circuit formed delta section, consisting crystal capacitors (C). crystal behaves like inductance, with resonant frequency being somewhat above actual resonant frequency crystal. impedance delta section, measured input output, typically amounts several tens kilohms because high crystal. When components correctly dimensioned, very small current (IO) flows between amplifier external components because high resistance circuit. However, there opposite effect result circuits having output impedances that ideally matched crystal; they should also several kilohms. addition, these circuits usually supply square-wave signal containing harmonics, which delta section longer represents high resistance. result correspondingly high output currents amplifier. improvement usually achieved resistor (RS) series with amplifier output (see Figure 11). Ideally, voltage waveform input resonant circuit will then sine wave. output correctly terminated high input impedance circuit, such that, this case, only very small current (II) flows. Capacitor (see Figure impedance only hundred ohms resonant frequency. Consequently, current (IS) flows resonant circuit that much higher than current line leading this part circuit. This loop must regarded considerably more critical; therefore, construction must compact, with extremely short lines. Figure suggests this done. capacitors resonant circuit placed directly beside crystal (X). Note that these components should close possible corresponding pins
Ground Input
Output
Figure Proposal Layout Metallization Oscillator crystal capacitors' part circuit board, radiated interference that results from them, largely under control development engineer. Nevertheless, also necessary that ground connection needed amplifier made near possible i.e., beside amplifier connections, possible. This ensures that when there also longer connections package, unavoidable current loops will enclose only small area.
Summary
This application report covers several important factors considered when designing circuit boards ensure subsystems. proposals based well-understood basic principles have been successfully implemented make electronic circuits immune self-generated interference (e.g., crosstalk) interference coupled into them from outside sources. Since radiation simply opposite irradiation, logical further development application these rules results circuits that fulfill requirements electromagnetic compatibility. implementation EMC-compatible circuit boards begins when circuit first being developed components being selected. wrong decisions made this early stage, they often must corrected later with considerable expenditure time effort, example, with costly screening. understanding circuit operation absolutely necessary when laying circuit boards ensure that appropriate measures taken. reduction effective areas antennas requires, example, that only signal line taken shortest route, corresponding return line well. Perhaps longer line, that taken parallel existing ground supply line, will better solution. Computer-aided layout programs have, until now, been unable provide useable results with respect improvement EMC. processes used these programs take electrical requirements into account. This means that experience development engineer needed decide where every critical connection should made. computer then serve intelligent draftsman.

SDYA011 October 1996 Copyright 1996, Texas Instruments Incorporat

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